Which element of `3^(rd)` period has the highest `I.E_(1)` ? Explain the variation of `I.E_(1)` in this period?

Explanation

In each period the last element i.e., the inert gas element has the highest I.E. In `3^(rd)` period , it is Argon.
Variation of I.E. in `3^(rd)` period:
The increasing order of IE of elements of `3^(rd)` period is `Na < Al < Mg < Si < S < P < Cl < Ar`. Mg and P have greater IE values than those of AL and S respectively.
Reason: In Mg, the 3s electrons are paired up `(1s^(2)2s^(2)2p^(6)3s^(2))`. So the outermost electron i.e., 3s electron is paired up and more-over it is present in the s-orbital, which is more penetrating. So it requires more energy to remove this electron. So Mg has higher I.E., than expected.
In P, the 3p orbitals are half-filled `(1s^(2)2s^(2)2p^(6)3s^(2)3p_(x)^(1)3p_(y)^(1)3p_(z)^(1))`. So the atom is quite stable. So it has higher I.E. than expected.