Complete the following statements :
(a) Magnesium has 2 valence electrons in the ……… shell
(b) The valency of nitrogen in `N_(2)` molecule is ……..
(c) Isotopes have different mass numbers because their nuclei contain different number of ........
(d) Some boron atoms have mass number 10 and some have mass number 11. These boron atoms with different mass numbers are called ........