The first ionisation enthalpy `(Delta_(i)H^(o-))` values of the third period elements, `Na, Mg` and `Si` are respectively `496, 737` and `786 kJ mol^(-1)`. Predict whether the first `Delta_(i)H^(o-)` value for `Al` will be more close to `575` or `760 kJ mol^(-1)`? Justify your answer.

Explanation

Arranging the elements Na, Mg and Si different groups in order of their increasing atomic numbers we have ,
`{:(1,,2,,13,,14),(Na,,Mg,,A1,,Si),(-,,-,,,,):}`
Since in case of A1, a 3- electron is to be lost while in Mg, a 2s- electron is to be lost therefore `Delta_(i)H_(1)` of A1 will be lower than that of Mg (737 kJ `mol^(-) )` because of the effective shielding of the 3p- electron from the nucleus by 3s- electrons . Therefore `Delta_(i)H_(1)` for A1 will be more close to 575 kJ `mol^(-1)`