IONISATION
ENERGY
A guide for A level students
2008
KNOCKHARDY PUBLISHING
SPECIFICATIONS
KNOCKHARDY PUBLISHING
IONISATION ENERGY
INTRODUCTION
This Powerpoint show is one of several produced to help students understand
selected topics at AS and A2 level Chemistry. It is based on the requirements of
the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may
be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available
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www.knockhardy.org.uk/sci.htm
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IONISATION ENERGY
CONTENTS
• What is Ionisation Energy?
• Definition of 1st Ionisation Energy
• What affects Ionisation Energy?
• General variation across periods
• Variation down groups
• Variation in the first twelve elements
• Successive Ionisation Energies
• Questions
• Check list
IONISATION ENERGY
Before you start it would be helpful to…
• Recall the electronic configurations of the first 36 elements
• Recall the properties of the three main sub-atomic particles
WHAT IS IONISATION ENERGY?
Ionisation Energy is a measure of the amount of energy
needed to remove electrons from atoms.
As electrons are negatively charged and protons in the
nucleus are positively charged, there will be an attraction
between them. The greater the pull of the nucleus, the
harder it will be to pull an electron away from an atom.
-
Attraction between
the nucleus and
an electron
WHAT IS IONISATION ENERGY?
-
Ionisation Energy is a measure of the amount of energy
needed to remove electrons from atoms.
As electrons are negatively charged and protons in the
nucleus are positively charged, there will be an attraction
between them. The greater the pull of the nucleus, the
harder it will be to pull an electron away from an atom.
Attraction between
the nucleus and
an electron
FIRST IONISATION ENERGY - Definition
The energy required to remove ONE MOLE of electrons (to infinity) from ONE
MOLE of gaseous atoms to form ONE MOLE of gaseous positive ions.
e.g.
Na(g)
Na+(g) + e-
Al(g)
Al+(g) + e-
Make sure you
write in the (g)
WHAT AFFECTS IONISATION ENERGY?
The value of the 1st Ionisation Energy depends on the electronic structure
Hydrogen
1310 kJ mol-1
Helium
Lithium
2370 kJ mol-1
519 kJ mol-1
The value for helium is higher than that for hydrogen because there are now two
protons in the nucleus. The nuclear charge is greater so the pull on the outer
electrons is larger. More energy will be needed to pull an electron out of the atom.
WHAT AFFECTS IONISATION ENERGY?
The value of the 1st Ionisation Energy depends on the electronic structure
Hydrogen
1310 kJ mol-1
Helium
Lithium
2370 kJ mol-1
519 kJ mol-1
The value for helium is higher than that for hydrogen because there are now two
protons in the nucleus. The nuclear charge is greater so the pull on the outer
electrons is larger. More energy will be needed to pull an electron out of the atom.
Lithium atoms have 3 protons so you would expect the pull on electrons to be
greater. However, the 1st Ionisation Energy of lithium is lower than that of helium
because…
• Filled inner shells exert a SHIELDING EFFECT; lowers the effective nuclear pull
• FURTHER AWAY from the nucleus = lower nuclear attraction for an electron
Variation in 1st Ionisation Energy - PERIODS
1st Ionisation Energy shows a ‘general increase’ across a given period
Variation in 1st Ionisation Energy - PERIODS
2500
2000
1500
1st Ionisation Energy values show a periodic
trend. There is a ‘general increase’ across a
period before the value drops dramatically for
the start of another period.
He
The values get smaller down groups as the
electron removed comes from an orbital further
from the nucleus - there is more shielding.
Ne
Ar
Kr
Xe
1000
500
0
Variation in 1st Ionisation Energy - GROUPS
GROUP I
Value decreases down the Group
despite an increased nuclear charge the outer s electron is easier to remove
this is due to increased shielding and greater distance from the nucleus
the outer electron is held less strongly and easier to remove
Li
Na
K
519 kJ mol-1
494 kJ mol-1
418 kJ mol-1
Variation in 1st Ionisation Energy - GROUPS
GROUP I
Value decreases down the Group
despite an increased nuclear charge the outer s electron is easier to remove
this is due to increased shielding and greater distance from the nucleus
the outer electron is held less strongly and easier to remove
Li
Na
K
519 kJ mol-1
494 kJ mol-1
GROUP II Similar trend to Group I
Group II values are greater than their Group I neighbours
increased nuclear charge = stronger pull on electron
more energy required to remove an electron
418 kJ mol-1
Variation in 1st Ionisation Energy
HYDROGEN
EXPLANATION
1st IONISATION ENERGY / kJmol-1
Despite having a nuclear
charge of only 1+,
Hydrogen has a relatively
high 1st Ionisation Energy
as its electron is closest to
the nucleus and has no
shielding.
