Question

An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a divalent halide.

Where in the periodic table are elements X and Y placed?

Answer 1

Placement of X and Y:

• Electronic configuration of X with atomic number $17$= $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^5$
• Electronic configuration of Y with atomic number $20$= $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}4{\mathrm{s}}^2$

The highest value of n in an electronic configuration gives us the value of the period.

• If the last electron enters in s-subshell i.e.; in Y, then if s has $1$ electron, group is $1$. If s has $2$ electrons, group is $2$.
• Example: In Y, there are $2$ electrons in $4\mathrm{s}$. So the group number is $2$.
• If the last electron enters in p-subshell i.e.; in X, then group number is given by $12$+ number of electrons in p-subshell.
• Example: in X, Group= $12+5=17$.

Element	Group	Period
X	17	3
Y	2	4

It can be found that X is Chlorine $\left(\mathrm{Cl}\right)$ and Y is Calcium $\left(\mathrm{Ca}\right)$.