The electrons in an atom are in continuous and constant motion in shells around the nucleus, represented by capital letters K,L, M,N, etc. The K shell is the innermost and the nearest to the nucleus followed by the L-shell, then the M-shell the N-shell etc. Each shell is at the energy level represented by the principal quantum number n. The K-shell is in the first energy level, where n=1. It has the lowest energy, and hence the most stable shell. The L-shell is in the second energy level n=2; the M-shell is in the third energy level n=3, while the N-shell is in the fourth energy level, n=4.
| Shell | K | L | M | N |
| Energy Level(n) | 1 | 2 | 3 | 4 |
SubShells & Atomic Orbitals
Each Shell has one or more subshells which are at energy sublevels represented by small letters of the alphabets s,p,d, or f. The number of sublevels in a main shell equals the value of n, the energy level hence
- K-shell(n=1), has only one subshell, 1s
- L-shell(n=2) has only two subshell 2s, 2p
- M-shell(n=3) has only three subshell 3s, 3p, 3d
- N-shell(n=4) has four subshell 4s, 4p. 4d, 4f
Each subshell has one or more atomic orbitals. An atomic orbital is a region around the nucleus where electrons are most likely to be found.
- An s-subshell has one orbital called s-orbital
- A p-subshell has three orbitals called p-orbitals
- A d-subshell has five orbitals called d-orbitals
- An f-subshell has seven orbitals called f-orbitals
| Subshells | s | p | d | f |
| Number of orbitals | 1 | 3 | 5 | 7 |
- An s-orbital is spherical in shape, and symmetrical about the nucleus
- A p-orbital has a dumbbell shape like figure eight
- An orbital can accommodate a maximum of two electrons
| Energy Level (n) (Shell) | Sub-Shell n | No of orbitals n2 (Total) | No of electrons per subshell | Maximum Electrons per Shell 2n2 |
| K | 1s | 1(1) | 2 | 2 |
| L | 2s 2p | 1 3(4) | 2 6 | 8 |
| M | 3s 3p 3d | 1 3 5(9) | 2 6 10 | 18 |
| N | 4s 4p 4d 4f | 1 3 5 7(16) | 2 6 10 14 | 32 |