1s
ATOMIC NUMBER
1
Variation in 1st Ionisation Energy
1st IONISATION ENERGY / kJmol-1
1s
HELIUM
EXPLANATION
Helium has a much higher
value because of the extra
proton in the nucleus. The
additional charge provides
a stronger attraction for the
electrons making them
harder to remove.
1s
ATOMIC NUMBER
2
Variation in 1st Ionisation Energy
1st IONISATION ENERGY / kJmol-1
1s
LITHIUM
EXPLANATION
There is a substantial drop
in the value for Lithium.
This is because the extra
electron has gone into an
orbital in the next energy
level. Despite the increased
nuclear charge, the
effective nuclear charge is
less because of the
shielding effect of filled
inner 1s energy level. The
2s electron is also further
away from the nucleus. It is
held less strongly and
needs less energy for
removal.
1s
1s 2s
ATOMIC NUMBER
3
Variation in 1st Ionisation Energy
1st IONISATION ENERGY / kJmol-1
1s
BERYLLIUM
EXPLANATION
The value for Beryllium is
higher than for Lithium due to
the increased nuclear
charge. There is no extra
shielding.
1s
1s 2s
1s 2s
ATOMIC NUMBER
4
Variation in 1st Ionisation Energy
1st IONISATION ENERGY / kJmol-1
1s
BORON
EXPLANATION
There is a DROP in the value
for Boron. This is because
the extra electron has gone
into one of the 2p orbitals.
The increased shielding
makes the electron easier to
remove
It was evidence such as this
that confirmed the existence
of sub-shells. If there hadn’t
been any sub-shell, the value
would have been higher than
that of Beryllium.
1s
1s 2s
1s 2s
2p
1s 2s
ATOMIC NUMBER
5
Variation in 1st Ionisation Energy
1st IONISATION ENERGY / kJmol-1
1s
CARBON
EXPLANATION
The value increases again
for Carbon due to the
increased nuclear charge.
1s
1s 2s
2p
1s 2s
1s 2s
2p
The extra electron does not
pair up with the previous one
in the same orbital but
occupies another of the 2p
orbitals. This gives a lower
energy configuration
because there is less
repulsion between the
negatively charged particles.
This is known as Hund’s
Rule.
1s 2s
ATOMIC NUMBER
6
Variation in 1st Ionisation Energy
1st IONISATION ENERGY / kJmol-1
1s
NITROGEN
EXPLANATION
The value increases again
for Nitrogen due to the
increased nuclear charge.
1s 2s
As before, the extra electron
goes into the vacant 2p
orbital. There are now three
unpaired electrons.
2p
1s
1s 2s
2p
1s 2s
1s 2s
2p
1s 2s
ATOMIC NUMBER
7
Variation in 1st Ionisation Energy
1st IONISATION ENERGY / kJmol-1
1s
OXYGEN
1s 2s
EXPLANATION
2p
1s
1s 2s
1s 2s
2p
2p
There is a DROP in the value
for Oxygen. The extra
electron has paired up with
one of the electrons already
in one of the 2p orbitals. The
repulsive force beteen the
two paired-up electrons
means that less energy is
required to remove one of
them.
1s 2s
1s 2s
2p
1s 2s
ATOMIC NUMBER
8
Variation in 1st Ionisation Energy
1st IONISATION ENERGY / kJmol-1
1s
FLUORINE
EXPLANATION
The value increases again
for Fluorine due to the
increased nuclear charge.
1s 2s
1s 2s
The 2p orbitals are almost
full.
2p
2p
1s
1s 2s
1s 2s
2p
2p
1s 2s
1s 2s
2p
1s 2s
ATOMIC NUMBER
9
Variation in 1st Ionisation Energy
1s
NEON
1st IONISATION ENERGY / kJmol-1
1s 2s
1s 2s
1s 2s
EXPLANATION
The value increases again
for Neon due to the
increased nuclear charge.
