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

Explanation

`rarr` In periods IE values increase from left to right
`rarr` Among `3^(rd)` period elements Argon (Ar) [Z=18] possess highest `IE_(1)` value
`rarr IE_(1)` values of `3^(rd)` period elements given below

Exceptions:
`rarr IE_(1)` of .Mg. is higher than .Al. because .Mg. has completely filled .s. subshells
`rarr IE_(1)` of .p. is higher than .s. because .p. has half filled .p. subshells.