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 energyThe energy required to remove one

electron from an atom in it’s gaseous state.

X(g) X+(g) + e-

First ionisation energy – removal of the first electron

Second ionisation energy – removal of the second electron

Third ionisation energy – removal of the third electron

X+(g) X2+(g) + e-

X2+(g) X3+(g) + e-

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Ionization energy definitions

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Plotting the successive ionization energies of magnesium clearly shows the existence of different energy levels, and the number of electrons at each level.

Successive ionization energies increase as more electrons are removed.

Evidence for energy levels

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.

electron removed

ion

izat

ion

en

erg

y

2

3

4

5

6

12th

11th

10th

9th

8th

7th

6th

5th

4th

3rd

2nd1

st

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More evidence for energy levels

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.

element

firs

t io

niz

atio

n e

ner

gy

(kJ

mo

l-1)

500

600

700

800

900

Be Mg Ca Sr Ba

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

400

• 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 energyExplanation

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.

A logarithmic plot is needed for successive ionisation energies due to the scale. log 1 = 10log 5 = 100,000

Successive Ionisations

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 2 4 6 8 10 12 14 16 18 20

electron removed

log10 of ionisation

energy

Notice the “jump” in energy needed to remove the 2nd electron

Successive ionisation of potassium

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 2 4 6 8 10 12 14 16 18 20

electron removed

log10 of ionisation

energy

Successive ionisation energies for potassium

The different “jumps” are evidence for the arrangement of electrons in energy levels and sub-levels

level 1

level 2

level 3

level 4

Question

Identify the groups that these atoms belong to

Group 4 – the jump is to remove the 5th electron

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0 1 2 3 4 5 6 7

electron removed

kJ/mol

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 1 2 3 4 5 6 7

electron removed

kJ/mol

Group 2 – the jump is to remove the 3rd electron

Question

Identify the groups that these atoms belong to

Group 3 – the jump is to remove the 4th electron

Group 5 – the jump is to remove the 6th electron

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 1 2 3 4 5 6 7

electron removed

kJ/mol

0

2000

4000

6000

8000

10000

12000

14000

0 1 2 3 4 5 6 7

electron removed

kJ/mol

Question

Identify the group that this atom belongs to

Group 1 – the jump is to remove the 2nd electron

The number of the electron whose removal causes a jump is one more than the group number that the element belongs to.

0

2000

4000

6000

8000

10000

12000

0 1 2 3 4 5 6 7

electron removed

kJ/mol

Write a general rule for identifying groups from the pattern in ionisation energy

**

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

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Trends in first ionization energies

0200400600800

1000120014001600

Na Mg Al Si P S Cl Ar

1st i

on

isat

ion

en

erg

y (k

J/m

ol)

Periodicity of ionisation energy

What trend would you expect ionisation energy to have as you move across a period? B

A

C

What does region “A” represent?

2 x s electrons

What does region “B” represent?

3 x p electrons

Which three p electrons are these?

px1 py

1 and pz1

What else do you notice about the

graph?

The slopes of A, B and C are almost the same

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What is shielding?

ShieldingAs you move down a group, the distance of the outer electrons from the nucleus increases

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

+

e_

2) nuclear charge

1) distance from nucleus

3) shielding (repulsion) by electrons in inner shells between nucleus and outer electron

A graph of the first ionisation energy plotted against the atomic number.

What trends can you see in the ionisation energy. (Use your periodic table to help

you!)

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.

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 energyExplanation

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.

• 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.”

STARTERCreate an energy level diagram of Boron and

Beryllium.

MAIN ACTIVITY

1 1s

4f

22s2p

4s3

3s3p

3d

44p4d

INC

REA

SIN

G E

NER

GY

/

DIS

TA

NC

E F

RO

M

NU

CLEU

S

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.

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.

Phosphorus: 1s2 2s2 2p6 3s2 3p3 ... and ... Sulphur: 1s2 2s2 2p6 3s2 3p4

Slight decrease in energy from

P to S

Ionisation Energies of Magnesium (Example)

Successive ionisation energy graph for aluminium

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Energy levels

Complete the Test Yourself Questions

• Page 77

• Questions 13-14

• Check your answers on page 560