Question

An atom has electronic configuration $\text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{6}}\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{6}}\text{3}{{\text{d}}^{3}}\text{4}{{\text{s}}^{2}}$. It will be place it in:
A. fifth group
B. fifteenth group
C. second group
D. third group

Answer 1

Hint: On the basis of electron configuration, we can find the period number, group number, number of electrons and to which block the element belongs. The group number of an atom can be told by the number of valence electrons present in the atom.

Complete answer:
The valence electrons will be correctly found by writing the electronic configuration according to Aufbau’ rule (shells with lower energies are filled first). The subshell $4\text{s}$ has lower energy than $3\text{d}$ sub-shell. The correct electronic configuration is $\text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{6}}\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{6}}\text{4}{{\text{s}}^{2}}\text{3}{{\text{d}}^{3}}$.
Let us determine the number of valence electrons in the element: The valence electrons are $\left( 2+3 \right)$ electrons. The valence electrons should have been 2 because the outermost shell is $4\text{s}$. But the electrons have also been filled in the next subshell after $4\text{s}$ su-shell. So, the overall electrons will be counted, that is the electrons in the ultimate and pen-ultimate subshell. So, the element has 5 valence electrons.
We know the number of valence electrons decides the group number of an element, so the group of the element will be the same as the number of valence electrons. The group number is 5 as the valence electrons are 5.
An atom has electronic configuration $\text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{6}}\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{6}}\text{3}{{\text{d}}^{3}}\text{4}{{\text{s}}^{2}}$. It will be placed in the fifth group.

The correct answer of this question is option ‘a’.

Additional Information:
The highest value or number of shells is the period number of the element. Like, $\text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{6}}\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{6}}\text{4}{{\text{s}}^{2}}\text{3}{{\text{d}}^{3}}$ has 4 as the maximum value, so, the element belongs to period 4.

Note:
The correct determination of valence electrons is necessary for deciding the group number of elements. Like, if there are 3 valence electrons in an element, then, corresponding group number will be 13 and not 3. This is because d-block is introduced after $3\text{p}$ sub-shell. So, till $3{{\text{p}}^{5}}$ configuration, group numbers are 13,14...17.