CBSE Class 10th Science | Chapter 5. Periodic Classification of Elements | Solved Exercises
Things to remember :
In Text Questions | Page 81 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Answer. Yes, Dobereiner's triads also exist in the columns of Newlands' Octave. For example, Li, Na, K is a triad of Dobereiner which also exists in the columns of Newlands' Octaves.
Question 2. What were the limitations of Döbereiner’s classification?
Answer.
Question 3. What were the limitations of Newlands’ Law of Octaves?
Answer.
Activity 5.1 | Page 84 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Looking at its resemblance to alkali metals and the halogen family, try to assign hydrogen a correct position in Mendeléev’s Periodic Table.
Answer. Mendeleev placed hydrogen with alkali metals while its some properties are similar to halogens. The position of hydrogen in Mendeleev's Periodic Table is correct because its properties are more similar to the alkali metals as it has the property of losing the electron, i.e., it is electropositive
Question 2. To which group and period should hydrogen be assigned?
Answer.First group and first period
Activity 5.2 | Page 85 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Consider the isotopes of chlorine, Cl-35 and Cl-37.
Question 1. Would you place them in different slots because their atomic masses are different?
Answer. No, the more fundamental base of classification is atomic number and not atomic mass
Question 2. Or would you place them in the same position because their chemical properties are the same?
Answer.Yes, both the isotopes are placed in same position because they have same chemical properties and same atomic number.
Intext Questions | Page 85 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements:
K, C, AI, Si, Ba.
Answer.
Question 2. Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (any two)
Answer. (i) Scandium (ii) Germanium
Question 3. What were the criteria used by Mendeléev in creating his Periodic Table?
Answer. (i) He arranged the elements on the basis of their increasing atomic masses.
(ii) Second criteria was the similarity in the chemical properties
(iii) The formulae of hydides and oxides formed by an element were treated as one of the basic property of an element for the classification used by Mendeleev.
Question 4. Why do you think the noble gases are placed in a separate group?
Answer.Nobel gases are placed in separate group because:
(i) These gases discovered very late as they are very inert
(ii) These gases could be placed in a new group without disturbing the existing order
Activity 5.3 | Page 85 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. How were the positions of cobalt and nickel resolved in the Modern Periodic Table?
Answer. Modern Periodic Table is based on the atomic number. Cobalt (27) is placed before Nickel (28) in Modern Periodic Table
Question 2. How were the positions of isotopes of various elements decided in the Modern Periodic Table?
Answer.The isotopes have same atomic number so they are placed in the same group in Modern Periodic Table
Question 3. Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?
Answer.No, it is not possible because atomic number is a whole number
Question 4. Where do you think should hydrogen be placed in the Modern Periodic Table?
Answer.I think that the place of hydrogen in the Modern Periodic Table is correct
Activity 5.4 | Page 87 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Look at the group 1 of the Modern Periodic Table, and name the elements present in it.
Answer. The elements in group 1 are:
Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (k), Rubidium (Rb), Cesium (Cs) and Francium (Fr).
Question 2. Write down the electronic configuration of the first three elements of group 1.
Answer.
(i) H (1) --> 1
(ii) Li(3) -->2, 1
(iii) Na (11) --> 2, 8, 1
Question 3.What similarity do you find in their electronic configurations?
Answer. All the elements have same number of valence electron, i.e. 1
Question 4. How many valence electrons are present in these three elements?
Answer. One (1)
Activity 5.5 | Page 87 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. If you look at the long form of the Periodic Table, you will find that the elements Li, Be, B, C, N, O, F, and Ne are present in the second period. Write down their electronic configuration.
Answer.
Question 2. Do these elements also contain the same number of valence electrons?
Answer. No
Question 3. Do they contain the same number of shells?
Answer. Yes
The elements of same periods have same number of shells but not same number of valence electrons, which increases by 1
Activity 5.6 | Page 88 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. How do you calculate the valency of an element from its electronic configuration?
Answer.
Valency of metal : It is same as the number of valence electron i.e. 1, 2, and 3.
Valency of non-metal : Valency of non-metals can be calculated by subtracting number of valence electrons from 8 (i.e. 8- number of valence electrons).
For example :
Question 2.What is the valency of magnesium with atomic number 12 and sulphur with atomic number 16?
Answer.Magnesium (12) --> 2, 8, 2
Sulphur (16) --> 2, 8, 6
The valency of Magnesium is same as valence electron, i.e. 2
The valency of Sulphur is 8-6 = 2 because it is non-metal.
Question 3. Similarly find out the valencies of the first twenty elements.
Answer.
Question 4. How does the valency vary in a period on going from left to right?
Answer. Valency first increases 1 to 4 then decreases from 4 to 0 (1, 2, 3, 4, 3, 2, 1, 0).
Question 5. How does the valency vary in going down a group?
Answer. Valency remains the same because valence electrons do not change on going down in a group.
