Question

Explain why $N {H_3}$ is basic while $Bi {H_3}$ is only feebly basic.

Answer 1

In this question, first understand the concept of atomic radii and its effect on the element. Atomic radius is the measure of the size of the atom, usually the distance from the centre of the nucleus to the boundary of the surrounding shell of electrons. Atomic size and basicity of an element are inversely proportional to each other .

Complete answer:
Nitrogen belongs to group $15$ of the periodic table. Their electronic configuration is $n{s^2} n {p^3}$. It is colourless, odourless and tasteless gas. It’s the most abundant element in the earth atmosphere and it’s also a constituent of all the living matter.
Now coming to the question which is asked :
$N {H_3}$ is more basic than $Bi {H_3}$ because nitrogen is a small element. The electronegativity of nitrogen is more than the hydrogen atom that is attached to it, due to which electrons are pulled towards nitrogen and the lone pair are accumulated or concentrated in a very small region. Which makes the charge density per unit volume very high and it behaves as an electron donor.
Now if we look at $Bi {H_3}$ the electronegativity of bismuth is not that much and it is not able to pull the electrons towards itself. So it is less basic than $N {H_3}$ .
If we look at the periodic table, group $15$ down the group the size of the central atom increases because of which the charge gets distributed over the element and the electron density is decreased. This is why the electron donating capacity of the group $15$ elements decreases down the group. Also when we come down the group the inert pair effect increases, due to which the electrons in the ‘S’ orbitals stability increases, which decreases their ability to donate their electrons. Since the donating ability of electrons decreases down the group, acidic nature increases and basic nature decreases.

Note: The basicity of an element is inversely proportional to its atomic size, that is an element with smaller size will have more basicity as compared to a larger element. Therefore the basicity decreases as we move down the group because the atomic size increases .