Question

How can you find out the group and the period of the element with electronic configuration $2,\;8,\;8,\;2$.

Answer 1

In a modern periodic table, the horizontal rows are known as periods while the vertical columns are termed as groups. There are a total of seven periods and eighteen groups in which the elements are placed systematically according to their increasing order of the atomic number.

Complete answer:
In the question we are given the electronic configuration in terms of energy levels. But in order to find the group number and period number of the element, we need to find the electronic configuration in terms of the orbitals.
Electronic configuration in terms of energy levels $= 2,\;8,\;8,\;2$
Electronic configuration in terms of orbitals $= 1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}$
According to the electronic configuration of the given element, the valence electron enters the fourth energy level i.e., the value of the principal quantum number is $4$ for the last electron. Therefore, the element is of the fourth period.
Because, the last electron enters the s - subshell. So, we can say that the element belongs to s – block. As the s – subshell of the given element is completely filled i.e., it has two electrons in its 4s subshell which can be compared to the general electronic configuration of group 2 elements i.e., $n{s^2}$. Therefore, we can conclude that the given element belongs to the second group of the periodic table.
Based on the observations, we can say that the given element is calcium which is placed in the second group and fourth period of the modern periodic table.

Note:
It is important to note that the electrons are filled in the 4s subshell before than the 3d subshell because, 4s subshell consist of lesser energy as compared to 3d subshell and according to Aufbau’s principle, the orbitals or subshells with lesser energy will be filled first.