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RULES TO FILL ELECTRONS IN SUB SHELL
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"; 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.[1] The various possible subshells are shown in the following table:
Subshell label    รข??    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 (theoretically)    (next in alphabet after f)[3]
The first column is the "subshell label", a lowercase-letter label for the type of subshell. For example, the "4s subshell" is a subshell of the fourth (N) shell, with the type ("s") described in the first row.
The second column is the azimuthal quantum number of the subshell. The precise definition involves quantum mechanics, but it is a number that characterizes the subshell.
The third column is the maximum number of electrons that can be put into a subshell of that type. For example, the top row says that each s-type subshell ("1s", "2s", etc.) can have at most two electrons in it. In each case the figure is 4 greater than the one above it.
The fourth column says which shells have a subshell of that type. For example, looking at the top two rows, every shell has an s subshell, while only the second shell and higher have a p subshell (i.e., there is no "1p" subshell).
The final column gives the historical origin of the labels s, p, d, and f. They come from early studies of atomic spectral lines. The other labels, namely g, h and i, are an alphabetic continuation following the last historically originated label of 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 one subshell do have exactly the same level of energy,[4] 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 (see Valence shells and Aufbau principle).
Number of electrons in each shell

Shell
name    Subshell
name    Subshell
max
electrons    Shell
max
electrons
K    1s    2    2
L    2s    2    2 + 6 = 8
2p    6
M    3s    2    2 + 6 + 10
= 18
3p    6
3d    10
N    4s    2    2 + 6 +
+ 10 + 14
= 32
4p    6
4d    10
4f    14
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. [5]
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.[6][7] 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).

+ 10 + 14
= 32
4p    6
4d    10
4f    14
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. [5]
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.[6][7] 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).


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