Ncert Exercises & Solutions

The first ionisation enthalpies of Na, Mg, Al, and Si are in the order of-

1. Na < Al < Mg < Si

2. Na > Mg > Al > Si

3. Na < Mg < Al < Si

4. Na > Mg > Al < Si

HINT: Left to right across the period ionisation enthalpy increases.

Explanation:

Follow the following steps to solve out such problems

Steps	Method	Apply
Step I	Write the electronic configuration to find position in the periodic table	${}_{11}{Na} = [Ne] 3 s^{1},$ ${}_{12}{Mg} = [Ne] 3 s^{2}$ ${}_{13}{Al} = [Ne] 3 s^{2} 3 p^{1},$ ${}_{14}{Si} = [Ne] 3 s^{2} 3 p^{2}$
Step II	Arrange them in the order as they are in the periodic table	$11$ $12$ $13$ $14$ $Na$ $Mg$ $Al$ $Si$
Step III	Follow the general trend and also keep in mind the exception	The IP increases along a period from left to right but IP of Mg is higher than that of Al due to completely filled 3s orbital in Mg.
Step IV	On the above basis find the order	The order of IP is Na < Al < Mg < Si