The atomic number of an element ‘X’ is 12, the element will show a valency of _______:
+ 2
The valency of an element is the number of electrons its atom loses, gains or shares to reach a stable octet (noble gas) configuration. Atoms that lose electrons form positive ions and show a positive valency.
Atomic number 12 means the atom has 12 electrons, arranged in shells K, L, M as 2, 8, 2. The element is magnesium, a metal of group 2 with two electrons in its outermost shell.
It is far easier for this atom to lose its two outer electrons than to gain six more. On losing them it forms the ion X2+, so its valency is + 2.
Check: after losing two electrons the configuration becomes 2, 8, the stable configuration of neon. Gaining two electrons (– 2) would give 2, 8, 4, which is not stable, and a valency of 6 would need six electrons to be gained or lost, which does not happen for this atom.
Hence, + 2.
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