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In hydrogen all orbitals in the same shell have the same energy:

1

2

3

s p d

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but once some orbitals contain electrons the energies of other orbitals are affected due to repulsion

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Hydrogen

1s1

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Helium

1s2

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Lithium

1s2 2s1

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Beryllium

1s2 2s2

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Boron

1s2 2s2 2p 1

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Carbon

1s2 2s2 2p 2

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Nitrogen

1s2 2s2 2p 3

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Oxygen

1s2 2s2 2p 4

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T ry to write down the electronic configurations of :

N eon (z = 10)

Sodium (z = 11)

Silicon (z = 14)

Potassium (z = 19)

1s2 2s2 2p 6

1s2 2s2 2p 6 3s1

1s2 2s2 2p 6 3s2 3p 2

1s2 2s2 2p 6 3s2 3p 6 4s 1

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Y ou may have e x pected the final electron in potassium to be in the3d sub-shell

1s2 2s2 2p 6 3s2 3p 6 3d 1

but potassium is so similar to sodium that the electronic structureshould also be similar with a outer s electron

T he e x planation is that the increase in energy of the 3d sub-shell puts it higher than the 4s.

1s2 2s2 2p 6 3s2 3p 6 4s 11s2 2s2 2p 6 3s1 N a

K

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1

2

3

s

p

d4

4f

NB 3d sub-shell higher than the 4s.

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1

2

3

s

p

d4

1s2 2s2 2p 6 3s2 3p 6 4s1

2 8 8 1

Potassium

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T his accounts for the odd series of lengths of periods in the

Periodic T able : 2, 8, 8, 18

when the ma x imum number of electrons per shell is

2, 8, 18, 36

B romine for e x ample is the 17 th element across the P T but has only 7 outer electrons.

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1

2

3

s

p

d4

B romine

1s2 2s2 2p 6 3s2 3p 6 3d 10 4s2 4p 5

2 8 18 7