Question

Why are there 18 electrons in the third shell?

Answer 1

1. An atom contains many electrons that are present in the fixed orbits.
2. These electrons are filled by using the concept of quantum numbers.
3. There are mainly four types of subshells present such as s, p, d, and f in which the number of electrons is filled up.
4. Each element has its own electronic configuration that depends upon its atomic number.
5. Example: Nickle, the atomic number of Nickle is $28$.
6. The electronic configuration of Nickle is$1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^{10}$.
7. 1,2,3 denotes the number of shells that contains the number of electrons.
8. The first two electrons are filled up in the first shell denoted as 1.
9. The second shell of such an atom contains $2+6=8$electrons.
10. The third shell of such an atom contains $2+6+10=18$ electrons.
11. After this the fourth shell is started and the electrons start filling in that shell.
12. The general formula used for the filling of electrons is the nth shell can hold up to $2\left(n^2\right)$ electrons in principle.
13. Thus by this example we can easily understand that the first second and third shells contain $2,8,18$ electrons respectively.