Question

why chlorine has highest ionization energy than sodium

Answer 1

The reason for this is chlorine has more protons than sodium resulting in a stronger net positive charge. This increased nuclear charge increases the pulling effect the nucleus has on the electrons in the outer shell making removal of electrons from chlorine difficult than in sodium. This is what is referred to as a high ionization energy.
2)Why atomic size of sodium is greater than that of chlorine.
The effect of more protons in chlorine than in sodium has the effect of increasing the pull of electrons towards the nucleus resulting in a smaller radius. This is why sodium has a greater atom size than chlorine.

Answer 2

na(2,8,1)(g) =na (2,8) +electron