IONISATION ENERGYOBJECTIVES:

To define the term ‘ionisation energy’

To describe and explain the trends in ionisation energy across period 3 and down group 2

FIRST THOUGHTS…

What do you think we might mean by ionisation energy?

KEY WORDS:

IONISATION ENERGY

NUCLEAR CHARGE

TREND

I CAN… I AM…

State what is meant by ionisation energy C

Describe the trend across period 3 and down group 2

B

Explain why this trend occurs A

Suggest how these trends provide evidence for the existence of electron levels and sub-levels

A*

IONISATION ENERGY

WHAT IS IONISATION ENERGY?

“The energy required to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions”

Energy required to remove electrons

Measured in kJmol-1

Given the abbreviation IE

Values tell us a lot about the electronic configuration of elements

IONISATION ENERGY

SUCCESSIVE IONISATION ENERGIES

We can measure the energies required to remove each electron in turn, from outer electrons to inner electrons

Values increase with each successive ionisation event

Why might thisbe?

IONISATION ENERGY

Successive Ionisation Energies of Na

SUCCESSIVE IONISATION ENERGIES

IONISATION ENERGY

To form a positive ion, energy is needed to overcome the attraction from the nucleus.

As each electron is removed from an atom, the remaining ion becomes more positively charged.

Removing the next electron away from an increasing positive charge is more difficult and the ionisation energy is even larger.

SUCCESSIVE IONISATION ENERGIES

Na (g) Na+ (g) + e- 1st IE = + 496 kJmol-1

Na+ (g) Na2+ (g) + e- 2nd IE = + 4563 kJmol-1

Na2+ (g) Na3+ (g) + e- 3rd IE = + 6913 kJmol-1

IONISATION ENERGY

Notice how successive ionisation energies are written

Start with the end product of previous ionisation event

MUST include the gaseous state!

SUCCESSIVE IONISATION ENERGIES

From this graph we can see evidence for electron shells

Na looks to have 1 electron that’s easy to remove furthest from nucleus

8 nearer to the nucleus bit harder to remove

2 very close to the nucleus nearest to +ve charge and hardest to remove

IONISATION ENERGY

IONISATION ENERGY

FACTORS AFFECTING IONISATION ENERGY

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.

Nuclear attraction of an electron depends on:

• Atomic radius

• Nuclear Charge

• Electron shielding or screening

IONISATION ENERGYATOMIC RADIUS:

Greater the atomic radius, the smaller the nuclear attraction experienced by the outer electrons NUCLEAR CHARGE:

The greater the nuclear charge, the greater the attractive force on the outer electrons

Hydrogen Helium Lithium

519 kJ mol-1

1310 kJ mol-1 2370 kJ mol-1

IONISATION ENERGY

ELECTRON SHIELDING

Inner shells of electrons repel the outer-shell electrons

Known as ‘Electron shielding’ or ‘screening’

More inner shells larger the screening and smaller the nuclear attraction of outer electrons

Hydrogen Helium Lithium

519 kJ mol-1

1310 kJ mol-1 2370 kJ mol-1

IONISATION ENERGY

TRENDS ACROSS PERIODS

Look at the graph

1) Describe the patterns you see

2) Can you suggest an explanation for them

IONISATION ENERGY

TRENDS ACROSS PERIODS

0

500

1000

1500

2000

2500 He

Ne

Ar

KrXe

There is a ‘general increase’ across a period before the value drops dramatically for the start of another period.

This is because nuclear charge is increasing

There is a ‘general increase’ across a period before the value drops dramatically for the start of another period.

This is because nuclear charge is increasing

ATOMIC NUMBER

1s

t IO

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l-1

EXPLANATION

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.

EXPLANATION

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.

HYDROGEN

1

1s

IONISATION ENERGY

1s

1s

ATOMIC NUMBER

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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.

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.

HELIUM

2

IONISATION ENERGY

1s 2s

1s

1s

ATOMIC NUMBER

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EXPLANATION

There is a substantial drop in the value for Lithium.

Despite the increased nuclear charge, there is electron shielding from the 1s orbital.

The 2s electron is also further away from the nucleus. It is held less strongly and needs less energy for removal.

EXPLANATION

There is a substantial drop in the value for Lithium.

Despite the increased nuclear charge, there is electron shielding from the 1s orbital.

The 2s electron is also further away from the nucleus. It is held less strongly and needs less energy for removal.

LITHIUM

3

IONISATION ENERGY

1s 2s

1s 2s

1s

1s

ATOMIC NUMBER

1s

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EXPLANATION

The value for Beryllium is higher than for Lithium due to the increased nuclear charge. There is no extra shielding.

EXPLANATION

The value for Beryllium is higher than for Lithium due to the increased nuclear charge. There is no extra shielding.

BERYLLIUM

4

IONISATION ENERGY

1s 2s 2p

1s 2s

1s 2s

1s

1s

ATOMIC NUMBER

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EXPLANATION

There is a DROPDROP 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.