2p
The 2p orbitals are now full
so the next electron in will
have to go into the higher
energy 3s orbital.
2p
2p
1s
1s 2s
1s 2s
2p
2p
1s 2s
1s 2s
2p
1s 2s
ATOMIC NUMBER
10
Variation in 1st Ionisation Energy
1s
SODIUM
1st IONISATION ENERGY / kJmol-1
1s 2s
1s 2s
1s 2s
EXPLANATION
There is a substantial drop in
the value for Sodium. This is
because the extra electron
has gone into an orbital in
the next energy level.
Despite the increased
nuclear charge, the effective
nuclear charge is less
because of the shielding
effect of filled inner 1s, 2s
and 2p energy levels.
2p
2p
2p
1s
1s 2s
1s 2s
2p
2p
1s 2s
1s 2s
2p
1s 2s
1s 2s
2p
3s
ATOMIC NUMBER
11
Variation in 1st Ionisation Energy
1s
MAGNESIUM
1st IONISATION ENERGY / kJmol-1
1s 2s
1s 2s
1s 2s
EXPLANATION
The value for Magnesium is
higher than for Sodium due
to the increased nuclear
charge. There is no extra
shielding.
2p
2p
The trend is similar to that at
the start of the 2nd period.
2p
1s
1s 2s
1s 2s
2p
2p
1s 2s
1s 2s
1s 2s
2p
3s
2p
1s 2s
1s 2s
2p
3s
ATOMIC NUMBER
12
Successive Ionisation Energies
Atoms with more than one electron can have them successively removed.
2nd I.E. The energy required to remove one mole of electrons (to infinity) from one mole
of gaseous unipositive ions to form one mole of gaseous dipositive ions.
e.g.
Trends
Na+(g)
Na2+(g) + e-
Al+(g)
Al2+(g) + e-
Make sure you
write in the (g)
Successive ionisation energies are always greater than the previous one
Reason :- the electron is being pulled away from a more positive species
Large increases occur when there is a change of shell
Reason :- there is a big decrease in shielding
Large increases can be used to predict the group of an unknown element
See next slide for an example
Successive Ionisation Energies of Calcium
I.E. kJmol-1 Electronic configuration
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
A
590
1145
4912
6474
8145
10496
12320
14207
18192
20385
57048
63333
70052
78792
86367
94000
104900
111600
494790
527759
1s2 2s2 2p6 3s2 3p6 4s2
1s2 2s2 2p6 3s2 3p6 4s1
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p6 3s2 3p4
1s2 2s2 2p6 3s2 3p3
1s2 2s2 2p6 3s2 3p2
1s2 2s2 2p6 3s2 3p1
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s1
1s2 2s2 2p6
1s2 2s2 2p5
1s2 2s2 2p4
1s2 2s2 2p3
1s2 2s2 2p2
1s2 2s2 2p1
1s2 2s2
1s2 2s1
1s2
1s1
A
The 3rd I.E. is significantly higher than the 2nd I.E. because the third electron
is coming out of a 3p orbital, nearer the nucleus and subjected to less
shielding. More energy is needed to overcome the attraction of the nucleus.
Successive Ionisation Energies of Calcium
I.E. kJmol-1 Electronic configuration
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
B
590
1145
4912
6474
8145
10496
12320
14207
18192
20385
57048
63333
70052
78792
86367
94000
104900
111600
494790
527759
1s2 2s2 2p6 3s2 3p6 4s2
1s2 2s2 2p6 3s2 3p6 4s1
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p6 3s2 3p4
1s2 2s2 2p6 3s2 3p3
1s2 2s2 2p6 3s2 3p2
1s2 2s2 2p6 3s2 3p1
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s1
1s2 2s2 2p6
1s2 2s2 2p5
1s2 2s2 2p4
1s2 2s2 2p3
1s2 2s2 2p2
1s2 2s2 2p1
1s2 2s2
1s2 2s1
1s2
1s1
B
The 11th I.E. is significantly higher than the 10th I.E. because the eleventh
electron is coming out of the second main energy level, not the third. It is
much nearer the nucleus and is subjected to less shielding.
