Radioactivity

and

Ionisation Energies

(AS Chemistry)

Mrs Nerys Stephens�Chemistry Teacher and �Head of Years 9, 10 & 11� Ysgol Bro Pedr

Spring 2024 AS Sessions:

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Session 1 – Radioactivity and Ionisation Energies (NS)

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Session 2 – Gravimetric Practical (PB)

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Session 3 – Instrumental Analysis (NS)

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Session 4 – Making up a Standard Solution and Titrations (PB)

RADIOACTIVE EMISSION

Radioactive emission occurs when an unstable nucleus becomes more stable by giving out energy and, in most cases, small particles such as electrons.

Radioactive materials give out radiation all the time. Radioactivity cannot be stopped, slowed down or sped up. It is a random process that does not depend on factors such as temperature. It is only dependent on the nature of the radioactive material.

There are four types of radioactive emission that you need to be aware of.

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RADIOACTIVE EMISSION

Alpha (α) particles

Alpha particles are made of two protons and two neutrons that are ejected from the nucleus at high speed. Alpha particles can also be described as helium nuclei ⁴₂He²⁺.

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Beta (β) particles

Beta particles are electrons (⁰_-1e) that are ejected from the nucleus at high speed.

RADIOACTIVE EMISSION

Positrons (β+)

Positrons are anti-electrons (⁰₁e) that are ejected from the nucleus at high speed. An anti-electron has the same mass as an electron but a different electrical charge: positive.

When a positron comes into contact with an electron, they obliterate each other and create gamma radiation

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RADIOACTIVE EMISSION

Gamma (γ) radiation

Gamma radiation is high energy electromagnetic radiation. Gamma radiation is emitted when a heavy nucleus rearranges itself into a more stable configuration.

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Gamma radiation is also emitted after a process called electron capture. Electron capture occurs when a proton-rich nucleus absorbs an inner electron. This electron combines with a proton to form a neutron.

Behaviour of Radioactive Particles

Deflection of radioactive particles in an electric field

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The charge on each particle is shown when they pass through an electric (or magnetic) field:

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Alpha particles are positive, heavy, and slow moving. Therefore, they are weakly attracted to the negative plate of an electric field.

Behaviour of Radioactive Particles

Deflection of radioactive particles in an electric field

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The charge on each particle is shown when they pass through an electric (or magnetic) field:

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Beta particles are light and fast moving. Therefore, they show considerable deviation towards the positive plate of the electric field.

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Gamma radiation is electromagnetic radiation of short wavelength. Therefore, it is unaffected by an electric field.

Penetrating Power of Radioactive Particles

Radioactive particles can be absorbed by substances in their path:

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Alpha particles are large and relatively slow moving. Therefore, they are the least penetrating particles. Alpha particles can be stopped by a single sheet of paper or the outer layer of human skin.

Penetrating Power of Radioactive Particles

Radioactive particles can be absorbed by substances in their path:

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Beta particles (and positrons) are small and fast moving. Therefore, they are more penetrating than alpha particles. Beta particles can be stopped by a thin layer of metal such as aluminium foil.

Penetrating Power of Radioactive Particles

Radioactive particles can be absorbed by substances in their path:

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Gamma radiation is high energy, short wavelength electromagnetic radiation and is the most penetrating of the three. It can only be stopped by a thick plate (2 cm or more) of a highly dense metal (such as lead).

Ionising Power of Radioactive Particles

Radioactive particles ionise substances (i.e., remove electrons) as they collide with them.

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Alpha particles are large and highly charged slow-moving particles. Therefore, they are the most ionising particles. Alpha particles remove electrons from atoms as they have a high positive charge that strongly attracts the electrons.

Ionising Power of Radioactive Particles

Radioactive particles ionise substances (i.e., remove electrons) as they collide with them.

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Beta particles are small, meaning they are less ionising than alpha particles. Beta particles collide with electrons in atoms, knocking them out and therefore ionising the atom.

Ionising Power of Radioactive Particles

Radioactive particles ionise substances (i.e., remove electrons) as they collide with them.

