Question

The atomic number of the elements $X$ and $Y$ are $20,26$ respectively. When these elements combine with chlorine, three compounds $XC{l}_2,YC{l}_2,YC{l}_3$ are formed.
Explain the reason for this, on the basis of the subshell based electronic configuration.

Answer 1

$X_{20}=1s^2,2s^2,2p^6,3p^6,4s^2$

$X_{26}=1s^2,2s^2,2p^6,3s^2,3p^6,4s^2,3d^6$

$Y$ is a transitional element, In chemical reactions, only two electrons in ${}^{\prime }{s}^{\prime }$ subshell or besides ${}^{\prime }{s}^{\prime }$ subshell electrons ${}^{\prime }{d}^{\prime }$ subshell electrons also take part.

The element $\text{X is calcium-}$ ${}_{20}Ca-1s^2,2s^2,2p^6,3s^2,3p^6,4s^2$

The element $\text{Y is iron-}$ ${}_{26}Fe-1s^2,2s^2,2p^6,3s^2,3p^6,4s^2,3d^6$

Calcium can lose only two electrons and form $CaC{l}_2$ But, due to the $d-$shell present in $Fe,$ when it forms $FeC{l}_2$, the configuration of $d-$shell in $FeC{l}_2$ is $d^6$. When it again loses an electron to form $FeC{l}_3$ it forms a stable half-filled $d^5$ configuration. That's why $Fe$ has a variable oxidation state