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Trends in Electron (electronic) Configuration

Key Concepts

Trends in number of valence electrons (electrons in the highest energy level):
- Elements in the same Group of the Periodic Table have the same number of valence shell electrons (electrons in the highest energy level).
- Going across a period of the Periodic Table from left to right the number of valence electrons increases.
Trends in energy of valence electrons:
- The valence shell electrons of elements in the same Period of the Periodic Table occupy the same energy level (shell).*
- Going down a Group of the Periodic Table from top to bottom, the energy of the valence shell electrons increases.

*This discussion excludes transition elements, actinides and lanthanides

Trends in the Number of Valence Electrons

H
1

symbol
simple electron configuration
(valence shell electrons, the number of electrons in the highest energy level, shown in bold)
He
2

Group 1
(IA) Group 2
(IIA) Group 13
(IIIA) Group 14
(IVA) Group 15
(VA) Group 16
(VIA) Group 17
(VIIA) Group 18
(VIIIA or 0)

Li
2,1 Be
2,2 B
2,3 C
2,4 N
2,5 O
2,6 F
2,7 Ne
2,8

Na
2,8,1 Mg
2,8,2 Al
2,8,3 Si
2,8,4 P
2,8,5 S
2,8,6 Cl
2,8,7 Ar
2,8,8

K
2,8,8,1 Ca
2,8,8,2 Ga
2,8,18,3 Ge
2,8,18,4 As
2,8,18,5 Se
2,8,18,6 Br
2,8,18,7 Kr
2,8,18,8

Rb
2,8,18,8,1 Sr
2,8,18,8,2 In
2,8,18,18,3 Sn
2,8,18,18,4 Sb
2,8,18,18,5 Te
2,8,18,18,6 I
2,8,18,18,7 Xe
2,8,18,18,8

Cs
2,8,18,18,8,1 Ba
2,8,18,18,8,2 Tl
2,8,18,32,18,3 Pb
2,8,18,32,18,4 Bi
2,8,18,32,18,5 Po
2,8,18,32,18,6 At
2,8,18,32,18,7 Rn
2,8,18,32,18,8

Fr
2,8,18,32,18,8,1 Ra
2,8,18,32,18,8,2

1 valence electron 2 valence electrons 3 valence electrons 4 valence electrons 5 valence electrons 6 valence electrons 7 valence electrons 8 valence electrons
(EXCEPTION He has 2 valence electrons)

Elements in the same Group have the same number of valence shell electrons.

Group 1
(IA) 2
(IIA) 13
(IIIA) 14
(IVA) 15
(VA) 16
(VIA) 17
(VIIA) 18
(VIIIA or 0)

No. valence electrons 1 2 3 4 5 6 7 8*

*Helium is an exception since it has only 2 valence electrons
Across a period of the Periodic Table from left to right the number of valence electrons increases.*

Group 1
(IA) 2
(IIA) 13
(IIIA) 14
(IVA) 15
(VA) 16
(VIA) 17
(VIIA) 18
(VIIIA or 0)

No. valence electrons 1 2 3 4 5 6 7 8*

lowest number of
valence electrons → → → → → → highest number of
valence electrons

*Helium is an exception since it has only 2 valence electrons

Trends in Energy of Valence Electrons

H
1s¹
symbol
position of valence shell electrons (energy level of valence electrons shown in bold)

Period Group 1
(IA) Group 2
IIA Group 13
(IIIA) Group 14
(IVA) Group 15
(VA) Group 16
(VIA) Group 17
(VIIA) Group 18
(VIIIA or 0) Energy Level of Valence Electrons

1 He
1s² 1
(K shell) Lowest Energy
↓

2 Li
2s¹ Be
2s² B
2s²2p¹ C
2s²2p² N
2s²2p³ O
2s²2p⁴ F
2s²2p⁵ Ne
2s²2p⁶ 2
(L shell) ↓

3 Na
3s¹ Mg
3s² Al
3s²3p¹ Si
3s²3p² P
3s²3p³ S
3s²3p⁴ Cl
3s²3p⁵ Ar
3s²3p⁶ 3
(M shell) ↓

4 K
4s¹ Ca
4s² Ga
4s²4p¹ Ge
4s²4p² As
4s²4p³ Se
4s²4p⁴ Br
4s²4p⁵ Kr
4s²4p⁶ 4
(N shell) ↓

5 Rb
5s¹ Sr
5s² In
5s²5p¹ Sn
5s²5p² Sb
5s²5p³ Te
5s²5p⁴ I
5s²5p⁵ Xe
5s²5p⁶ 5
(O shell) ↓

6 Cs
6s¹ Ba
6s² Tl
6s²6p¹ Pb
6s²6p² Bi
6s²6p³ Po
6s²6p⁴ At
6s²6p⁵ Rn
6s²6p⁶ 6
(P shell) ↓

7 Fr
7s¹ Ra
7s² 7
(Q shell) ↓
Highest Energy

The valence shell electrons of elements in the same period of the Periodic Table occupy the same energy level:
Period Energy Level of
Valence Electrons Valence Shell

1 1 K

2 2 L

3 3 M

4 4 N

5 5 O

6 6 P

7 7 Q

Down a Group of the Periodic Table from top to bottom, the energy of the valence electrons increases:
Energy Level of
Valence Electrons
in a Group

2 lowest energy

3 ↓

4 ↓

5 ↓

6 ↓

7 highest energy