Transcript Successive and First Ionisation Energies

Title: Lesson 7 Successive and First
Ionisation Energies
Learning Objectives:
• Understand why different elements have different ionisation energies
• Know what happens to successive ionisation energies of an element
• Describe the relevance of ionisation energies to electron sub shells
Ionisation energy
The energy required to remove one electron
from an atom in it’s gaseous state.
First ionisation energy – removal of the first electron
X(g)
X+(g) + e-
Second ionisation energy – removal of the second electron
X+(g)
X2+(g) + e-
Third ionisation energy – removal of the third electron
X2+(g)
X3+(g) + e-
Ionization energy definitions
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ionization energy
Evidence for energy levels
Plotting the successive ionization energies of magnesium
clearly shows the existence of different energy levels, and
the number of electrons at each level.
6
Successive ionization
energies increase as more
5
electrons are removed.
Large jumps in the ionization
energy reveal where electrons
are being removed from the
next principal energy level,
such as between the 2nd and
3rd, and 10th and 11th ionization
energies for magnesium.
4
3
2
electron removed
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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.
Even though the nuclear
charge increases down the
group, the first ionization
energy decreases.
900
800
700
600
500
400
Be
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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.
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• Patterns of successive ionisation energies are evidence for electron configuration
in atoms.
• For example, Aluminium:
1st ionisation energy
2nd ionisation energy
and so on…
Q. What can we
see happening
in the graph?
Successive ionisation energy
Explanation
As more electrons are removed the pull of the
protons holds the remaining electrons more
tightly so more energy is needed to remove
them.
By looking to see where the ‘large jumps’ occur
in the successive ionisation energies, the
number of valence (free) electrons and period
number can be determined.
Successive Ionisations
A logarithmic plot is
needed for successive
ionisation energies
due to the scale.
log 1 = 10
log 5 = 100,000
6.0
5.5
5.0
4.5
log10 of
ionisation 4.0
energy
Successive
ionisation of
potassium
3.5
3.0
2.5
2.0
0
2
4
6
8
10
12
electron removed
Notice the “jump” in energy needed to
remove the 2nd electron
14
16
18
20
Successive ionisation energies for potassium
6.0
5.5
level 1
5.0
4.5
level 2
log10 of
ionisation 4.0
energy
3.5
level 3
3.0
2.5
level 4
2.0
0
2
4
6
8
10
12
14
16
18
20
electron removed
The different “jumps” are evidence for the arrangement of electrons in
energy levels and sub-levels
Question
Identify the groups that
these atoms belong to
50000
20000
45000
18000
40000
16000
35000
14000
30000
12000
kJ/mol 25000
kJ/mol 10000
20000
8000
15000
6000
10000
4000
5000
2000
0
0
0
1
2
3
4
electron removed
Group 4 – the jump is to
remove the 5th electron
5
6
7
0
1
2
3
4
electron removed
Group 2 – the jump is to
remove the 3rd electron
5
6
7
Question
Identify the groups that
these atoms belong to
20000
14000
18000
12000
16000
10000
14000
12000
8000
kJ/mol
kJ/mol 10000
6000
8000
6000
4000
4000
2000
2000
0
0
0
1
2
3
4
electron removed
Group 3 – the jump is to
remove the 4th electron
5
6
7
0
1
2
3
4
5
electron removed
Group 5 – the jump is to
remove the 6th electron
6
7
Question
Write a general rule for identifying
groups from the pattern in
ionisation energy
Identify the group that this
atom belongs to
The number of the electron
whose removal causes a
jump is one more than the
group number that the
element belongs to.
12000
10000
8000
kJ/mol 6000
4000
2000
0
0
1
2
3
4
5
6
7
electron removed
Group 1 – the jump is to
remove the 2nd electron
**
Key Points:
1. There is an increase in successive ionisation energies. The process
becomes more difficult as there is increasing attraction between the
higher charged positive ions and the oppositely charged electron.
2. There are jumps when electrons are removed from levels closer to the
nucleus. Electrons are removed from 3p first then 3s. On the 4th
ionisation energy, electrons are removed from the second energy level.
 nearer to the nucleus
 more exposed to the positive charge
 needs more energy to remove electron
Trends in first ionization energies
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What trend would you expect
ionisation energy to have as
you move across a period?
What does region
“A” represent?
2xs
electrons
What does region
“B” represent?
3xp
electrons
Which three p
electrons are these?
What else do you
notice about the
graph?
1st ionisation energy
(kJ/mol)
Periodicity of ionisation energy
1600
1400
1200
1000
800
600
400
200
0
C
B
A
Na
Mg
Al
px1 py1 and pz1
The slopes of A, B and C are
almost the same
Si
P
S
Cl
Ar
What is shielding?
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Shielding
_
As you move down a group, the
distance of the outer electrons
from the nucleus increases
e
1) distance from nucleus
+
2) nuclear charge
3) shielding (repulsion) by electrons
in inner shells between nucleus
and outer electron
The inner electrons also shield
the outer electrons from the full
effect of the positive nuclear
charge and repel each other.
They are less tightly bound to
the nucleus and so are more
easily removed
A graph of the first ionisation energy plotted
against the atomic number.
The first ionisation
energy is the energy
change when one
mole of gaseous
atoms forms one
mole of gaseous ions
with a single positive
charge.
What trends can you see in the ionisation
energy. (Use your periodic table to help you!)

IE generally increases from left to right, as nuclear charge
increases.

(Electrons removed from same main energy level, increase of
electrostatic attraction between the nucleus and outer electrons).

IE decreases down a group (new energy level, further away from
the nucleus, less energy required).

Regular discontinuities across period (evidence for sub shells)
Ionisation energy
Explanation
The highest value is for helium because the
two electrons are in the lowest level and are
held tightly by the two protons.
For lithium it is easier to remove an electron
suggesting the third electron is in a higher
energy level than the first two.
The graph generally increases until Neon, then
drops sharply for sodium.
The graph provides evidence that the levels
can contain different numbers of electrons
before they become full.
MAIN
STARTER
ACTIVITY
INCREASING ENERGY /
DISTANCE FROM
NUCLEUS
Create an energy level diagram of Boron and Beryllium.
4
3
2
1
4f
4d
4p
3d
4s
3p
3s
2p
2s
1s
• THE AFBAU (BUILDING UP) PRINCIPLE
• “Electrons enter the lowest available energy level.”
• HUND’S RULE OF MAXIMUM MULTIPLICITY
• “When in orbitals of equal energy, electrons will
try to remain unpaired.”
Use your
diagrams to
explain why
there is a
decrease in
ionisation
energy
between Be
and B.
Explanation of increase across period
Going across Period 3:
 more protons in each nucleus so the nuclear charge in each element increases
 the force of attraction between the nucleus and outer electron is increased
 negligible increase in shielding because each successive electron enters the same
energy level
 more energy is needed to remove the outer electron.
1s2
2s2
2p6
3s2
Phosphorus:
Sulphur: 1s2 2s2 2p6 3s2 3p4
3p3
... and ...
Slight
decrease in
energy from
P to S
 The 3p electrons in phosphorus are all unpaired.
 In sulphur, two of the 3p electrons are paired.
 There is some repulsion between paired electrons in the same sub-level.
 This reduces the force of their attraction to the nucleus.
 less energy is needed to remove one of these paired electrons than is needed to
remove an unpaired electron from phosphorus.
Successive ionisation energy graph for aluminium
Ionisation Energies of Magnesium (Example)
Energy levels
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Complete the Test Yourself Questions
• Page 77
• Questions 13-14
• Check your answers on page 560