Topic 7: Atomic and nuclear physics

Question 1

Describe a model of the atom that features a small nucleus surrounded by electrons

Question 2

How are electrons kept in orbit?

Answer 2

Electrons are kept in orbit around the nucleus as a result of the electrostatic attraction between the electrons and the nucleus.

Question 3

Describe the Geiger-Marsden experiment

Answer 3

1. Beam of alpha particles aimed at thin gold foil
2. Passage through foil detected
3. Expected that particles would pass straight through
4. Some of the particles emerged at different angles and some reflected back
5. It was realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of negative charge in the atom
6. As a result, the plum pudding model was replaced by the nuclear model
7. Rutherford concluded that the atom must have a tiny nucleus with electrons whizzing around it and that the nucleus had a positive charge to balance the negative charge of the electrons
8. He thought that almost the whole mass of an atom was concentrated in the nucleus, so it must be incredibly dense

Question 4

Outline one limitation of the simple model of the nuclear atom

Answer 4

Accelerating charges are known to lose energy. If the orbiting electrons were to lose energy they would spiral into the nucleus. The Rutherford model cannot explain how atoms are stable.

Question 5

Outline evidence for the existence of atomic energy levels.

Answer 5

1. Rutherford model was developed further by Niels Bohr who suggested that the electrons orbit the nucleus rather like a planet orbits the sun
2. Radius of Bohr’s electrons depended on the energy they had
3. He also suggested that they could only move in certain orbits: when the electrons moved from a high energy state to a lower energy state they emitted a photon of light and the frequency of the light depends on the difference between the energy levels
4. As there are a fixed number of energy levels only a few wavelengths of light are given out, resulting in a line spectrum
5. Each individual element has distinct energy levels and therefore the emission spectra can be used to identify them

Question 6

Define: nuclide

Answer 6

An atom characterised by its proton number and atomic number:

^A_ZX

Question 7

Define: isotope

Answer 7

Nuclei with the same atomic number but different mass number (due to a different number of neutrons)

Question 8

Define: nucleon

Answer 8

A proton or a neutron making up a nucleus

Question 9

Define: nucleon/mass number, A

Answer 9

The number of nucleons in a nucleus

Question 10

Define: proton/atomic number, Z

Answer 10

The number of protons in a nucleus.

Question 11

Define: neutron number, N

Answer 11

The number of neutrons in a nucleus

Question 12

Describe the interactions in a nucleus

Answer 12

• According to our knowledge of electrostatics a nucleus should not be stable; protons are positive charges so should repel each other and so there must be another force in the nucleus that overcomes the electrostatic repulsion and hold the nucleus together
• This force is called the strong nuclear force
• Strong nuclear forces must be very strong to overcome the electrostatic forces and must also have a very small range as they are not observed outside of the nucleus
• Neutrons have some involvement in strong nuclear forces: small nuclei have equal numbers of protons and neutrons, but larger nuclei, which are harder to hold together, have a greater ratio of neutrons to protons

Question 13

Describe the phenomenon of alpha decay.

Answer 13

• Alpha decay is one process that unstable atoms can use to become more stable. During alpha decay, an atom’s nucleus sheds two protons and two neutrons in an alpha particle.
• Since an atom loses two protons during alpha decay, it changes from one element to another.

Question 14

Describe the phenomenon of beta decay.

Answer 14

Beta particles are electrons emitted from the nucleus. The electron is formed when a neutron decays. At the same time, another particle is emitted called an antineutrino.

Question 15

Describe the phenomenon of gamma decay.

Answer 15

Gamma rays are unlike the other two radiations in that they are part of the electromagnetic spectrum. After their emission, the nucleus has less energy but its mass number and its atomic number have not changed. It is said to have changed from an excited state to a lower energy state.

Question 16

Effect on photographic film of alpha, beta and gamma radiation

Answer 16

Alpha - yes

Beta - yes

Gamma - yes

Question 17

Approximate number of ion pairs produced in air for alpha, beta and gamma radiation.

Answer 17

Alpha - 10⁴ per mm travelled

Beta - 10² per mm travelled

Gamma - 1 per mm travelled

Question 18

Typical material needed to absorb alpha, beta and gamma radiation.

Answer 18

Alpha - 10^-2 mm aluminium; piece of paper

Beta - a few mm aluminium

Gamma - 10 cm lead

Question 19

Penetration ability of alpha, beta and gamma radiation

Answer 19

Alpha - low

Beta - medium

Gamma - high

Question 20

Typical path length in air of alpha, beta and gamma radiation

Answer 20

Alpha - a few cm

Beta - less than one m

Gamma - infinite

Question 21

Speed of alpha, beta and gamma radiation

Answer 21

Alpha - about 10⁷ m s^-1

Beta - about 10⁸ m s^-1, very variable

Gamma - 3 X 10⁸ m s^-1

Question 22

Outline the biological effects of ionising radiation.

Answer 22

At the molecular level, an ionisation could cause damage directly to a biologically important molecule such as DNA or RNA. This could cause it to cease functioning. Alternatively, an ionisation in the surrounding medium is enough to interfere with the complex chemical reactions called metabolic pathways taking place.

Molecular damage can result in a disruption to the functions that are taking place within the cells that make up the organism. As well as potentially causing the cell to die, this could just prevent cells from dividing and multiplying. On top of this, it could be the cause of the transformation of the cell into a malignant form.

As all body tissues are built up of cells, damage to these can result in damage to the body systems that have been affected. The non-functioning of these systems can result in death. If malignant cells continue to grow, then this is called cancer.

Question 23

Explain why some nuclei are stable while others are unstable.

Answer 23

• The stability of a particular nuclide depends greatly on the numbers of neutrons present.
• For small nuclei, the number of neutrons tends to equal the number of protons.
• For large nuclei there are more neutrons than protons.
• Nuclides above the band of stability have too many neutrons and will decay with either alpha or beta decay.
• Nuclides below the band of stability have too few neutrons and will tend to emit positrons

Question 24

Describe the process of radioactive decay

Answer 24

Radioactive decay is a random process and is not affected by external conditions. For example, increasing the temperature of a sample of radioactive material does not affect the rate of decay. This means that there is no way of knowing whether or not a particular nucleus is going to decay within a certain period of time. All we know is the chances of a decay happening in that time.

Although the process is random, the large numbers of atoms involved allows us to make some accurate predictions. If we start with a given number of atoms, then we can expect a certain number to decay within the next minute. If there were more atoms in the sample, we would expect the number decaying to be larger. On average, the rate of decay of a sample is proportional to the number of atoms in the sample. This proportionality means that radioactive decay is an exponential process. The number of atoms of a certain element, N, decreases exponentially over time.

Question 25

Define: radioactive half life

Answer 25

The time taken for half the number of nuclides present in a sample to decay. The time taken for the rate of decay of a particular sample of nuclides to halve.

Question 26

Define: artificial transmutation

Answer 26

The conversion of one isotope to another. This can be done through a nuclear reaction whereby a nucleus is bombarded with a nucleon, an alpha particle or another small nucleus.

Question 27

Define: unified atomic mass unit

Answer 27

One twelfth the mass of a carbon-12 atom.

Question 28

Define: mass defect

Answer 28

The difference between the mass of a nucleus and the mass of its component nucleons

Question 29

Define: binding energy

Answer 29

The amount of energy that is released when a nucleus is assembled from its component nucleons.

Question 30

Define: binding energy per nucleon

Answer 30

Total binding energy for the nucleus divided by the total number of nucleons