Activity 5.7 | Page 88 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Atomic radii of the elements of the second period are given below:
Answer.
Question 1. Arrange them in decreasing order of their atomic radii.
Answer.
Question 2. Are the elements now arranged in the pattern of a period in the Periodic Table?
Answer. Yes
Question 3.Which elements have the largest and the smallest atoms?
Answer. Lithium (Li) has Largest atoms (152 pm)
Oxygen (O) has smallest atoms (77 pm)
Question 4. How does the atomic radius change as you go from left to right in a period?
Answer. The atomic radius decreases as we go left to right in a period
Activity 5.8 | Page 89 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Study the variation in the atomic radii of first group elements given below and arrange them in an increasing order.
Answer.
Question 2. Name the elements which have the smallest and the largest atoms.
Answer. Na (Sodium) has smallest atom
Ca (Calcium) has largest atom.
Question 3. How does the atomic size vary as you go down a group?
Answer.Atomic size increases as we go down a group
Activity 5.9 | Page 89 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Examine elements of the third period and classify them as metals and non-metals.
Answer. Elements of third period are : Na(11), Mg (12), Al (13), Si (14), P (15), S (16), Cl (17), Ar (18)
Question 2. On which side of the Periodic Table do you find the metals?
Answer.On the left side
Question 3. On which side of the Periodic Table do you find the non- metals?
Answer. On the right side
Activity 5.10 | Page 89 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. How do you think the tendency to lose electrons will change in a group?
Answer. The tendency of lose electrons increases in a group on going down
Question 2. How will this tendency change in a period?
Answer. The tendency of lose electrons decreases as we go left to right in a period.
Activity 5.11 | Page 90 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1.How would the tendency to gain electrons change as you go from left to right across a period?
Answer. The tendency of gaining electrons increases as we go left to right in a period upto 17th group. It decreases in 18th group.
Question 2. How would the tendency to gain electrons change as you go down a group?
Answer. The tendency of gaining the electrons decreases as we go down a group.
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Intext Questions | Page 90 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?
Answer.(i) In Modern Periodic Table, the place of hydrogen is justify because it is electropositive and so it is placed in first group with metals.
(ii) In Modern Periodic Table, the elements are arranged on the basis of atomic number. So the place of isotopes is also justified as the atomic number remain same of all the isotopes.
(iii) The order of heavy and light elements is also corrected in the Modern Periodic Table
(iv) The position of inert gases is also justified.
Question 2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer. Calcium (ca) and Barium (Ba), as these two elements belong to same group as magnesium and have same valence electrons as magnesium has.
Question 3. Name :
(a) three elements that have a single electron in their outermost shells.
Answer. Lithium (li), Sodium (Na),Potassium (k),
(b) two elements that have two electrons in their outermost shells.
Answer. Magnesium (mg), Calcium (ca), Barium (Ba)
(c) three elements with filled outermost shells.
Answer. Helium (He), Neon (Ne), Argon (Ar).
Question 4.(a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
Answer. Lithium, sodium and potassium atoms have same number of electrons in their outermost shell and have same valency
Question 4.(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Answer. Helium and neon both have their outermost shell filled
Question 5. In the Modern Periodic Table, which are the metals among the first ten elements?
Answer. Lithium, Beryllium, Boron are the metals in Modern Periodic Table among the first ten elements.
Question 6. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?
Ga, Ge, As, Se, Be
Answer. Be has the maximum metallic characteristics because all other elements are situated at the right hand side in periodic table than Be. Due to the position their metallic characteristics decreases as we go from left to right.
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Question 1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic Table. (a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
Answer.(c) The atoms lose their electrons more easily.
(As we move from left to right across the periods of the periodic table, the non-metallic character increases. Hence, the tendency to lose electrons decreases.)
Question 2. Element X forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as :
(a) Na (b) Mg (c) AI (d) Si
Answer. (b) Mg, X would most likely be in the same group of the Periodic Table as magnesium (Mg) due to strong ionic bonding between Magnesium and Chlorine. .
Question 3. Which element has:
(a) two shells, both of which are completely filled with electrons?
Answer. Neon (Ne) , Neon has two completely filled shells with 2 electrons in K shell and 8 electrons in L shell.
(b) the electronic configuration 2, 8, 2?
Answer. Magnesium (Mg)
(c) a total of three shells, with four electrons in its valence shell?
Answer. Silicon (Si). Silicon has a total of three shells. K shell has 2 electrons, L has 8 and M i.e. valence shell has 4 electrons. .
(d) a total of two shells, with three electrons in its valence shell?
Answer. Boron (B). It has a two shells, with 3 electrons in its L i.e. valence shell and 2 electrons in K shell
(e) twice as many electrons in its second shell as in its first shell?
Answer. Carbon (C) has electronic configuration of 2 electrons in K shell and 4 electrons in L shell. Clearly, it has twice as many electrons in its second shell as in its first shell .
