Periodic Classifications of Periodic Table NCERT SOL


In-text Solved Activity

Activity 5.1 | Page 84

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 however its some properties were similar to halogens.

The position of hydrogen in Mendeleev’s Periodic table is correct because its properties are much similar to the alkali metals. Since it has the property of losing the electron.

Question 2. To which group and period should hydrogen be assigned?

Answer.The first group and the first period 

Activity 5.2 | Page 85

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 basis of classification is atomic number and not atomic mass.

Question 2. Would you place them in the same position because their chemical properties are the same?

Answer. Yes, both the isotopes are placed in the same position because they have the same chemical properties and the same atomic number. 

Intext Questions | Page 85

Question 1. Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements:
K, C, AI, Si, Ba.


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 were the similarity in the chemical properties
(iii) The formulae of hydrides 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?


Nobel gases are placed in a 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

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 the same atomic number so they are placed in the same group in the 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 the 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

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. 
(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 the same number of a 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

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. 

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 the same periods have the same number of shells but not the same number of valence electrons, which increases by 1 

Activity 5.6 | Page 88

Question 1. How do you calculate the valency of an element from its electronic configuration?
Valency of metal: It is the same as the number of electrons present in the outermost shell or valence electron i.e. 1, 2, and 3.
Valency of non-metal: Valency of non-metals can be calculated by subtracting the 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. 

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

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

Question 1. Arrange them in decreasing order of their atomic radii.

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 the Largest atoms (152 pm) 
Oxygen (O) has the 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 |

Question 1. Study the variation in the atomic radii of first group elements given below and arrange them in 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 the smallest atom. Ca (Calcium) has the 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

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

Question 1. How do you think the tendency to lose electrons will change in a group?

Answer. The tendency of losing electrons increases in a group on going down 

Question 2. How will this tendency change in a period?

Answer. The tendency of losing electrons decreases as we go left to right in a period.

Activity 5.11 | Page 90

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 up to the 17th group. It decreases in the 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. 

Intext Questions | Page 90

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 justified because it is electropositive and so it is placed in the first group with metals.
(ii) In the 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 the 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 the same group as magnesium and have the same valence electrons as magnesium has.

Question 3. Name :
(a) three elements that have a single electron in their outermost shells.

 Lithium (li), Sodium (Na),Potassium (k), 

(b) two elements that have two electrons in their outermost shells.

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 the same number of electrons in their outermost shell and have the 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 is 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 the periodic table than Be. Due to the position, their metallic characteristics decreases as we go from left to right.