Question

Arrange the following elements in increasing order of their first ionisation energies.

• A. Lithium
• B. Boron
• C. Carbon
• D. Oxygen

Answer 1

Ionisation energy is the amount of energy required to remove the outermost electron from the isolated gaseous atom.
Li, B, C, O belong to the same period and the electrons are added to same shell.

Nuclear charge increases from Li to O and thus, the atomic size decreases. For electrostatic force - $F\propto q$ and $F\propto 1/r^2$.
This states that more energy is required to overcome greater magnitude of force.

Hence, ionisation energy follows the order: Li < B < C < O.