Question 4.(a) What property do all elements in the same column of the Periodic Table as boron have in common?
Answer. Both the elements are metals and show the following common properties :
(i) Both are good conductor of electricity.
(ii) Both show malleability
Question 4.(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?
Answer. Both the elements are non-metal and show following common properties :
(i) Both are brittle
(ii) Both are bad conductor of electricity.
Question 5.An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
Answer. The atomic number of element is 17
(b) To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.)
Answer. It belongs chemically to F (9) because the electronic configuration of F is 2, 7 and having same valence electrons.
Question 6. The position of three elements A, B and C in the Periodic Table are shown below –
(a) State whether A is a metal or non-metal.
Answer. C is non-metal because it belongs to 17th group.
(b) State whether C is more reactive or less reactive than A.
Answer. C is less reactive than A because the reactivity of non-metal decreases from top to bottom.
(c) Will C be larger or smaller in size than B?
Answer. The size of C is smaller than B because B and C belong to the same period and the size decreases in a period on going left to right
(d) Which type of ion, cation or anion, will be formed by element A?
Answer.A forms anion because C is non-metals for anion (Negative Ion)
Question 7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?
Answer. Atomic number of Nitrogen is 7 and electronic configuration is 2, 5.
Atomic number of Phosphorus is 15 and electronic configuration is 2,8, 5.
Phosphorus will be more electronegative because phosphorus and nitrogen both are non-metals. Phosphorus is situated in the lower side than Nitrogen. In non-metals, as we go top to bottom the electronegativity is increased.
Question 8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Answer. The electronic configuration is related to the position of element in periodic table. The numbers of electrons in outermost shell show the number of group and the number of shells show the number of periods.
Question 9.In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?
Answer. Atomic number of Calcium is 20 and its electronic configuration is 2, 8, 8, 2.
To see the resemblance with Calcium, first we have to check the Electronic Configuration of respective elements.
From above, it is clear, elements with atomic number 12 and 38 has same electronic configuration of valence shell as that of Calcium, and therefor, will have physical and chemical properties resembling calcium
Question 10. Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.
Answer.
(i) Mendeleev's Periodic Table is based on atomic mass while Modern Periodic Table is based on atomic number.
(ii) In Medeleev's table there is no place for inert gases, while in modern Periodic Table they are classified.
(iii) In Modern Periodic Table all the anomalies of Mendleev's Periodic Table are removed.
(iv) There are 18 groups in Modern Periodic Table.
Question 1: The elements of 3rd period will have _ _ _ _ _.
a. 1 shell
b. 3 shells
c. 2 shells
d. 0 shell
Answer : b. 3 shells
Question 2: Which element has zero electron affinity in the 3rd period ?
a. Ar
b. Al
c. P
d. S
Answer : a. Ar
Question 3: An element has configuration 2, 8, 1. It belongs to, _ _ _ _ _.
a. 17 group and 3rd period
b. 3 group and 1st period
c. 1 group and 8th period
d. 1 group and 3rd period
Answer : d. 1 group and 3rd period
Question 4: At the end of each period the valence shell is _ _ _ _ _.
a. incomplete
b. half filled
c. completely filled
d. singly occupied
Answer : c. completely filled
Question 5: The number of electrons in the valence shell is equal to its _ _ _
a. atomic mass
b. period number
c. group number
d. atomic volume
Answer : c. group number
Question 6: The non-metallic element present in the third period other than sulphur and chlorine is _ _ _ _ _.
a. phosphorus
b. oxygen
c. fluorine
d. nitrogen
Answer : a. phosphorus
Question 7: The family of elements to which potassium belongs is _ _ _ _ _.
a. halogens
b. alkaline earth metals
c. alkali metals
d. noble gases
Answer : c. alkali metals
Question 8: Which of the following factors does not affect the metallic character of an element?
a. Atomic size
b. Ionisation potential
c. Electronegativity
d. Atomic radius
Answer : c. Electronegativity
Question 9: Lanthanides and actinides are also called _ _ _ _ _.
a. normal elements
b. transition elements
c. noble gases
d. inner transition elements
Answer : d. inner transition elements
Question 10: The family of elements to which calcium belongs is _ _ _ _ _.
a. alkali metals
b. halogens
c. noble gases
d. alkaline earth metals
Answer : d. alkaline earth metals
Question 11: The modern periodic table is given by _ _ _ _ _.
a. Mendeleev
b. Einstein
c. Bohr
d. Mosley
Answer : d. Mosley
Question 12: The family of elements having seven electrons in the outermost shell is _ _ _ _ .
a. alkali metals
b. alkaline earth metals
c. halogens
d. noble gases
Answer : c. halogens
Question 13: A liquid non-metal is _ _ _ _ _.
a. phosphorous
b. mercury
c. bromine
d. nitrogen
Answer : c. bromine
Question 14: The first alkali metal is _ _ _ _ _.
