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Electron Configuration

Key Concepts

  • The number of electrons in an atom of an element corresponds to the element's Atomic Number (Z) and is equal to the number of protons in the nucleus of the atom.
        H atom, Z = 1.
        Number of protons in H atom = Z = 1
        Number of electrons in H atom = Z = 1

  • The number of electrons in the ion of an element corresponds to the element's Atomic Number (Z) minus the charge on the ion:
        H+, Z = 1, charge = +1. Number of electrons in ion = Z - charge = 1 - (+1) = 0
        H-, Z = 1 charge = -1. Number of electrons in ion = Z - charge = 1 - (-1) = 2

  • Simple Electron Configuration shows the number of electrons in each energy level

  • Lowest Energy levels are filled first
        (ie, first energy level is filled first followed by the second then the third etc)

    Energy Level Maximum Number of Electrons
    First 2
    Second 8
    Third 18
    Fourth 32

  • Filled energy levels correspond to the electron configuration of Group 18 (VIII) elements
        (the Noble or Inert Gases)

  • Subshell Electron Configuration shows the number of electrons in each subshell
        (or sub-level) within each energy level

    Group of the Periodic Table subshell being filled Maximum Number of Electrons
    1 and 2
    (IA and IIA)
    s 2
    13, 14 , 15 , 16 , 17 , 18
    (IIIA,IVA,VA,VIA,VIIA,VIII)
    p 6
    Transition Metals d 10
    Lanthanides and Actinides f 14

  • Orbital Notation shows the number of electrons in each orbital within an energy level

Simple Electron Configuration

Maximum number of electrons in each energy level (shell):
1st(K)=2, 2nd(L)=8, 3rd(M)=18, 4th(N)=32

The helium atom has 2 electrons in the first energy level.
Its simple electron configuration is 2

The neon atom has 10 electrons,
2 electrons in the first energy level and 8 electrons in the second energy level.
Its simple electron configuration is 2,8

The argon atom has 18 electrons,
2 electrons in the first energy level, 8 electrons in the second energy level, 8 electrons in the thrid energy level.
Its simple electron configuration is 2,8,8

Period Order for Filling Energy Levels Energy Level Shell
1 H   He First K
2 Li Be   B C N O F Ne Second L
3 Na Mg   Al Si P S Cl Ar Third M
4 K Ca Transition Metals : third energy level filling Ga Ge As Se Br Kr Fourth N
5 Rb Sr Transition Metals : fourth energy level filling In Sn Sb Te I Xe Fifth O
6 Cs Ba Transition Metals : fifth energy level filling Tl Pb Bi Po At Rn Sixth P
7 Fr Ra Transition Metals : sixth energy level filling Seventh Q
 
  Lanthanides : fourth energy level filling  
  Actinides : fifth energy level filling  

Examples

Atom Z No. Electrons
= Z
Simple
Electron
Configuration
Ion No. Electrons
= Z - charge
Simple
Electron
Configuration
H 1 1 1 H- 1 - (-1) = 2 2
He 2 2 2
Li 3 3 2,1 Li+ 3 - 1 = 2 2
Be 4 4 2,2 Be2+ 4 - 2 = 2 2
B 5 5 2,3 B3+ 5 - 3 = 2 2
C 6 6 2,4 C4+ 6 - 4 = 2 2
N 7 7 2,5 N3- 7 - (-3) = 10 2,8
O 8 8 2,6 O2- 8 - (-2) = 10 2,8
F 9 9 2,7 F- 9 - (-1) = 10 2,8
Ne 10 10 2,8
Na 11 11 2,8,1 Na+ 11 - 1 = 10 2,8
Mg 12 12 2,8,2 Mg2+ 12 - 2 = 10 2,8
Al 13 13 2,8,3 Al3+ 13 - 3 = 10 2,8
Si 14 14 2,8,4 Si4+ 14 - 4 = 10 2,8
P 15 15 2,8,5 P3- 15 - (-3) = 18 2,8,8
S 16 16 2,8,6 S2- 16 - (-2) = 18 2,8,8
Cl 17 17 2,8,7 Cl- 17 - (-1) = 18 2,8,8
Ar 18 18 2,8,8

Sub-shell Electron Configuration

Order for filling subshells: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p
where the number indicates the energy level and the letter indicates the subshell being filled.

