Question

Which one of the following is different from the others with respect to valency?
A.Potassium
B.Ammonium
C.Barium
D.Lithium

Answer 1

Hint: The combining capacity of an atom or molecule is measured by its valency. Elements belonging to the same group of the periodic table tend to have the same valency. It is related to the number of electrons present in the outer shell.

Complete step by step answer:
The combining power/capacity of an element reflects on its valency. We can follow this following method in order to calculate valency of an element. Suppose, for a compound, the number of electrons in the outermost shell is between one to four, then the compound is said to have positive valency. On the other hand, for compounds having electrons four, five, six, or seven, the valency can be determined by subtracting the number of electrons from eight. The atomic number of Potassium is 19, with electronic configuration 2,8,8,1 .So, it needs to lose 1 electron in order to attain stability. Therefore, it has the valency of +1
Lithium has an atomic number of 3, having electronic configuration 2, 1. So, it needs to lose only one electron in order to achieve stable configuration. So, its valency will be 1.
Barium belongs to group II, and it has 2 electrons in its outermost shell, i.e., 6s shell. It can readily lose these 2 electrons in order to attain stability. So, barium has valency of 2.
Ammonium has the chemical formula of ${\left[ {N{H_4}} \right]^ + }$ . Its valency is +1 most of the time, because it consists of one free electron on the central metal atom nitrogen.

So, the correct option is C.

Note:
If we know the valency of an element, we can easily write the chemical formula of any compound, consisting of those elements.