a. hydrogen
b. lithium
c. sodium
d. francium
Answer : b. lithium
Question 15: A purple coloured solid halogen is _ _ _ _ _.
a. chlorine
b. bromine
c. iodine
d. astatine
Answer : c. iodine
Question 16: Six elements A, B, C, D, E and F have the following atomic numbers (A = 12, B = 17, C = 18, D = 7, E = 9 and F = 11). Among these elements, the element, which belongs to the 3rd period and has the highest ionization potential, is _ _ _ _ _.
a. C
b. A
c. B
d. F
Answer : a. C
Question 17: The least reactive element in group 17 is _ _ _ _ _.
a. fluorine
b. chlorine
c. bromine
d. iodine
Answer : d. iodine
Question 18: The statement that is not true about electron affinity is
a. It causes energy to be released
b. It causes energy to be absorbed
c. It is expressed in electron volts
d. It involves formation of an anion
Answer : b. It causes energy to be absorbed
Question 19: The valency of chlorine with respect to oxygen is _ _ _ _ _.
a. 1
b. 7
c. 3
d. 5
Answer : b. 7
Question 20: Elements belonging to groups 1 to 17 are called _ _ _ _ _.
a. noble gases
b. transition elements
c. normal elements
d. inner transition elements
Answer : c. normal elements
Question 21: Four elements along a period have atomic number (11, 13, 16 and 17). The most metallic among these has an atomic number of _ _ _ _ _.
a. 12
b. 16
c. 17
d. 11
Answer : d. 11
Question 22: A factor that affects the ionisation potential of an element is _ _ _ _.
a. atomic size
b. electron affinity
c. electro-negativity
d. neutrons
Answer : a. atomic size
Question 23: The property of an element in the periodic table depends on its, _ _ _ _ _.
a. electronic configuration
b. atomic size
c. atomic mass
d. number of protons
Answer : a. electronic configuration
Question 24: The element, which has the highest electron affinity in the 3rd period is _ _ _ _ .
a. Na
b. Mg
c. Cl
d. Si
Answer : c. Cl
Question 25: Down a group, the electron affinity _ _ _ _ _.
1. decreases
2. increases
3. remains same
4. increases and then decreases
Answer : 1. decreases
- From only 30 elements known around year 1800, today we have list of 114 known elements
- Elements are classified on the basis of similarities in their properties.
- Döbereiner grouped the elements into triads and Newlands gave the Law of Octaves.
- Mendeléev arranged the elements in increasing order of their atomic masses and according to their chemical properties.
- Mendeléev even predicted the existence of some yet to be discovered elements on the basis of gaps in his Periodic Table.
- Anomalies in arrangement of elements based on increasing atomic mass could be removed when the elements were arranged in order of increasing atomic number, a fundamental property of the element discovered by Moseley.
- Elements in the Modern Periodic Table are arranged in 18 vertical columns called groups and 7 horizontal rows called periods.
- Elements thus arranged show periodicity of properties including atomic size, valency or combining capacity and metallic and non-metallic character.
In Text Questions | Page 81 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Answer. Yes, Dobereiner's triads also exist in the columns of Newlands' Octave. For example, Li, Na, K is a triad of Dobereiner which also exists in the columns of Newlands' Octaves.
Question 2. What were the limitations of Döbereiner’s classification?
Answer.
- Dobereiner could not classify all the elements known at that time.
- Dobereiner could identify only three triads from the elements known at that time.
- So the system of classification into triads was not accepted.
Question 3. What were the limitations of Newlands’ Law of Octaves?
Answer.
- Newlands' Law of Octaves was applicable only up to calcium. After calcium the eighth element did not possess properties similar to that of the first.
- Newlands assume that only 56 elements existed in nature and no more elements would be discovered in the future.
- Newlands' Octaves contains only few elements having similar properties.
Activity 5.1 | Page 84 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Looking at its resemblance to alkali metals and the halogen family, try to assign hydrogen a correct position in Mendeléev’s Periodic Table.
Answer. Mendeleev placed hydrogen with alkali metals while its some properties are similar to halogens. The position of hydrogen in Mendeleev's Periodic Table is correct because its properties are more similar to the alkali metals as it has the property of losing the electron, i.e., it is electropositive
Question 2. To which group and period should hydrogen be assigned?
Answer.First group and first period
Activity 5.2 | Page 85 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Consider the isotopes of chlorine, Cl-35 and Cl-37.
Question 1. Would you place them in different slots because their atomic masses are different?
Answer. No, the more fundamental base of classification is atomic number and not atomic mass
Question 2. Or would you place them in the same position because their chemical properties are the same?
Answer.Yes, both the isotopes are placed in same position because they have same chemical properties and same atomic number.
Intext Questions | Page 85 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements:
K, C, AI, Si, Ba.