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Gamma radiation is high energy, short wavelength electromagnetic radiation which is the least ionising. Gamma radiation only ionises atoms if the electrons absorb the energy of the gamma radiation. However, this is unlikely given that gamma radiation has such a short wavelength

Nuclear Equations

It is very important that you understand the effect that radioactive emission has on the mass and atomic number of the atom that emits it.

​

When constructing nuclear equations, the atomic and mass numbers must balance on both sides of the equation

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Gamma radiation has no effect on the mass and atomic number of the atom that emits it. This is because it is a form of energy and is not an atomic particle

Nuclear Equations

It is very important that you understand the effect that radioactive emission has on the mass and atomic number of the atom that emits it.

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When an alpha particle is emitted, the new atom formed has a mass number 4 lower and an atomic number 2 lower than the original atom:

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Nuclear Equations

It is very important that you understand the effect that radioactive emission has on the mass and atomic number of the atom that emits it.

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When a beta particle is emitted, the new atom formed has an atomic number 1 higher than the original atom because a neutron is changed into a proton. The mass number is unchanged:

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Nuclear Equations

It is very important that you understand the effect that radioactive emission has on the mass and atomic number of the atom that emits it.

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When a positron is emitted, the new atom formed has an atomic number 1 lower than the original atom because a proton is changed into a neutron. The mass number is unchanged

Nuclear Equations

It is very important that you understand the effect that radioactive emission has on the mass and atomic number of the atom that emits it.

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In electron capture, the atomic number goes down by 1 due to the changing of the proton into a neutron.

Question:

Which series of decays would change protactinium-238 into uranium-234?

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It can be seen that the atomic number increases by 1 (protactinium has an atomic number of 91, whilst uranium has an atomic number of 92).

Question:

This means that for protactinium-238 to change into uranium-234, the atom must undergo one alpha decay and three beta decays.

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Initial alpha decay will produce actinium-234.

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Question:

The first beta decay produces thorium-234:

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followed by protactinium-234:

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finally producing uranium-234 after the third and final beta decay.

Half-life

The radioactivity of a substance decreases over time as it becomes more stable.

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Half-life is the time it takes for the count-rate (or mass) of a radioactive substance to fall to half of its original value.

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This time remains the same throughout the decay of the substance, meaning it is easy to predict the radioactivity of a substance in the future

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Half-life

There are three types of question that involve half-life in AS Chemistry. In these questions, you may be asked to work out:

• the time it takes for the radioactivity of a sample to fall to a specified fraction of its original value

• the mass of a radioisotope remaining after a specified time when you know the starting mass

• the half-life of a radioisotope if you know how long it takes for a sample to fall to a specified fraction of its original value.

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Worked examples:

1. The half-life of Uranium-238 is 2400 years. How long does it take a sample of 0.8g to fall to 0.1g?

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To calculate the time, we must work out how many half-lives it takes to reduce the mass from 0.8g to 0.1g.

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0.8g → 0.4g → 0.2g → 0.1g

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There are three arrows here, showing three half-lives. So, the time taken is three half-lives:

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Time = 3 x 2400 = 7200 years

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Worked examples:

2. In 84 seconds, the activity of thorium falls to 1/16th of its original value. What is its half-life?

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To calculate the half-life, we must work out how many half-lives it takes to reduce the activity to 1/16th of its original value.

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1 → 1/2 → 1/4 → 1/8 → 1/16

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There are four arrows here, showing four half-lives. So, the half-life is the total time divided by four:

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Half-life = 84 ÷ 4 = 21 seconds

Worked examples:

3. An isotope of iodine, ¹³¹I, has a half-life of 8 days. If you started with 8.0g of ¹³¹I, calculate the mass remaining after 40 days.

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To calculate the mass, we must work out how many half-lives there are in the time specified.

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Half-lives = 40 ÷ 8 = 5 half lives

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We then need to reduce the initial mass by half five times.

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8.0g → 4.0g → 2.0g → 1.0g → 0.5g → 0.25g

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Therefore, the mass left after 40 days is 0.25g

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Knowledge Organisers (wjec.co.uk)

IONISATION ENERGY

What is Ionisation Energy?

