Question

The given table shows the elements with the same number of electrons in its valence shell.

Elements	$A$	$B$	$C$
M.P.	$63.0$	$180.0$	$97.0$

State:

(i) Whether these elements belong to same group or period.

Answer 1

• The number of valence electrons defines the group of the element, not the period.
• The elements having the same number of valence electrons are placed in the same group.
• Since the elements, $A,B,$ and $C$ have the same number of valence electrons, they must belong to the same group but not the same period.

For example, $Li,Na,K,Rb,$ and $Cs$ have $1$ valence electron thus, they are placed in group $1$.