Answer.
| K | ------> | K2O |
| C | ------> | CO2 |
| Al | ------> | Al2O3 |
| Si | ------> | SiO2 |
| Ba | ------> | BaO |
Question 2. Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (any two)
Answer. (i) Scandium (ii) Germanium
Question 3. What were the criteria used by Mendeléev in creating his Periodic Table?
Answer. (i) He arranged the elements on the basis of their increasing atomic masses.
(ii) Second criteria was the similarity in the chemical properties
(iii) The formulae of hydides and oxides formed by an element were treated as one of the basic property of an element for the classification used by Mendeleev.
Question 4. Why do you think the noble gases are placed in a separate group?
Answer.Nobel gases are placed in separate group because:
(i) These gases discovered very late as they are very inert
(ii) These gases could be placed in a new group without disturbing the existing order
Activity 5.3 | Page 85 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. How were the positions of cobalt and nickel resolved in the Modern Periodic Table?
Answer. Modern Periodic Table is based on the atomic number. Cobalt (27) is placed before Nickel (28) in Modern Periodic Table
Question 2. How were the positions of isotopes of various elements decided in the Modern Periodic Table?
Answer.The isotopes have same atomic number so they are placed in the same group in Modern Periodic Table
Question 3. Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?
Answer.No, it is not possible because atomic number is a whole number
Question 4. Where do you think should hydrogen be placed in the Modern Periodic Table?
Answer.I think that the place of hydrogen in the Modern Periodic Table is correct
Activity 5.4 | Page 87 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Look at the group 1 of the Modern Periodic Table, and name the elements present in it.
Answer. The elements in group 1 are:
Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (k), Rubidium (Rb), Cesium (Cs) and Francium (Fr).
Question 2. Write down the electronic configuration of the first three elements of group 1.
Answer.
(i) H (1) --> 1
(ii) Li(3) -->2, 1
(iii) Na (11) --> 2, 8, 1
Question 3.What similarity do you find in their electronic configurations?
Answer. All the elements have same number of valence electron, i.e. 1
Question 4. How many valence electrons are present in these three elements?
Answer. One (1)
Activity 5.5 | Page 87 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. If you look at the long form of the Periodic Table, you will find that the elements Li, Be, B, C, N, O, F, and Ne are present in the second period. Write down their electronic configuration.
Answer.
| Li (3) | ---> | 2,1 |
| Be (4) | ---> | 2, 2 |
| B (5) | ---> | 2, 3 |
| C (6) | ---> | 2, 4 |
| N (7) | ---> | 2, 5 |
| O (8) | ---> | 2, 6 |
| F (9) | ---> | 2, 7 |
| Ne (10) | ---> | 2, 8 |
Question 2. Do these elements also contain the same number of valence electrons?
Answer. No
Question 3. Do they contain the same number of shells?
Answer. Yes
The elements of same periods have same number of shells but not same number of valence electrons, which increases by 1
Activity 5.6 | Page 88 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. How do you calculate the valency of an element from its electronic configuration?
Answer.
Valency of metal : It is same as the number of valence electron i.e. 1, 2, and 3.
Valency of non-metal : Valency of non-metals can be calculated by subtracting number of valence electrons from 8 (i.e. 8- number of valence electrons).
For example :
| 8 - 4 = 4 | 8 - 5 = 3 | 8 - 6 = 2 |
| 8 - 7 = 1 | 8 - 8 = 0 |
Question 2.What is the valency of magnesium with atomic number 12 and sulphur with atomic number 16?
Answer.Magnesium (12) --> 2, 8, 2
Sulphur (16) --> 2, 8, 6
The valency of Magnesium is same as valence electron, i.e. 2
The valency of Sulphur is 8-6 = 2 because it is non-metal.
Question 3. Similarly find out the valencies of the first twenty elements.
Answer.
| Sr No | Elements | Atomic No. | Configuration | Valency |
| 1. | H | 1 | 1 | 1 |
| 2. | He | 2 | 2 | 0 |
| 3. | Li | 3 | 2, 1 | 1 |
| 4. | Be | 4 | 2, 2 | 2 |
| 5. | B | 5 | 2, 3 | 3 |
| 6 | C | 6 | 2, 4 | 8-4 = 4 |
| 7. | N | 7 | 2, 5 | 8-5 = 3 |
| 8. | O | 8 | 2,6 | 8-6 = 2 |
| 9. | F | 9 | 2, 7 | 8-7 = 1 |
| 10 | Ne | 10 | 2, 8 | 8-8 = 0 |
| 11. | Na | 11 | 2, 8, 1 | 1 |
| 12. | Mg | 12 | 2, 8, 2 | 2 |
| 13. | Al | 13 | 2, 8, 3 | 3 |
| 14. | Si | 14 | 2, 8, 4 | 8-4= 4 |
| 15. | P | 15 | 2, 8, 5 | 8-5= 3 |
| 16. | S | 16 | 2, 8, 6 | 8-6= 2 |
| 17. | Cl | 17 | 2, 8, 7 | 8-7= 1 |
| 18. | Ar | 18 | 2, 8, 8 | 8-8= 0 |
| 19. | K | 19 | 2, 8, 8, 1 | 1 |
| 20. | Ca | 20 | 2, 8, 8, 2 | 2 |
Question 4. How does the valency vary in a period on going from left to right?
