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Chemistry Revision
Unit 1

Spin Diagram

The Pauli exclusion principle and spin

Electron Configuration

Unit 1
Chemistry –
Electronic
Arrangement
sub-level

principal energy
level, n

max no
...

electrons

Ionisation

Energy levels

Models of atoms
At GCSE level, the model of the
atom states that a nucleus is
surrounded by shells of electrons
...


At A-level, this model is slightly different
...


What is ionization energy?
Ionization is a process in which atoms lose or gain electrons
and become ions
...

M(g) → M+(g) + e-(g)
The second ionization (I2) energy involves the removal of a
second electron:
M+(g) → M2+(g) + e-(g)
Looking at trends in ionization energies can reveal useful
evidence for the arrangement of electrons in atoms and ions
...

6
Successive ionization
energies increase as more
5
electrons are removed
...


4
3
2
electron removed

More evidence for energy levels

first ionization energy
(kJ mol-1)

The first ionization energies of group 2 elements also show
evidence for the existence of different principal energy levels
...


900
800
700
600

500
400
Be

Mg

Ca
Sr
element

This means electrons are
being removed from
successively higher
energy levels, which lie
further from the nucleus
and are less attracted to
Ba
the nucleus
...


1000

However, aluminium’s
800
value is below that of
magnesium
...

element

All principal energy levels contain one or more sub-levels,
with different but exact energy values
...
Each holds a
different number of
electrons
...


sub-level max no
...

electrons
1s
2
2s, 2p
8
3s, 3p, 3d
18
4s, 4p, 4d, 4f
32

Levels and sub-levels

Blocks of the periodic table

Order of sub-levels

The Aufbau principle
As part of his work on electron configuration, Niels Bohr
developed the Aufbau principle, which states how
electrons occupy sub-levels
...

This means the 1s sub-level
is filled first, followed by 2s,
2p, 3s and 3p
...


Writing electron configuration

Electron configuration: true or false?

Electron configuration of transition metals
Although the 3d sub-level is in
a lower principal energy level
than the 4s sub-level, it is
actually higher in energy
...


Example: what is the electron structure of vanadium?
1
...
Fill sub-levels, remembering
4s is filled before 3d

1s22s22p63s23p64s23d3

Electronic configuration: atoms

Electron configuration of ions
When writing the electron
configuration of ions, it is
important to add or subtract the
appropriate number of electrons
...


For negative ions
add electrons
...


Example: what is the electron structure of O2-?
1
...
Add or remove electrons due to charge

8 + 2 = 10

3
...

Example: what is the electron structure of Ni2+?
1
...
Fill sub-levels, remembering
4s is filled before 3d

1s22s22p63s23p64s23d8

3
...
Remove electrons starting
with 4s

1s22s22p63s23p63d8

Electron configuration: ions

Electron orbitals
It is impossible to exactly locate the position of an electron
within an energy sub-level
...
These
regions are called orbitals
...

sub-level no
...

electrons
s
1
2
p

3

6

d

5

10

f

7

14

The Pauli exclusion principle and spin
The Pauli exclusion principle states that each orbital
may contain no more than two electrons
...
The spins of electrons in the
same orbital must be opposite, i
...
one ‘up’ and one ‘down’
...
Orbitals are
represented by squares,
and electrons by arrows
pointing up or down
...

Hund’s rule states that single electrons
occupy all empty orbitals within a sub-level
before they start to form pairs in orbitals
...
The most stable
configuration is with single electrons in different orbitals
...


1600
1400
1200

1000
800
600
400
Na Mg Al Si P S Cl Ar
element

However, sulfur’s value
is below that of
phosphorus
...


Evidence for Hund’s rule: P vs
...


The lower first ionisation energy for sulfur is because it has
a pair of electrons in one of the 3p orbitals
...


phosphorus

sulfur

Electron configuration of Cr and Cu
The electron configurations of chromium and copper are
exceptions to the normal rules of orbital filling:
chromium

copper

1s22s22p63s23p64s13d5

1s22s22p63s23p64s13d10

In each case the 4s orbital contains one electron
...

With larger atoms like this it can be useful to shorten the
electron arrangement
...


Creating spin diagrams

What’s the keyword?



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