Successive Ionisation Energies of Calcium
I.E. kJmol-1 Electronic configuration
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
C
590
1145
4912
6474
8145
10496
12320
14207
18192
20385
57048
63333
70052
78792
86367
94000
104900
111600
494790
527759
1s2 2s2 2p6 3s2 3p6 4s2
1s2 2s2 2p6 3s2 3p6 4s1
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p6 3s2 3p4
1s2 2s2 2p6 3s2 3p3
1s2 2s2 2p6 3s2 3p2
1s2 2s2 2p6 3s2 3p1
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s1
1s2 2s2 2p6
1s2 2s2 2p5
1s2 2s2 2p4
1s2 2s2 2p3
1s2 2s2 2p2
1s2 2s2 2p1
1s2 2s2
1s2 2s1
1s2
1s1
C
The 19th I.E. is significantly higher than the 18th I.E. because the electron being
removed is from the first main energy level. It is much nearer the nucleus and is
subjected to no shielding - its value is extremely large.
Successive Ionisation Energies of Calcium
I.E. kJmol-1 Electronic configuration
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
590
1145
4912
6474
8145
10496
12320
14207
18192
20385
57048
63333
70052
78792
86367
94000
104900
111600
494790
527759
SUMMARY
1s2 2s2 2p6 3s2 3p6 4s2
1s2 2s2 2p6 3s2 3p6 4s1
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p6 3s2 3p4
1s2 2s2 2p6 3s2 3p3
1s2 2s2 2p6 3s2 3p2
1s2 2s2 2p6 3s2 3p1
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s1
1s2 2s2 2p6
1s2 2s2 2p5
1s2 2s2 2p4
1s2 2s2 2p3
1s2 2s2 2p2
1s2 2s2 2p1
1s2 2s2
1s2 2s1
1s2
1s1
C
B
A
Wherever there has been a large increase in Ionisation Energy
there has been a change in energy level from which the electron
has been removed.
QUESTION TIME
Q.1
Which has the higher value, the 3rd I.E. of aluminium or the 3rd I.E. of magnesium?
CLICK HERE FOR
THE ANSWER
QUESTION TIME
Q.2 Which has the higher value, the 1st I.E. of sodium or the 2nd I.E. of magnesium?
CLICK HERE FOR
THE ANSWER
QUESTION TIME
Q.1
Which has the higher value, the 3rd I.E. of aluminium or the 3rd I.E. of magnesium?
Ans The 3rd I.E. of magnesium
EXPLANATION
The 3rd I.E. of aluminium involves the following change...
Al2+(g)
1s2 2s2 2p6 3s1
Al3+(g)
1s2 2s2 2p6
The 3rd I.E. of magnesium involves the following change…
Mg2+(g)
Mg3+(g)
1s2 2s2 2p6
1s2 2s2 2p5
Despite magnesium having 12 protons in its nucleus and aluminium
having 13, more energy is required to remove the third electron from
magnesium. This is because the electron being removed is coming
from an orbital closer to the nucleus. There is less shielding and
therefore a greater effective nuclear charge. The electron is thus held
more strongly.
Q.2
QUESTION TIME
Q.2 Which has the higher value, the 1st I.E. of sodium or the 2nd I.E. of magnesium?
Ans
The 2nd I.E. of magnesium
EXPLANATION
The 1st I.E. of sodium involves the following change
Na(g)
1s2 2s2 2p6 3s1
Na+(g)
1s2 2s2 2p6
The 2nd I.E. of magnesium involves the same change in electron configuration…
Mg+(g)
1s2 2s2 2p6 3s1
Mg2+(g)
1s2 2s2 2p6
However, magnesium has 12 protons in its nucleus, whereas sodium only has
11. The greater nuclear charge means that the electron being removed is held
more strongly and more energy must be put in to remove it.
REVISION CHECK
What should you be able to do?
Recall the definition of 1st Ionisation Energy
Understand why energy is needed to remove an electron from an atom / ion
Write equations representing 1st Ionisation Energy
Know the trend in 1st Ionisation Energy across periods
Explain, in terms of electron configuration, the trend across a given period
Know the trend in 1st Ionisation Energy down groups
Explain the trend down a given group
Know, and explain, why successive Ionisation Energies get bigger
Explain why there is sometimes a large jump between successive values
Predict which group an element is in from its Ionisation Energies
CAN YOU DO ALL OF THESE?
YES
NO
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relevant topic(s) again
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IONISATION
ENERGY
THE END
© 2008 JONATHAN HOPTON & KNOCKHARDY PUBLISHING