Answer. Valency first increases 1 to 4 then decreases from 4 to 0 (1, 2, 3, 4, 3, 2, 1, 0).
Question 5. How does the valency vary in going down a group?
Answer. Valency remains the same because valence electrons do not change on going down in a group.
Activity 5.7 | Page 88 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Atomic radii of the elements of the second period are given below:
| Period II elements : | B | Be | O | N | Li | C |
| Atomic radius (pm) : | 88 | 111 | 66 | 74 | 152 | 77 |
Answer.
Question 1. Arrange them in decreasing order of their atomic radii.
Answer.
| Period II elements : | Li | Be | B | C | N | O |
| Atomic radius (pm) : | 152 | 111 | 88 | 77 | 74 | 66 |
Question 2. Are the elements now arranged in the pattern of a period in the Periodic Table?
Answer. Yes
Question 3.Which elements have the largest and the smallest atoms?
Answer. Lithium (Li) has Largest atoms (152 pm)
Oxygen (O) has smallest atoms (77 pm)
Question 4. How does the atomic radius change as you go from left to right in a period?
Answer. The atomic radius decreases as we go left to right in a period
Activity 5.8 | Page 89 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Study the variation in the atomic radii of first group elements given below and arrange them in an increasing order.
| Group 1 Elements : | Na | Li | Rb | Cs | K |
| Atomic Radius (pm) : | 186 | 152 | 244 | 262 | 231 |
| Group 1 Elements : | Na | Li | K | Rb | Cs |
| Atomic Radius (pm) : | 86 | 152 | 231 | 244 | 262 |
Question 2. Name the elements which have the smallest and the largest atoms.
Answer. Na (Sodium) has smallest atom
Ca (Calcium) has largest atom.
Question 3. How does the atomic size vary as you go down a group?
Answer.Atomic size increases as we go down a group
Activity 5.9 | Page 89 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. Examine elements of the third period and classify them as metals and non-metals.
Answer. Elements of third period are : Na(11), Mg (12), Al (13), Si (14), P (15), S (16), Cl (17), Ar (18)
| Elements | Atomic No. | Configuration | Metals / Non Metals |
| Na | 11 | 2, 8, 1 | Metal |
| Mg | 12 | 2, 8, 2 | Metal |
| Al | 13 | 2, 8, 3 | Metal |
| Si | 14 | 2, 8, 4 | Non-Metal |
| P | 15 | 2, 8, 5 | Non-Metal |
| S | 16 | 2, 8, 6 | Non-Metal |
| Cl | 17 | 2, 8, 7 | Non-Metal |
| Ag | 18 | 2, 8, 8 | Non-Metal |
| The elements having 1,2,3 valence electrons are metals while the elements having 4,5,6,7,8 valence electrons are non- metals. | |||
Question 2. On which side of the Periodic Table do you find the metals?
Answer.On the left side
Question 3. On which side of the Periodic Table do you find the non- metals?
Answer. On the right side
Activity 5.10 | Page 89 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. How do you think the tendency to lose electrons will change in a group?
Answer. The tendency of lose electrons increases in a group on going down
Question 2. How will this tendency change in a period?
Answer. The tendency of lose electrons decreases as we go left to right in a period.
Activity 5.11 | Page 90 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1.How would the tendency to gain electrons change as you go from left to right across a period?
Answer. The tendency of gaining electrons increases as we go left to right in a period upto 17th group. It decreases in 18th group.
Question 2. How would the tendency to gain electrons change as you go down a group?
Answer. The tendency of gaining the electrons decreases as we go down a group.
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Intext Questions | Page 90 | Chapter 5. Periodic Classification of Elements| CBSE Class 10th Science
Question 1. How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?
Answer.(i) In Modern Periodic Table, the place of hydrogen is justify because it is electropositive and so it is placed in first group with metals.
(ii) In Modern Periodic Table, the elements are arranged on the basis of atomic number. So the place of isotopes is also justified as the atomic number remain same of all the isotopes.
(iii) The order of heavy and light elements is also corrected in the Modern Periodic Table
(iv) The position of inert gases is also justified.
Question 2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer. Calcium (ca) and Barium (Ba), as these two elements belong to same group as magnesium and have same valence electrons as magnesium has.
Question 3. Name :
(a) three elements that have a single electron in their outermost shells.
Answer. Lithium (li), Sodium (Na),Potassium (k),
(b) two elements that have two electrons in their outermost shells.
