Q.Describe the periodic trend of ionisation potential.

Ionisation potential rises across a period (more nuclear charge, smaller atoms) and falls down a group (more shielding, larger atoms), with a few stability-based exceptions.

Ionisation potential (ionisation energy) is the minimum energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state. Its periodic trend is explained by two competing factors: effective nuclear charge and atomic radius.

Across a period (left to right): ionisation potential generally increases. As we move across a period, electrons are added to the same principal shell while protons are also added to the nucleus, so the effective nuclear charge increases and the atomic radius decreases. The valence electrons are held more tightly, so more energy is needed to remove one — ionisation potential rises steadily from the alkali metal on the left to the noble gas on the right of each period.

Down a group (top to bottom): ionisation potential generally decreases. Even though nuclear charge increases going down a group, the number of electron shells also increases, so the valence electron is much farther from the nucleus and is more strongly shielded by inner-shell electrons. The net effect (larger radius + more shielding) outweighs the increased nuclear charge, so the outermost electron is held less tightly and is easier to remove — ionisation potential falls.

…