Question

In the $3^{rd}$ period, the first ionisation potential is of the order

• A. ${ Na > Mg > Al > Si > P }$
• B. ${ Mg > Na > Si > P > Al }$
• C. ${ Na < Mg < Al < Si < P }$
• D. ${ Na < Al < Mg < Si < P }$

Answer 1

The Correct Option is D

Ionisation potential increases across a period with increasing atomic number. But ${ Mg }\, (1s^22s^22p^63s^2)$ has higher ionisation enthalpy than ${Al }(1s^22s^22p^63s^23p^1)$ due to stable fully filled orbitals of magnesium. Thus, the correct order is :
1st $I.E ({kJmol^{-1}}) : {$\underset{\mathrm{496}}{{Na}}$ < $\underset{\mathrm{577}}{{Al}}$ < $\underset{\mathrm{737}}{{Mg}}$ < $\underset{\mathrm{786}}{{Si}}$ < $\underset{\mathrm{1011} }{{P}}$}$