Trends in Electronic Configuration Chemistry Tutorial

Electronic configuration is also referred to as electron configuration.

If you consider the electronic configuration of an atom of each element in the Periodic Table you will see a number of patterns which are referred to as periodic trends, or just trends.

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).¹

⚛ Going down a Group of the Periodic Table from top to bottom, the energy of the valence shell electrons increases.

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Trends in the Number of Valence Electrons

The table below for the main group elements is set out just like the Periodic Table of the elements.

The electronic configuration in shell notation is given for an atom of each of the elements.

The electrons in the valence shell (highest energy level) are given in red.

Can you see a pattern, or trend, in the electronic configuration of the atoms in each group and in each period?

H
1

symbol
electron configuration in shells
(valence shell electrons, the number of electrons in the highest energy level, shown in red)

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)

You should see two patterns, or trends:

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)

Number of
valence electrons 1 2 3 4 5 6 7 8 ²

Across a period of the Periodic Table from left to right the number of valence electrons increases.²

Group	1 (IA)	2 (IIA)	13 (IIIA)	14 (IVA)	15 (VA)	16 (VIA)	17 (VIIA)	18 (VIIIA or 0)
Number of valence electrons	1	2	3	4	5	6	7	8 ²
Trend:	lowest number of valence electrons	→	→	→	→	→	→	highest number of valence electrons

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Trends in Energy of Valence Electrons

In the table below, the electronic configuration of only the valence electrons is given using subshell notation.

Can you find a pattern, or trend, in the energy levels occupied by the valence electrons?

H
1s¹

symbol
position of valence shell electrons (energy level of valence electrons shown in red)

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

Trend

He
1s²

1
(K shell)

Lowest Energy
↓

Li
2s¹

Be
2s²

B
2s²2p¹

C
2s²2p²

N
2s²2p³

O
2s²2p⁴

F
2s²2p⁵

Ne
2s²2p⁶

2
(L shell)

↓

Na
3s¹

Mg
3s²

Al
3s²3p¹

Si
3s²3p²

P
3s²3p³

S
3s²3p⁴

Cl
3s²3p⁵

Ar
3s²3p⁶

3
(M shell)

↓

K
4s¹

Ca
4s²

Ga
4s²4p¹

Ge
4s²4p²

As
4s²4p³

Se
4s²4p⁴

Br
4s²4p⁵

Kr
4s²4p⁶

4
(N shell)

↓

Rb
5s¹

Sr
5s²

In
5s²5p¹

Sn
5s²5p²

Sb
5s²5p³

Te
5s²5p⁴

I
5s²5p⁵

Xe
5s²5p⁶

5
(O shell)

↓

Cs
6s¹

Ba
6s²

Tl
6s²6p¹

Pb
6s²6p²

Bi
6s²6p³

Po
6s²6p⁴

At
6s²6p⁵

Rn
6s²6p⁶

6
(P shell)

↓

Fr
7s¹

Ra
7s²

7
(Q shell)

↓
Highest Energy

You should see two patterns, or trends, in the energy levels of the valence electrons:

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 Trend

2 lowest energy

3 ↓

4 ↓

5 ↓

6 ↓

7 highest energy

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

Energy Level of Valence Electrons in a Group	Trend
2	lowest energy
3	↓
4	↓
5	↓
6	↓
7	highest energy

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Footnotes:

1. This discussion will only talk about main group elements.
This discussion excludes transition metals, actinoids (actinides) and lanthanoids (lanthanides).

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