EXPLANATION

There is a DROPDROP 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.

BORON

5

IONISATION ENERGY

1s 2s 2p

1s 2s 2p

1s 2s

1s 2s

1s

1s

ATOMIC NUMBER

1s

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l-1 EXPLANATION

The value increases again for Carbon due to the increased nuclear charge.

EXPLANATION

The value increases again for Carbon due to the increased nuclear charge.

CARBON

6

IONISATION ENERGY

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s 2s

1s

1s

ATOMIC NUMBER

1s

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EXPLANATION

The value increases again for Nitrogen due to the increased nuclear charge.

As before, the extra electron goes into the vacant 2p orbital. There are now three unpaired electrons.

EXPLANATION

The value increases again for Nitrogen due to the increased nuclear charge.

As before, the extra electron goes into the vacant 2p orbital. There are now three unpaired electrons.

NITROGEN

7

IONISATION ENERGY

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s 2s

1s

1s

ATOMIC NUMBER

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EXPLANATION

There is a DROPDROP 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 between the two paired-up electrons means that less energy is required to remove one of them.

EXPLANATION

There is a DROPDROP 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 between the two paired-up electrons means that less energy is required to remove one of them.

OXYGEN

8

IONISATION ENERGY

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s 2s

1s

1s

ATOMIC NUMBER

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EXPLANATION

The value increases again for Fluorine due to the increased nuclear charge.

The 2p orbitals are almost full.

EXPLANATION

The value increases again for Fluorine due to the increased nuclear charge.

The 2p orbitals are almost full.

FLUORINE

9

IONISATION ENERGY

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s 2s

1s

1s

ATOMIC NUMBER

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EXPLANATION

The value increases again for Neon due to the increased nuclear charge.

The 2p orbitals are now full so the next electron in will have to go into the higher energy 3s orbital.

EXPLANATION

The value increases again for Neon due to the increased nuclear charge.

The 2p orbitals are now full so the next electron in will have to go into the higher energy 3s orbital.

NEON

10

IONISATION ENERGY

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s 2s

1s

1s

ATOMIC NUMBER

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1s 2s 2p 3s

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.

This means there is an extra shielding effect of filled inner 1s, 2s and 2p energy levels.

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.

This means there is an extra shielding effect of filled inner 1s, 2s and 2p energy levels.

SODIUM

11

IONISATION ENERGY

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s 2s

1s

1s

ATOMIC NUMBER

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1s 2s 2p 3s

1s 2s 2p 3s

EXPLANATION

The value for Magnesium is higher than for Sodium due to the increased nuclear charge. There is no extra shielding.

The trend is similar to that at the start of the 2nd period.

EXPLANATION

The value for Magnesium is higher than for Sodium due to the increased nuclear charge. There is no extra shielding.

The trend is similar to that at the start of the 2nd period.

MAGNESIUM

12

IONISATION ENERGY

IONISATION ENERGY

TREND IN PERIOD 3

Draw a graph for the first ionisation energy of elements in period 3

For each point draw the box and arrow electron configuration

For each point explain why the first ionisation energy either increases or decreases

Na Mg Al Si P S Cl Ar

496 738 578 789 1012 1000 1251 1521

IONISATION ENERGY

TREND IN PERIOD 3

Drop from Mg Al:

Mg: 1s2 2s2 2p6 3s2

Al: 1s2 2s2 2p6 3s2 3p1

Drop from P S:

P: 1s2 2s2 2p6 3s2 3p3

S: 1s2 2s2 2p6 3s2 3p4

The outer electron in Al has moved into the 3p orbital. It takes less energy to remove an electron from here than the 3s

In P each of the 3p orbitals has 1 electron. In S one must have 2. The repulsion between these 2 paired electrons makes it easier to remove one

IONISATION ENERGYTREND DOWN GROUPS

General trend is a decrease as we go down a group

The outer electron is in a level that gets further from the nucleus

Nuclear charge increases however this is more than cancelled out by the electron shielding from lower energy shells

IONISATION ENERGY

A: The 2nd I.E. of magnesium

The 1st I.E. of sodium involves the following change

Na(g) Na+(g)

1s2 2s2 2p6 3s1 1s2 2s2 2p6

The 2nd I.E. of magnesium involves the same change in electron configuration…

Mg+(g) Mg2+(g)

1s2 2s2 2p6 3s1 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.

EXTENSION QUES: Which has the higher value, the 1st I.E. of sodium or the 2nd I.E. of magnesium?

IONISATION ENERGYHOMEWORK:

Revise for an end of chapter test next lesson

How low can you go?? Write what you can do and what grade this is show some proof you can do this!

I CAN… I AM…

State what is meant by ionisation energy C

Describe the trend across period 3 and down group 2

B

Explain why this trend occurs A

Suggest how these trends provide evidence for the existence of electron levels and sub-levels

A*

IONISATION ENERGY