Arrange each set of atoms in order of decreasing IE₁:

(a) Na, Li, K (b) Be, F, C (c) Cl, Ar, Na (d) Cl, Br, Se

(d) Cl, Br, Se

Explanation:

Ionisation energy is defined as the minimum energy required to remove the loosely bound valence electron of a neutral gaseous atom or molecule. It is usually endothermic process.

Accorfing to the trends down the group and across the period:

Ionisation energy generally increases as one moves from left to right in a given period while ionisation generally decreases as one moves from top to bottom in a given group. This is of the magnitude of the effective Nuclear charge and the Number of electron shells.

From the elements above:

Ionisation energy decreases down the group while it increases across the period.

F > C > Be

Li > Na > K

Ar > Cl > Na

Cl > Br > Se