Question

An element has atomic number 19. Where would you expect this element in the periodic table and why?

Answer 1

Hint: For finding the position of an element whose atomic number is known we should always look at the electronic configuration first. Then we can find in which group or period it is present.

Complete step by step solution
Let us first look at the electronic configuration of the element with atomic no. 19.
So according to the rules electronic configuration of elements with atomic no. 19 is:
$1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}$ , so according to this electronic configuration,
No. of valence electrons=1
In periodic table, the vertical columns are called groups and the horizontal rows are called periods.
The Groups were divided from 1 to 8 on the basis of Valence electrons. However, the transition elements were excluded from this, because these elements will possess Variable charges in its ionic form.
For example, if you see the $Na$ , it belongs to Group 1 because it has only one valence electron. Similarly, in case of $O$ , it belongs to Group 6 because it has only six valence electrons.
So according to this element with atomic no. must be in group 1 because it is having 1 valence electron.
The Periods were divided on the basis of location of valence electrons in the energy levels.
For example, if you see the $Li$ , it'll be placed in period 2, because its valence electron is in 2s subshell. Similarly, in case of $Na$ , it’ll be placed in period 3, because its valence electron is in 3s subshell.
So according to this the element must be in period 4th period because it is having valence electrons in 4s.

So the element with atomic no. 19 is in group 1st and period 4th.

Note:
Valence electrons are the electrons present in the highest energy level, here the highest energy level is 4s, and hence only one electron is present there. The element with 19 atomic no. is Potassium.