Just as the orbitals page shows electron configurations in terms of orbitals, this page shows how electrons are configured in terms of electron shells.

Shells get filled with electrons as shown in the above graphic.
To use that graphic, start at the “K” shell, then follow that arrow to the beginning of the next arrow to the “L” shell and then to the next “L” shell,
and to the “M” shell and so on.
An example will show this quite well.

How many electrons are in each shell of a cobalt atom (atomic number 27)?
Using the above “Shell-Filling Diagram”, the shells get filled with electrons as follows:

K – 2 L – 2 L – 6 M – 2 M – 6 N – 2 M – 7

Adding up the electrons (K – 2) (L – 2) (L – 6) etc., once you get to element 20, the shells no longer fill alphabetically.
(After 2 electrons are added to the “N” shell, it is the “M” shell that gets filled with 7 electrons.)

We may then total the electrons for each shell:

K – 2 L – 8 M – 15 N – 2

Another way to determine the electrons in each shell of an atom is to look at the periodic table below.
Each row of that table consists of three lines:

  • the atomic number
  • the atomic symbol
  • the shell (capital letter) and the sub-shell (lower-case letter) being filled with electrons.

For a third way to see the electrons per shell, scroll down to the next table.

****** 1 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Row
  1 		1
H
Ks		 2
He
Ks
Row
  2		 3
Li
Ls		 4
Be
Ls		 5
B
Lp		 6
C
Lp		 7
N
Lp		 8
O
Lp		 9
F
Lp		 10
Ne
Lp
Row
  3		 11
Na
Ms		 12
Mg
Ms		 13
Al
Mp		 14
Si
Mp		 15
P
Mp		 16
S
Mp		 17
Cl
Mp		 18
Ar
Mp
Row
  4		 19
K
Ns		 20
Ca
Ns		 21
Sc
Md		 22
Ti
Md		 23
V
Md		 24
Cr
Md		 25
Mn
Md		 26
Fe
Md		 27
Co
Md		 28
Ni
Md		 29
Cu
Md		 30
Zn
Md		 31
Ga
Np		 32
Ge
Np		 33
As
Np		 34
Se
Np		 35
Br
Np		 36
Kr
Np
Row
  5		 37
Rb
Os		 38
Sr
Os		 39
Y
Nd		 40
Zr
Nd		 41
Nb
Nd		 42
Mo
Nd		 43
Tc
Nd		 44
Ru
Nd		 45
Rh
Nd		 46
Pd
Nd		 47
Ag
Nd		 48
Cd
Nd		 49
In
Op		 50
Sn
Op		 51
Sb
Op		 52
Te
Op		 53
I
Op		 54
Xe
Op
Row
  6		 55
Cs
Ps		 56
Ba
Ps		 57
La
Nf		 58
Ce
Nf		 59
Pr
Nf		 60
Nd
Nf		 61
Pm
Nf		 62
Sm
Nf		 63
Eu
Nf		 64
Gd
Nf		 65
Tb
Nf		 66
Dy
Nf		 67
Ho
Nf		 68
Er
Nf		 69
Tm
Nf		 70
Yb
Nf		 71
Lu
Od		 72
Hf
Od		 73
Ta
Od		 74
W
Od		 75
Re
Od		 76
Os
Od		 77
Ir
Od		 78
Pt
Od		 79
Au
Od		 80
Hg
Od		 81
Tl
Pp		 82
Pb
Pp		 83
Bi
Pp		 84
Po
Pp		 85
At
Pp		 86
Rn
Pp
Row
  7		 87
Fr
Qs		 88
Ra
Qs		 89
Ac
Of		 90
Th
Of		 91
Pa
Of		 92
U
Of		 93
Np
Of		 94
Pu
Of		 95
Am
Of		 96
Cm
Of		 97
Bk
Of		 98
Cf
Of		 99
Es
Of		 100
Fm
Of		 101
Md
Of		 102
No
Of		 103
Lr
Pd		 104
Rf
Pd		 105
Db
Pd		 106
Sg
Pd		 107
Bh
Pd		 108
Hs
Pd		 109
Mt
Pd		 110
Ds
Pd		 111
Rg
Pd		 112
Cn
Pd		 113
Nh
Qp		 114
Fl
Qp		 115
Mc
Qp		 116
Lv
Qp		 117
Ts
Qp		 118
Og
Qp

No doubt, this is the easiest way to determine which shells are filled with electrons for any element.
The table below shows the electrons per shell for each of the 118 known elements.
There are 20 elements that do not fill the shells of atoms with electrons according to the procedure discussed above.
Such elements in this table are listed twice.
The first listing is the “theoretical” shell structure.
The second listing shows the actual shell structure. Here, the element names are in green italics and are capitalized.

