CBSE Class 11-science Answered
Let us recall the Bohr model. According to Bohr's model, electrons occupy certain stable orbits or shells. Each shell has definite energy. The maximum number of electrons present in the shell is given by the formula (2n2), where n is the orbit number or shell number.
Each shell is composed of one or more subshells, which are themselves composed of atomic orbitals. For example, the first (K) shell has one subshell, called 1s; the second (L) shell has two subshells, called 2s and 2p; the third shell has 3s, 3p, and 3d; the fourth shell has 4s, 4p, 4d and 4f; the fifth shell has 5s, 5p, 5d, and 5f and can theoretically hold more but the 5f subshell, although occupied in actinides, is not filled in any element occurring naturally.
Number of electrons in each shell
Each subshell is constrained to hold 4ℓ + 2 electrons at most, namely:
- Each s subshell holds at most 2 electrons
- Each p subshell holds at most 6 electrons
- Each d subshell holds at most 10 electrons
- Each f subshell holds at most 14 electrons
Therefore, the K shell, which contains only an s subshell, can hold up to 2 electrons; the L shell, which contains an s and a p, can hold up to 2 + 6 = 8 electrons, and so forth; that's why nth shell can hold up to 2n2 electrons.
Shell |
Subshell |
Subshell |
Shell |
K |
1s |
2 |
2 |
L |
2s |
2 |
2 + 6 = 8 |
2p |
6 |
||
M |
3s |
2 |
2 + 6 + 10 |
3p |
6 |
||
3d |
10 |
||
N |
4s |
2 |
2 + 6 + |
4p |
6 |
||
4d |
10 |
||
4f |
14 |