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• Definition of 1^st Ionisation Energy
• What affects Ionisation Energy?
• General variation across periods
• Variation down groups
• Variation in the first twelve elements
• Successive Ionisation Energies

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.

WHAT IS IONISATION ENERGY?

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.

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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

Helium

Lithium

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.

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519 kJ mol^-1

1310 kJ mol^-1

2370 kJ mol^-1

WHAT AFFECTS IONISATION ENERGY?

Hydrogen

Helium

Lithium

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

519 kJ mol^-1

1310 kJ mol^-1

2370 kJ mol^-1

1st Ionisation Energy shows a ‘general increase’ across a given period

Variation in 1st Ionisation Energy - PERIODS

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.

The values get smaller down groups as the electron removed comes from an orbital further from the nucleus - there is more shielding.

Variation in 1st Ionisation Energy - PERIODS

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

519 kJ mol^-1

Li

494 kJ mol^-1

418 kJ mol^-1

Na

K

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

519 kJ mol^-1

Li

494 kJ mol^-1

418 kJ mol^-1

Na

K

Variation in 1st Ionisation Energy - GROUPS

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

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

Variation in 1st Ionisation Energy

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

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

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.

Variation in 1st Ionisation Energy

HELIUM

2

1s 2s

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

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.

Variation in 1st Ionisation Energy

LITHIUM

3

1s 2s

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

EXPLANATION

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

Variation in 1st Ionisation Energy

BERYLLIUM

4

1s 2s 2p

1s 2s

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

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.

Variation in 1st Ionisation Energy

BORON

5

1s 2s 2p

1s 2s 2p

1s 2s

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

Variation in 1st Ionisation Energy

EXPLANATION

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

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.

CARBON

6

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

Variation in 1st Ionisation Energy

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

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

Variation in 1st Ionisation Energy

EXPLANATION

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.

OXYGEN

8

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

Variation in 1st Ionisation Energy

EXPLANATION

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

The 2p orbitals are almost full.

FLUORINE

9

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

Variation in 1st Ionisation Energy

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

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

1s 2s 2p 3s

Variation in 1st Ionisation Energy

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.

SODIUM

11

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s

1s

ATOMIC NUMBER

1st IONISATION ENERGY / kJmol^-1

1s 2s 2p 3s

Variation in 1st Ionisation Energy

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

Successive Ionisation Energies

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Atoms with more than one electron can have them successively removed.

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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.

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e.g. Na⁺(g) Na²⁺(g) + e^-

Al⁺(g) Al²⁺(g) + e^-

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Trends Successive ionisation energies are always greater than the previous one

Reason :- the electron is being pulled away from a more positive species

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Large increases occur when there is a change of shell

Reason :- there is a big decrease in shielding

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Large increases can be used to predict the group of an unknown element

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Again, make sure you write in the (g)

I.E. kJmol^-1 Electronic configuration

1 590 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

2 1145 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

3 4912 1s² 2s² 2p⁶ 3s² 3p⁶

4 6474 1s² 2s² 2p⁶ 3s² 3p⁵

5 8145 1s² 2s² 2p⁶ 3s² 3p⁴

6 10496 1s² 2s² 2p⁶ 3s² 3p³

7 12320 1s² 2s² 2p⁶ 3s² 3p²

8 14207 1s² 2s² 2p⁶ 3s² 3p¹

9 18192 1s² 2s² 2p⁶ 3s²

10 20385 1s² 2s² 2p⁶ 3s¹

11 57048 1s² 2s² 2p⁶

12 63333 1s² 2s² 2p⁵

13 70052 1s² 2s² 2p⁴

14 78792 1s² 2s² 2p³

15 86367 1s² 2s² 2p²

16 94000 1s² 2s² 2p¹

17 104900 1s² 2s²

18 111600 1s² 2s¹

19 494790 1s²

20 527759 1s¹

A

Successive Ionisation Energies of Calcium

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.

