Learning Outcomes
• Atomic radii (covalent radii only). Explanations for
general trends in values: (i) down a group (ii) across a
period (covalent radii of main group elements only).
• First ionisation energies.
• Explanations for general trends in values: (i) down a
group (ii) across a period (main group elements) and
for exceptions to the general trends across a period.
• Second and successive ionisation energies.
• Evidence for energy levels provided by successive
ionisation energy values.
Atomic radii
Atomic radii trends
• In general, the atomic radii values
decrease across the period and increase
down the group
Atomic radius
• Half the distance
• Between the nuclei of 2 atoms of the same
element
• Joined by a single covalent bond
trends
Reasons for increase down a
group
• The additional electrons are going into a
new shell which is further from the
nucleus
• Screening effect of inner electrons
Screening effect
Reasons for decrease across a
period
• Increasing nuclear charge.
• No increase in the screening effect
IONISATION ENERGY
• Some elements lose electrons very easily,
e.g. sodium and potassium
• Silver and gold have very little tendency to
lose their electrons and hence are very
unreactive
definition
The first ionisation energy is the energy required to
remove the most loosely held electron from one
mole of gaseous atoms to produce 1 mole of
gaseous ions each with a charge of 1+.
Na loses an electron
equation
Na  Na
+
+
e
I. E. in groups I and II
Chlorine increases in size
Ionisation Energy
decreases going down a
group
Reasons for decrease down
• Increasing atomic radius.
• Screening effect of inner electrons
Ionisation Energy
increases across a
period
Reasons increase across
• Increasing nuclear charge.
• Decreasing atomic radius
IE and periodic table.
Exceptions to the general trends
• Beryllium and nitrogen have higher values than
expected
• Reasons:
• Be  1S2 2S2 (Full orbitals give greater stability)
• N  1S2 2S2 2Sx1 2Py1 2Pz1 (3 half filled
orbitals give greater stability)
EVIDENCE FOR EXISTENCE
OF ENERCY LEVELS
• Suppose we measure the first, second,
third, etc. up to the nineteenth ionisation
energy of potassium
• K = 1S2 2S2 2P6 3S2 3P6 4S1
• K = 2,8,8,1
• First ionisation energy has the lowest
ionisation energy value. Electron in the 4s
sublevel is easiest to remove
Potassium ionisation
• 1st ionisation energy: K(g) → K+(g) + e–
• 2nd ionisation energy: K+(g) → K2+(g) + e–
n=1
n=2
n
Potassium IE’s
ionisation energy
(kJ mol–1)
Sucessive Ionisation of Potassium
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
419
3051
4412
5877
7975
9649
11343
14942
16964
48577
54433
60701
68896
75950
83152
93403
99771
444911
476075
500000
450000
400000
350000
300000
250000
200000
150000
100000
50000
0
1 2 3
4 5 6
7 8 9
10 11 12
13 14 15
16 17 18
19
Series1
Potassium IE’s
Sucessive Ionisation of
Potassium
500000
450000
400000
350000
300000
A
250000
200000
150000
B
C
100000
50000
0
1 2 3 4
5 6 7
8 9 10
11 12 13
Series1
14 15 16
17 18 19
• A, one electron has
been removed from
potassium
• The second electron
is much more difficult
to remove since this
electron is being
removed from the K+
ion
Potassium IE’s
Sucessive Ionisation of
Potassium
500000
450000
400000
350000
300000
250000
200000
150000
B
C
100000
50000
0
1 2 3 4
5 6 7
8 9 10
11 12 13
Series1
14 15 16
17 18 19
• K+ This ion has
eight electrons in
the outer shell (
B,C)
• The full outer
sublevel (3p6) has
extra stability and
therefore will
require more
energy to remove
electrons from it.
(B,C)
Potassium
IE’s
Sucessive Ionisation of
Potassium
• B to C we are
removing eight more
electrons
• (point C on the
graph), there is
another sudden jump
D is being removed
from a shell which is
closer to the nucleus
500000
450000
400000
350000
300000
D
250000
200000
B
C
150000
100000
50000
0
1 2 3
4 5 6
7 8 9
10 11 12
13
Series1
14 15 16
17 18 19