In order to achieve an octet in the outermost shell, atoms react. This is done by gaining, sharing or losing electrons. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives the combining capacity of the element which is called the valency of an atom.

(i) Valency of chlorine. The Atomic number of chlorine atom is 17. This means it has 17 protons. In an atom, the number of protons is always equal to the number of electrons. Hence, the electronic configuration of chlorine atom is following:

K shell -2, L shell -8 and M shell-7.

A chlorine atom can accept 1 more electron to achieve the nearest noble gas configuration of 8 valence electrons. Therefore, the valency of chlorine is 1.

(ii) Valency of sulphur. The atomic number of sulphur is 16. The sulphur atom has 16 protons and 16 electrons.Its electron configuration is K shell-2, L shell- 8 and M shell- 6. A sulphur atom has 6 electrons in its valence shell. A sulphur atom can accept 2 more electrons to achieve the nearest noble gas configuration of eight valence electrons. Therefore, the valency of sulphur is 2.

(iii) Valency of magnesium. The atomic number of magnesium is 12. The magnesium atom has 12 protons and 12 electrons. Its electron configuration is K shell-2, L shell-8 and M shell- 2 electrons. A magnesium atom has 2 electrons in its outermost shell. A magnesium atom can lose 2 outermost electrons to achieve the nearest noble gas electron configuration of 8 valence electrons, so the valency of magnesium is 2.