  Element     Symbol     Atomic #   K L M N O P Q
Hydrogen H 1 1 ** ** ** ** ** **
Helium He 2 2 ** ** ** ** ** **
Lithium Li 3 2 1 ** ** ** ** **
Beryllium Be 4 2 2 ** ** ** ** **
Boron B 5 2 3 ** ** ** ** **
Carbon C 6 2 4 ** ** ** ** **
Nitrogen N 7 2 5 ** ** ** ** **
Oxygen O 8 2 6 ** ** ** ** **
Fluorine F 9 2 7 ** ** ** ** **
Neon Ne 10 2 8 ** ** ** ** **
Sodium Na 11 2 8 1 ** ** ** **
Magnesium Mg 12 2 8 2 ** ** ** **
Aluminum Al 13 2 8 3 ** ** ** **
Silicon Si 14 2 8 4 ** ** ** **
Phosphorus P 15 2 8 5 ** ** ** **
Sulfur S 16 2 8 6 ** ** ** **
Chlorine Cl 17 2 8 7 ** ** ** **
Argon Ar 18 2 8 8 ** ** ** **
Potassium K 19 2 8 8 1 ** ** **
Calcium Ca 20 2 8 8 2 ** ** **
Scandium Sc 21 2 8 9 2 ** ** **
Titanium Ti 22 2 8 10 2 ** ** **
Vanadium V 23 2 8 11 2 ** ** **
Chromium Cr 24 2 8 12 2 ** ** **
CHROMIUM Cr 24 2 8 13 1 ** ** **
Manganese Mn 25 2 8 13 2 ** ** **
Iron Fe 26 2 8 14 2 ** ** **
Cobalt Co 27 2 8 15 2 ** ** **
Nickel Ni 28 2 8 16 2 ** ** **
Copper Cu 29 2 8 17 2 ** ** **
COPPER Cu 29 2 8 18 1 ** ** **
Zinc Zn 30 2 8 18 2 ** ** **
Gallium Ga 31 2 8 18 3 ** ** **
Germanium Ge 32 2 8 18 4 ** ** **
Arsenic As 33 2 8 18 5 ** ** **
Selenium Se 34 2 8 18 6 ** ** **
Bromine Br 35 2 8 18 7 ** ** **
Krypton Kr 36 2 8 18 8 ** ** **
Rubidium Rb 37 2 8 18 8 1 ** **
Strontium Sr 38 2 8 18 8 2 ** **
Yttrium Y 39 2 8 18 9 2 ** **
Zirconium Zr 40 2 8 18 10 2 ** **
Niobium Nb 41 2 8 18 11 2 ** **
NIOBIUM Nb 41 2 8 18 12 1 ** **
Molybdenum Mo 42 2 8 18 12 2 ** **
MOLYBDENUM Mo 42 2 8 18 13 1 ** **
Technetium Tc 43 2 8 18 13 2 ** **
Ruthenium Ru 44 2 8 18 14 2 ** **
RUTHENIUM Ru 44 2 8 18 15 1 ** **
Rhodium Rh 45 2 8 18 15 2 ** **
RHODIUM Rh 45 2 8 18 16 1 ** **
Palladium Pd 46 2 8 18 16 2 ** **
PALLADIUM Pd 46 2 8 18 18 ** ** **
Silver Ag 47 2 8 18 17 2 ** **
SILVER Ag 47 2 8 18 18 1 ** **
Cadmium Cd 48 2 8 18 18 2 ** **
Indium In 49 2 8 18 18 3 ** **
Tin Sn 50 2 8 18 18 4 ** **
Antimony Sb 51 2 8 18 18 5 ** **
Tellurium Te 52 2 8 18 18 6 ** **
Iodine I 53 2 8 18 18 7 ** **
Xenon Xe 54 2 8 18 18 8 ** **
Cesium Cs 55 2 8 18 18 8 1 **
Barium Ba 56 2 8 18 18 8 2 **
Lanthanum La 57 2 8 18 19 8 2 **
LANTHANUM La 57 2 8 18 18 9 2 **
Cerium Ce 58 2 8 18 20 8 2 **
CERIUM Ce 58 2 8 18 19 9 2 **
Praseodymium Pr 59 2 8 18 21 8 2 **
  Element     Symbol     Atomic #   K L M N O P Q
