# Periodic Classification of Elements | Activities in Text Book with Solution

Activity 5.6

How do you calculate the valency of an element from its electronic configuration?

• Valency of metal: It is same as the number of valence electron.
• Valency of non-metal: It is calculated by subtracting the number of valence electrons from 8.
What is the valency of magnesium with atomic number 12 and sulphur with atomic number 16?

• Magnesium: 2, 8, 2. It is a metal. So valency= 2.
• Sulphur: 2, 8, 6. It is a non-metal. So valency= 8-6 = 2
Similarly, find out the valencies of the first twenty elements.

 Elements Atomic No. E. Config. Valency H 1 1 1 He 2 2 0 Li 3 2, 1 1 Be 4 2, 2 2 B 5 2, 3 3 C 6 2, 4 8 – 4 = 4 N 7 2, 5 8 – 5 = 3 O 8 2,6 8 – 6 = 2 F 9 2, 7 8 – 7 = 1 Ne 10 2, 8 8 – 8 = 0 Na 11 2, 8, 1 1 Mg 12 2, 8, 2 2 Al 13 2, 8, 3 3 Si 14 2, 8, 4 8 – 4= 4 P 15 2, 8, 5 8 – 5= 3 S 16 2, 8, 6 8 – 6= 2 Cl 17 2, 8, 7 8 – 7= 1 Ar 18 2, 8, 8 8 – 8= 0 K 19 2, 8, 8, 1 1 Ca 20 2, 8, 8, 2 2

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).

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.