Question

Which of the following is the incorrect set of atomic number and valency?

• A. Sodium: atomic number-11, valency-1
• B. Nitrogen: atomic number-7, valency-1
• C. Magnesium: atomic number-12, valency-2
• D. Carbon: atomic number-6, valency-4

Answer 1

The correct option is B Nitrogen: atomic number-7, valency-1

Atomic number is the number of protons present in the nucleus of an atom.

For a neutral atom, number of protons is equal to the number of electrons. Valency can be calculated from a short-cut method as given below.

1. If valence electrons are four or less, valency = number of valence electrons
2. If valence electrons are more than four, valency = 8-valence electrons

$$\begin{array}{cccc}\text{Element}& \text{Atomic no.}& \text{Electronic configuration}& \text{Valency}\\ \text{Sodium}& \text{11}& \text{2,8,1}& \text{1}\\ \text{Nitrogen}& \text{7}& \text{2, 5}& \text{(8-5)=3}\\ \text{Magnesium}& \text{12}& \text{2,8,2}& \text{2}\\ \text{Carbon}& \text{6}& \text{2,4}& \text{4}\end{array}$$Hence, valency of nitrogen is 3 and not 1.