IONISATION ENERGY AND ELECTRONEGATIVITY
Ionisation energy
 Defined as the energy required to remove the least tightly bound
electron from one mole of isolated gaseous atoms or ions.
 Measured in kJ mol-1.
 The first ionisation energy is the energy required to remove the first
electron, from an atom, the second ionisation energy removes the
second electron from the ion, etc.
Ne(g)
Ne+(g)
Ne+(g) + e -Ne++(g) + e --
H = +2087 kJ mol-1
H = +6128 kJ mol-1
Notice the second ionisation energy is greater than the first. This
will always be the case since electrons are being removed from a
positive ion. Ionisation energies are always positive i.e. it is
always an endothermic reaction.
 Removal of all electrons from an atom allows us to deduce the
electron configuration.
 Studying graphs of ionisation energies with atomic number show up
definite periodic trends.
 For atoms in the same group the ionisation energy decreases
as you go down. The further away from the nucleus the electron
is the easier it is to remove.
 The general trend across a period is of increasing ionisation
energies.
 The zigzag shape of the graph indicates that some electron
numbers are more stable than others. There is added stability
associated with filled and half-filled levels, sublevels and
orbitals.
1st Ionisation Energies for H to Ar
2500
He
Ne
2000
Energy (kJ/mol)
F
Ar
1500
N
H
O
Cl
C
P
1000
S
Be
B
Si
Mg
Al
Li
500
Na
0
0
2
4
6
8
10
12
14
16
18
20
Atomic Number
Electronegativity
 Chemists use the term electronegativity to measure the tendency of
each atom in a molecule to attract shared electrons to itself. It is
calculated from the ionisation energy, electron affinity and bond
dissociation energies. (Electron affinity is the energy released when
an electron is added to a neutral atom in a gaseous state)
 Noble gases do not form compounds and therefore do not have
electronegativity values.
 Elements at the top right of the periodic table have the highest
electronegativity values. i.e. fluorine is greatest, followed by oxygen
and nitrogen.
 No atom has an electronegativity of zero.
 Differences in electronegativity lead to different types of bond
formation.
Electronegativity and Bond Types
Differences in
electronegativity
0.0
% Ionic character
0
0.5
6
Effectively
non-polar
1.0
22
1.5
2.0
43
Polar
covalent
63
2.5
79
Strongly
polar
3.0
3.5
89
95
Effectively
ionic
The bonding in most chemical species is not 100% ionic or 100%
covalent, although we talk about ‘ionic compounds’ or ‘covalent
compounds’. Instead, the bonding is somewhere along a continuum of
bonding, from covalent to ionic. Some supposed ‘ionic’ compounds,
such as AlCl3, are covalent in nature as shown by their properties such
as melting point and solubility. Even some metals with so-called metallic
bonding, may have a certain % covalent character in their bonds. e.g.
tin. The bonds in Cl2 or H2 are certainly 100% covalent, but it is unlikely
that a 100% ionic bond exists in any compound.
Covalent bonds will have partial ionic character when the elements
involved in the bond have differing electronegativities.
It is unwise to use the above values as ‘absolutes’ in terms of
determining whether a bond in polar or non-polar, as you will notice that
there are gaps. Better to take the view that if atoms in a bond are
different then there is a degree of polarity and that the only truly nonpolar bonds are those containing two identical non-metal atoms.