An atom of phosphorus has an electronic configuration of 2, 8, 5. What is its valency and how does it attain stability?
Phosphorus has a valency of 3, as it gains three electrons to achieve a stable configuration.
Phosphorus (atomic number 15) has the configuration 2, 8, 5, so it has 5 electrons in its outermost shell.
To reach a stable octet (8 electrons, like argon), it is far easier for phosphorus to gain or share 3 electrons than to lose all 5. So its valency = 8 − 5 = 3. For example, it forms phosphide ion \(P^{3-}\) with metals and shares three electrons in \(PH_3\).
Valency is not simply the number of valence electrons when there are more than 4 of them; it equals 8 minus that number. Losing five electrons, or losing three, would need a very large amount of energy, and losing three would still leave 2 electrons in the outer shell, which is not a stable configuration.
Hence, phosphorus has a valency of 3, as it gains three electrons to achieve a stable configuration.
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