Question

The first ionization energy of boron is less than that of beryllium because

• A. Boron has higher nuclear charge.
• B. Atomic size of boron is more than that of beryllium.
• C. Boron has only one electron in p-subshell.
• D. Atomic size of boron is less than that of beryllium.

Answer 1

The correct option is C

Boron has only one electron in p-subshell.

This is an exception to the general electronegativity trend. It can be explained by looking at the electron configurations of both elements.

Be:$[He{]}^2{s}^2$

B:$\left[He\right]2s^{2}2p^1$

When you remove an electron from beryllium, you are taking away an electron from the 2s orbital. When you remove an electron from boron, you are taking an electron from the 2p orbital. The 2p electrons have more energy than the 2s, so it is easier to remove them as they can more strongly resist the effective nuclear charge of the nucleus.