Question

Class X ICSE
match the atomic number 2,4,8,15 and 19 with each of the following :

1) A solid non metal belonging to the third period.
2) A metal of valency 1
3) A gaseous element with valency 2.
4) An element belonging to group 2.
5) a rare gas.

Answer 1

Elements are 2- Helium
4- Beryllium
8- Oxygen
15-- Phosphorus.
19-- Potassium.

1) A solid non metal belonging to the third period--- 15- Phosphorus
1s² 2s² 2p⁶ 3s² 3p⁵

2) A metal of valency 1 ---- Potassium 
1s² 2s² 2p⁶ 3s² 3p7

3)A gaseous element with valency 2 -- Oxygen
1s² 2s² 2p⁴

4)An element belonging to group 2. -- Beryllium- 1s² 2s²

5)a rare gas-- Helium

Answer 2

1 ) 15 phosphorous
2) 19 potassium
3) 8 oxygen
4) 4 beryllium
5) 2 helium