Question

Choose the correct order of decreasing energies required to put an electron in the outermost shell of the elements mentioned.

• A. Li < Be < B < C
• B. Be < Li < B < C
• C. C > Li > B > Be
• D. Be < B < Li < C

Answer 1

The correct option is C C > Li > B > Be

The amount of energy released will be higher when the atom becomes more stable on adding an electron.
On adding an electron in its valence shell carbon will have an electronic configuration $1s^{2}2s^{2}2p^3$ which is half filled and much more stable compared to $1s^{2}2s^{2}2p^2$. Beryllium destabilizes the most, because it had a fully filled valence shell electronic configuration. Lithium also becomes stable: $1s^{2}2s^2$. But the amount of energy released will be lower than carbon because the effective nuclear charge in carbon is higher.