Subshells

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"; and so on. The various possible subshells are shown in the following table:

Subshell label	ℓ	Max electrons	Shells containing it	Historical name
s	0	2	Every shell	sharp
p	1	6	2nd shell and higher	principal
d	2	10	3rd shell and higher	diffuse
f	3	14	4th shell and higher	fundamental
g	4	18	5th shell and higher	(next in alphabet after f)

Although it is commonly stated that all the electrons in a shell have the same energy, this is an approximation. However, the electrons in a subshell do have exactly the same level of energy, with later subshells having more energy per electron than earlier ones. This effect is great enough that the energy ranges associated with shells can overlap.


Number of electrons in each subshell

	s	p	d	f	g	Total
K	2					2
L	2	6				8
M	2	6	10			18
N	2	6	10	14		32
O	2	6	10	14	18	50

An atom's electron shells are filled according to the following theoretical constraints:

• 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
• Each g subshell holds at most 18 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. The general formula is that the nth shell can in principle hold up to 2n² electrons.

Although that formula gives the maximum in principle, in fact that maximum is only achieved (by known elements) for the first four shells (K,L,M,N). No known element has more than 32 electrons in any one shell.^[5][6] This is because the subshells are filled according to the Aufbau principle. The first elements to have more than 32 electrons in one shell would belong to the g-block of period 8 of the periodic table. These elements would have some electrons in their 5g subshell and thus have more than 32 electrons in the O shell (fifth principal shell).