Answer. Magnesium (mg), Calcium (ca), Barium (Ba)
(c) three elements with filled outermost shells.
Answer. Helium (He), Neon (Ne), Argon (Ar).
Question 4.(a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
Answer. Lithium, sodium and potassium atoms have same number of electrons in their outermost shell and have same valency
Question 4.(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Answer. Helium and neon both have their outermost shell filled
Question 5. In the Modern Periodic Table, which are the metals among the first ten elements?
Answer. Lithium, Beryllium, Boron are the metals in Modern Periodic Table among the first ten elements.
Question 6. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?
Answer. Be has the maximum metallic characteristics because all other elements are situated at the right hand side in periodic table than Be. Due to the position their metallic characteristics decreases as we go from left to right.
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Solved Exercises
Chapter 5. Periodic Classification of Elements| | CBSE Class 10th Science| Page 91-92)Question 1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic Table. (a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
Answer.(c) The atoms lose their electrons more easily.
(As we move from left to right across the periods of the periodic table, the non-metallic character increases. Hence, the tendency to lose electrons decreases.)
Question 2. Element X forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as :
(a) Na (b) Mg (c) AI (d) Si
Answer. (b) Mg, X would most likely be in the same group of the Periodic Table as magnesium (Mg) due to strong ionic bonding between Magnesium and Chlorine. .
Question 3. Which element has:
(a) two shells, both of which are completely filled with electrons?
Answer. Neon (Ne) , Neon has two completely filled shells with 2 electrons in K shell and 8 electrons in L shell.
(b) the electronic configuration 2, 8, 2?
Answer. Magnesium (Mg)
(c) a total of three shells, with four electrons in its valence shell?
Answer. Silicon (Si). Silicon has a total of three shells. K shell has 2 electrons, L has 8 and M i.e. valence shell has 4 electrons. .
(d) a total of two shells, with three electrons in its valence shell?
Answer. Boron (B). It has a two shells, with 3 electrons in its L i.e. valence shell and 2 electrons in K shell
(e) twice as many electrons in its second shell as in its first shell?
Answer. Carbon (C) has electronic configuration of 2 electrons in K shell and 4 electrons in L shell. Clearly, it has twice as many electrons in its second shell as in its first shell .
Question 4.(a) What property do all elements in the same column of the Periodic Table as boron have in common?
Answer. Both the elements are metals and show the following common properties :
(i) Both are good conductor of electricity.
(ii) Both show malleability
Question 4.(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?
Answer. Both the elements are non-metal and show following common properties :
(i) Both are brittle
(ii) Both are bad conductor of electricity.
Question 5.An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
Answer. The atomic number of element is 17
(b) To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.)
| N(7) | F(9) | P(15) | Ar(18) |
Answer. It belongs chemically to F (9) because the electronic configuration of F is 2, 7 and having same valence electrons.
Question 6. The position of three elements A, B and C in the Periodic Table are shown below –
| Group 16 | Group 17 |
| - | - |
| - | A |
| - | - |
| B | C |
(a) State whether A is a metal or non-metal.
Answer. C is non-metal because it belongs to 17th group.
(b) State whether C is more reactive or less reactive than A.
Answer. C is less reactive than A because the reactivity of non-metal decreases from top to bottom.
(c) Will C be larger or smaller in size than B?
Answer. The size of C is smaller than B because B and C belong to the same period and the size decreases in a period on going left to right
(d) Which type of ion, cation or anion, will be formed by element A?
Answer.A forms anion because C is non-metals for anion (Negative Ion)
Question 7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?
Answer. Atomic number of Nitrogen is 7 and electronic configuration is 2, 5.
Atomic number of Phosphorus is 15 and electronic configuration is 2,8, 5.
Phosphorus will be more electronegative because phosphorus and nitrogen both are non-metals. Phosphorus is situated in the lower side than Nitrogen. In non-metals, as we go top to bottom the electronegativity is increased.
Question 8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Answer. The electronic configuration is related to the position of element in periodic table. The numbers of electrons in outermost shell show the number of group and the number of shells show the number of periods.
Question 9.In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?
Answer. Atomic number of Calcium is 20 and its electronic configuration is 2, 8, 8, 2.
To see the resemblance with Calcium, first we have to check the Electronic Configuration of respective elements.
| Elements with given Atomic Number | Electronic Configuration |
| Element with Atomic Number - 12 | 2, 8, 2 |
| Element with Atomic Number - 19 | 2, 8, 8, 1 |
| Element with Atomic Number - 21 | 2, 8, 8, 3 |
| Element with Atomic Number - 38 | 2, 8, 18, 8, 2 |
Question 10. Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.
Answer.
(i) Mendeleev's Periodic Table is based on atomic mass while Modern Periodic Table is based on atomic number.
(ii) In Medeleev's table there is no place for inert gases, while in modern Periodic Table they are classified.
(iii) In Modern Periodic Table all the anomalies of Mendleev's Periodic Table are removed.
