(a) The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom. The number of electrons lost or gained (or shared) by one atom of an element to achieve the nearest inert gas electron configuration, gives us the valency of the element.
(b) (i) Atomic number of magnesium =12
Electronic configuration is 2, 8, 2.
A magnesium atom can lose its 2 valence electrons to achieve the inert gas electronic
configuration of neon (2, 8).
Therefore, valency of magnesium = 2.
(ii) The atomic number of sulphur =16.
Electronic configuration = 2, 8, 6.
A sulphur atom cannot lose 6 electrons to achieve inert gas electronic configuration clue to energy consideration. It can gain 2 electrons to achieve the nearest inert gas electronic configuration of argon (2, 8, 8).
Thus, the valency of sulphur = 2.
(c) On going from left to right along a period (short period), the valency of elements increases from 1 to 4 and then decreases to 0 (zero).
(d) All the elements in a group have the same valency.
