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 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
  I and II	s	2
  III, IV, V, VI, VII, 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):
1^st(K)=2, 2^nd(L)=8, 3^rd(M)=18, 4^th(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
H	1	1	1
He	2	2	2
Li	3	3	2,1
Be	4	4	2,2
B	5	5	2,3
C	6	6	2,4
N	7	7	2,5
O	8	8	2,6
F	9	9	2,7
Ne	10	10	2,8
Na	11	11	2,8,1
Mg	12	12	2,8,2
Al	13	13	2,8,3
Si	14	14	2,8,4
P	15	15	2,8,5
S	16	16	2,8,6
Cl	17	17	2,8,7
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

Ion	No. Electrons = Z - charge	Subshell Electron Configuration
H-	1 - (-1) = 2	1s2
Li+	3 - 1 = 2	1s2
Be2+	4 - 2 = 2	1s2
B3+	5 - 3 = 2	1s2
C4+	6 - 4 = 2	1s2
N3-	7 - (-3) = 10	1s2 2s2 2p6
O2-	8 - (-2) = 10	1s2 2s2 2p6
F-	9 - (-1) = 10	1s2 2s2 2p6
Na+	11 - 1 = 10	1s2 2s2 2p6
Mg2+	12 - 2 = 10	1s2 2s2 2p6
Al3+	13 - 3 = 10	1s2 2s2 2p6
Si4+	14 - 4 = 10	1s2 2s2 2p6
P3-	15-(-3)=18	1s2 2s2 2p6 3s2 3p6
S2-	16-(-2)=18	1s2 2s2 2p6 3s2 3p6
Cl-	17-(-1)=18	1s2 2s2 2p6 3s2 3p6

 

Atom	Z	No. Electrons = Z	Subshell Electron Configuration
H	1	1	1s1
He	2	2	1s2
Li	3	3	1s2 2s1
Be	4	4	1s2 2s2
B	5	5	1s2 2s2 2p1
C	6	6	1s2 2s2 2p2
N	7	7	1s2 2s2 2p3
O	8	8	1s2 2s2 2p4
F	9	9	1s2 2s2 2p5
Ne	10	10	1s2 2s2 2p6
Na	11	11	1s2 2s2 2p6 3s1
Mg	12	12	1s2 2s2 2p6 3s2
Al	13	13	1s2 2s2 2p6 3s2 3p1
Si	14	14	1s2 2s2 2p6 3s2 3p2
P	15	15	1s2 2s2 2p6 3s2 3p3
S	16	16	1s2 2s2 2p6 3s2 3p4
Cl	17	17	1s2 2s2 2p6 3s2 3p5
Ar	18	18	1s2 2s2 2p6 3s2 3p6