Question

The covalency of the central atom is maximum in:
(A) $HCN$
(B) $N{H_4}^ +$
(C) $PC{l_5}$
(D) ${H_2}O$

Answer 1

Covalency is the measure of the number of bonds formed by the central atom of a covalent compound. Covalent bonds are formed by simply sharing their electrons and therefore do not involve the formation of ions.

Complete Step By Step Answer:
Compounds other than binary compounds contain a central atom which is attached to two or more groups or atoms that may or may not be the same.
Hydrogen cyanide consists of three elements but is a binary compound made up of hydrogen and cyanide molecules. A single bond connects the two groups.
Ammonium ion is formed when ammonia acts as a base and accepts a proton from any acidic source. The central atom is a nitrogen atom that is connected to three neutral hydrogen atoms and one proton thus its covalency is four.
Phosphorus pentachloride is formed from the reaction of chlorine gas and white phosphorus. Phosphorus is in its maximum oxidation state and is covalently bonded to five monovalent chlorine atoms. Thus the covalency is five.
Water molecule is formed when a divalent oxygen atom combines with two hydrogen atoms to give a bent geometry. There are two covalent bonds that connect the three atoms together and therefore the covalency is two.
$\Rightarrow$ Thus, maximum covalency is observed in the case of Phosphorus pentachloride which is five and therefore option (c) is correct.

Note:
Covalency should not be confused with the oxidation state or formal charge. For calculation of oxidation states the number of electrons shared and the electronegativity of the neighboring atoms is taken into account however covalency simply deals with the total number of bonds formed.