Maximum number of electrons in each subshell: s=2, p=6, d=10, f=14
A superscipt number after the letter of the subshell shows how many electrons occupy that subshell.

The helium atom has 2 electrons, both in the s subshell of the first energy level.
Its sub-shell electron configuration is 1s2

The neon atom has 10 electrons.
2 electrons in the s subshell of the first energy level: 1s2
8 electrons in second energy level, made up of 2 electrons in an s subshell and 6 electrons in a p subshell: 2s2 2p6
Its subshell electron configuration is 1s2 2s2 2p6

The argon atom has 18 electrons.
2 electrons in the s sub-shell of the first energy level: 1s2
8 electrons in the second energy level made up of 2 electrons in an s subshell and 6 electrons in a p subshell: 2s2 2p6
8 electrons in the third energy level made up of 2 electrons in an s subshell and 6 electrons in a p subshell: 3s2 3p6
Its subshell electron configuration is: 1s2 2s2 2p6 3s2 3p6

Period s subshell d subshell p subshell Energy Level
1 H   He First
2 Li Be   B C N O F Ne Second
3 Na Mg   Al Si P S Cl Ar Third
4 K Ca Transition Metals : third energy level filling Ga Ge As Se Br Kr Fourth
5 Rb Sr Transition Metals : fourth energy level filling In Sn Sb Te I Xe Fifth
6 Cs Ba Transition Metals : fifth energy level filling Tl Pb Bi Po At Rn Sixth
7 Fr Ra Transition Metals : sixth energy level filling Seventh
 
  f subshell Lanthanides : fourth energy level filling  
  f subshell Actinides : fifth energy level filling  

Examples

Atom Z No. Electrons
= Z
Subshell Electron Configuration Ion No. Electrons
= Z - charge
Subshell Electron Configuration
H 1 1 1s1 H- 1 - (-1) = 2 1s2
He 2 2 1s2
Li 3 3 1s2 2s1 Li+ 3 - 1 = 2 1s2
Be 4 4 1s2 2s2 Be2+ 4 - 2 = 2 1s2
B 5 5 1s2 2s2 2p1 B3+ 5 - 3 = 2 1s2
C 6 6 1s2 2s2 2p2 C4+ 6 - 4 = 2 1s2
N 7 7 1s2 2s2 2p3 N3- 7 - (-3) = 10 1s2 2s2 2p6
O 8 8 1s2 2s2 2p4 O2- 8 - (-2) = 10 1s2 2s2 2p6
F 9 9 1s2 2s2 2p5 F- 9 - (-1) = 10 1s2 2s2 2p6
Ne 10 10 1s2 2s2 2p6
Na 11 11 1s2 2s2 2p6 3s1 Na+ 11 - 1 = 10 1s2 2s2 2p6
Mg 12 12 1s2 2s2 2p6 3s2 Mg2+ 12 - 2 = 10 1s2 2s2 2p6
Al 13 13 1s2 2s2 2p6 3s2 3p1 Al3+ 13 - 3 = 10 1s2 2s2 2p6
Si 14 14 1s2 2s2 2p6 3s2 3p2 Si4+ 14 - 4 = 10 1s2 2s2 2p6
P 15 15 1s2 2s2 2p6 3s2 3p3 P3- 15-(-3)=18 1s2 2s2 2p6 3s2 3p6
S 16 16 1s2 2s2 2p6 3s2 3p4 S2- 16-(-2)=18 1s2 2s2 2p6 3s2 3p6
Cl 17 17 1s2 2s2 2p6 3s2 3p5 Cl- 17-(-1)=18 1s2 2s2 2p6 3s2 3p6
Ar 18 18 1s2 2s2 2p6 3s2 3p6


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