Neodymium Nd 60 2 8 18 22 8 2 **
Promethium Pm 61 2 8 18 23 8 2 **
Samarium Sm 62 2 8 18 24 8 2 **
Europium Eu 63 2 8 18 25 8 2 **
Gadolinium Gd 64 2 8 18 26 8 2 **
GADOLINIUM Gd 64 2 8 18 25 9 2 **
Terbium Tb 65 2 8 18 27 8 2 **
Dysprosium Dy 66 2 8 18 28 8 2 **
Holmium Ho 67 2 8 18 29 8 2 **
Erbium Er 68 2 8 18 30 8 2 **
Thulium Tm 69 2 8 18 31 8 2 **
Ytterbium Yb 70 2 8 18 32 8 2 **
Lutetium Lu 71 2 8 18 32 9 2 **
Hafnium Hf 72 2 8 18 32 10 2 **
Tantalum Ta 73 2 8 18 32 11 2 **
Tungsten W 74 2 8 18 32 12 2 **
Rhenium Re 75 2 8 18 32 13 2 **
Osmium Os 76 2 8 18 32 14 2 **
Iridium Ir 77 2 8 18 32 15 2 **
Platinum Pt 78 2 8 18 32 16 2 **
PLATINUM Pt 78 2 8 18 32 17 1 **
Gold Au 79 2 8 18 32 17 2 **
GOLD Au 79 2 8 18 32 18 1 **
Mercury Hg 80 2 8 18 32 18 2 **
Thallium Tl 81 2 8 18 32 18 3 **
Lead Pb 82 2 8 18 32 18 4 **
Bismuth Bi 83 2 8 18 32 18 5 **
Polonium Po 84 2 8 18 32 18 6 **
Astatine At 85 2 8 18 32 18 7 **
Radon Rn 86 2 8 18 32 18 8 **
Francium Fr 87 2 8 18 32 18 8 1
Radium Ra 88 2 8 18 32 18 8 2
Actinium Ac 89 2 8 18 32 19 8 2
ACTINIUM Ac 89 2 8 18 32 18 9 2
Thorium Th 90 2 8 18 32 20 8 2
THORIUM Th 90 2 8 18 32 18 10 2
Protactinium Pa 91 2 8 18 32 21 8 2
PROTACTINIUM Pa 91 2 8 18 32 20 9 2
Uranium U 92 2 8 18 32 22 8 2
URANIUM U 92 2 8 18 32 21 9 2
Neptunium Np 93 2 8 18 32 23 8 2
NEPTUNIUM Np 93 2 8 18 32 22 9 2
Plutonium Pu 94 2 8 18 32 24 8 2
Americium Am 95 2 8 18 32 25 8 2
Curium Cm 96 2 8 18 32 26 8 2
CURIUM Cm 96 2 8 18 32 25 9 2
Berkelium Bk 97 2 8 18 32 27 8 2
Californium Cf 98 2 8 18 32 28 8 2
Einsteinium Es 99 2 8 18 32 29 8 2
Fermium Fm 100 2 8 18 32 30 8 2
Mendelevium Md 101 2 8 18 32 31 8 2
Nobelium No 102 2 8 18 32 32 8 2
Lawrencium Lr 103 2 8 18 32 32 9 2
LAWRENCIUM Lr 103 2 8 18 32 32 8 3
Rutherfordium Rf 104 2 8 18 32 32 10 2
Dubnium Db 105 2 8 18 32 32 11 2
Seaborgium Sg 106 2 8 18 32 32 12 2
Bohrium Bh 107 2 8 18 32 32 13 2
Hassium Hs 108 2 8 18 32 32 14 2
Meitnerium Mt 109 2 8 18 32 32 15 2
Darmstadtium Ds 110 2 8 18 32 32 16 2
Roentgenium Rg 111 2 8 18 32 32 17 2
Copernicium Uub 112 2 8 18 32 32 18 2
Nihonium Nh 113 2 8 18 32 32 18 3
Flerovium Fl 114 2 8 18 32 32 18 4
Moscovium Mc 115 2 8 18 32 32 18 5
Livermorium Lv 116 2 8 18 32 32 18 6
Tennesine Ts 117 2 8 18 32 32 18 7
Oganesson Og 118 2 8 18 32 32 18 8