A

I.E. kJmol^-1 Electronic configuration

1 590 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

2 1145 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

3 4912 1s² 2s² 2p⁶ 3s² 3p⁶

4 6474 1s² 2s² 2p⁶ 3s² 3p⁵

5 8145 1s² 2s² 2p⁶ 3s² 3p⁴

6 10496 1s² 2s² 2p⁶ 3s² 3p³

7 12320 1s² 2s² 2p⁶ 3s² 3p²

8 14207 1s² 2s² 2p⁶ 3s² 3p¹

9 18192 1s² 2s² 2p⁶ 3s²

10 20385 1s² 2s² 2p⁶ 3s¹

11 57048 1s² 2s² 2p⁶

12 63333 1s² 2s² 2p⁵

13 70052 1s² 2s² 2p⁴

14 78792 1s² 2s² 2p³

15 86367 1s² 2s² 2p²

16 94000 1s² 2s² 2p¹

17 104900 1s² 2s²

18 111600 1s² 2s¹

19 494790 1s²

20 527759 1s¹

B

Successive Ionisation Energies of Calcium

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.

B

I.E. kJmol^-1 Electronic configuration

1 590 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

2 1145 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

3 4912 1s² 2s² 2p⁶ 3s² 3p⁶

4 6474 1s² 2s² 2p⁶ 3s² 3p⁵

5 8145 1s² 2s² 2p⁶ 3s² 3p⁴

6 10496 1s² 2s² 2p⁶ 3s² 3p³

7 12320 1s² 2s² 2p⁶ 3s² 3p²

8 14207 1s² 2s² 2p⁶ 3s² 3p¹

9 18192 1s² 2s² 2p⁶ 3s²

10 20385 1s² 2s² 2p⁶ 3s¹

11 57048 1s² 2s² 2p⁶

12 63333 1s² 2s² 2p⁵

13 70052 1s² 2s² 2p⁴

14 78792 1s² 2s² 2p³

15 86367 1s² 2s² 2p²

16 94000 1s² 2s² 2p¹

17 104900 1s² 2s²

18 111600 1s² 2s¹

19 494790 1s²

20 527759 1s¹

C

Successive Ionisation Energies of Calcium

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.

C

I.E. kJmol^-1 Electronic configuration

1 590 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

2 1145 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

3 4912 1s² 2s² 2p⁶ 3s² 3p⁶

4 6474 1s² 2s² 2p⁶ 3s² 3p⁵

5 8145 1s² 2s² 2p⁶ 3s² 3p⁴

6 10496 1s² 2s² 2p⁶ 3s² 3p³

7 12320 1s² 2s² 2p⁶ 3s² 3p²

8 14207 1s² 2s² 2p⁶ 3s² 3p¹

9 18192 1s² 2s² 2p⁶ 3s²

10 20385 1s² 2s² 2p⁶ 3s¹

11 57048 1s² 2s² 2p⁶

12 63333 1s² 2s² 2p⁵

13 70052 1s² 2s² 2p⁴

14 78792 1s² 2s² 2p³

15 86367 1s² 2s² 2p²

16 94000 1s² 2s² 2p¹

17 104900 1s² 2s²

18 111600 1s² 2s¹

19 494790 1s²

20 527759 1s¹

C

Successive Ionisation Energies of Calcium

SUMMARY

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.

B

A

QUESTION TIME

Ans

The 3rd I.E. of magnesium

EXPLANATION

The 3rd I.E. of aluminium involves the following change...

Al²⁺(g) Al³⁺(g)

1s² 2s² 2p⁶ 3s¹ 1s² 2s² 2p⁶

The 3rd I.E. of magnesium involves the following change…

Mg²⁺(g) Mg³⁺(g)

1s² 2s² 2p⁶ 1s² 2s² 2p⁵

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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.

Which has the higher value, the 3rd I.E. of aluminium or the 3rd I.E. of magnesium?

Q.1

QUESTION TIME

Ans

The 2nd I.E. of magnesium

EXPLANATION

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.

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

Q.2

Knowledge Organisers (wjec.co.uk)

Any Questions?

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These can be addressed in the live session at the end of this series of lessons.

Spring 2024 AS Sessions:

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Session 1 – Radioactivity and Ionisation Energies (NS)

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Session 2 – Gravimetric Practical (PB)

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Session 3 – Instrumental Analysis (NS)

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Session 4 – Making up a Standard Solution and Titrations (PB)