(iv) There are 18 groups in Modern Periodic Table.
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Multiple Choice Questions (M.C.Q .)
CBSE Class 10th Science | Chapter 5. Periodic Classification of Elements
Question 1: The elements of 3rd period will have _ _ _ _ _.
a. 1 shell
b. 3 shells
c. 2 shells
d. 0 shell
Answer : b. 3 shells
Question 2: Which element has zero electron affinity in the 3rd period ?
a. Ar
b. Al
c. P
d. S
Answer : a. Ar
Question 3: An element has configuration 2, 8, 1. It belongs to, _ _ _ _ _.
a. 17 group and 3rd period
b. 3 group and 1st period
c. 1 group and 8th period
d. 1 group and 3rd period
Answer : d. 1 group and 3rd period
Question 4: At the end of each period the valence shell is _ _ _ _ _.
a. incomplete
b. half filled
c. completely filled
d. singly occupied
Answer : c. completely filled
Question 5: The number of electrons in the valence shell is equal to its _ _ _
a. atomic mass
b. period number
c. group number
d. atomic volume
Answer : c. group number
Question 6: The non-metallic element present in the third period other than sulphur and chlorine is _ _ _ _ _.
a. phosphorus
b. oxygen
c. fluorine
d. nitrogen
Answer : a. phosphorus
Question 7: The family of elements to which potassium belongs is _ _ _ _ _.
a. halogens
b. alkaline earth metals
c. alkali metals
d. noble gases
Answer : c. alkali metals
Question 8: Which of the following factors does not affect the metallic character of an element?
a. Atomic size
b. Ionisation potential
c. Electronegativity
d. Atomic radius
Answer : c. Electronegativity
Question 9: Lanthanides and actinides are also called _ _ _ _ _.
a. normal elements
b. transition elements
c. noble gases
d. inner transition elements
Answer : d. inner transition elements
Question 10: The family of elements to which calcium belongs is _ _ _ _ _.
a. alkali metals
b. halogens
c. noble gases
d. alkaline earth metals
Answer : d. alkaline earth metals
Question 11: The modern periodic table is given by _ _ _ _ _.
a. Mendeleev
b. Einstein
c. Bohr
d. Mosley
Answer : d. Mosley
Question 12: The family of elements having seven electrons in the outermost shell is _ _ _ _ .
a. alkali metals
b. alkaline earth metals
c. halogens
d. noble gases
Answer : c. halogens
Question 13: A liquid non-metal is _ _ _ _ _.
a. phosphorous
b. mercury
c. bromine
d. nitrogen
Answer : c. bromine
Question 14: The first alkali metal is _ _ _ _ _.
a. hydrogen
b. lithium
c. sodium
d. francium
Answer : b. lithium
Question 15: A purple coloured solid halogen is _ _ _ _ _.
a. chlorine
b. bromine
c. iodine
d. astatine
Answer : c. iodine
Question 16: Six elements A, B, C, D, E and F have the following atomic numbers (A = 12, B = 17, C = 18, D = 7, E = 9 and F = 11). Among these elements, the element, which belongs to the 3rd period and has the highest ionization potential, is _ _ _ _ _.
a. C
b. A
c. B
d. F
Answer : a. C
Question 17: The least reactive element in group 17 is _ _ _ _ _.
a. fluorine
b. chlorine
c. bromine
d. iodine
Answer : d. iodine
Question 18: The statement that is not true about electron affinity is
a. It causes energy to be released
b. It causes energy to be absorbed
c. It is expressed in electron volts
d. It involves formation of an anion
Answer : b. It causes energy to be absorbed
Question 19: The valency of chlorine with respect to oxygen is _ _ _ _ _.
a. 1
b. 7
c. 3
d. 5
Answer : b. 7
Question 20: Elements belonging to groups 1 to 17 are called _ _ _ _ _.
a. noble gases
b. transition elements
c. normal elements
d. inner transition elements
Answer : c. normal elements
Question 21: Four elements along a period have atomic number (11, 13, 16 and 17). The most metallic among these has an atomic number of _ _ _ _ _.
a. 12
b. 16
c. 17
d. 11
Answer : d. 11
Question 22: A factor that affects the ionisation potential of an element is _ _ _ _.
a. atomic size
b. electron affinity
c. electro-negativity
d. neutrons
Answer : a. atomic size
Question 23: The property of an element in the periodic table depends on its, _ _ _ _ _.
a. electronic configuration
b. atomic size
c. atomic mass
d. number of protons
Answer : a. electronic configuration
Question 24: The element, which has the highest electron affinity in the 3rd period is _ _ _ _ .
a. Na
b. Mg
c. Cl
d. Si
Answer : c. Cl
Question 25: Down a group, the electron affinity _ _ _ _ _.
1. decreases
2. increases
3. remains same
4. increases and then decreases
Answer : 1. decreases
