Give reasons for the following

1)molecules of hydrogen and chlorine have single covalent bonds between their atoms while oxygen has a double covalent and nitrogen a triple covalent bond respectively.

The number of bonds formed by atoms are the feature of their valency. Hydrogen and Chlorine have the valency of 1 as both of them need 1 electron to attain stable configuration while Oxygen and Nitrogen have their valency as 2 and 3 respectively, hence Oxygen form 2 and Nitrogen forms 3 bonds.