Question

When the ionisation energy v/s atomic number is plotted for the elements, of atomic number $11$ to $18$ , two peaks are observed for the elements $X$ and $Y$ in between the curve. What is the difference between atomic number of element $X$ and element $Y$ ?

Answer 1

For element $11$ to $18$ is :
Atomic Number $=$ Peak is observed at $Mg$ (Atomic number)$=12$ and
$P$ (Atomicnumber ) $=15$
Two peaks will be observed one at $15$ $=1s^{2},2s^{2},2p^{6},3s^{2},3p^{3}$
(Half-filled extra stable)
Another at $12=Mg=1s^{2},2s^{2},2p^{6},3s^{2}$
(fully-filled extra stable)
It is because $Mg$ has completely filled orbitals and $P$ has half filled orbitals.
Difference between Atomic number of $Mg$ and $P$ is $15-12=3$
For alkaline earth metal and Nitrogen family member $I.E.$ is extra high so peak appear and their difference of atomic number $=3$ .
According to ionization enthalpy Vs. atomic number graph: