Chapter 4: Atomic structure

Question 1

How were electrons discovered?

Answer 1

In 1897, J J Thompson discovered that electrons were emitted from the surface of hot metals. He showed that electrons were negatively charged and much smaller then atoms. This led to a change in the accepted atomic theory

Question 2

What is the plum pudding model?

Answer 2

The model for the atom proposed by Thompson in 1904 that states that atoms were made up of a ball of positive charge with negative electrons inside it

Question 3

How was radioactivity discovered?

Answer 3

In 1896, Henry Becquerel placed some uranium salts next to a photographic plate in a thick black bag. When the plate was later developed, it had been affected as though it had been exposed to light. Becquerel realised that particles had been emitted from the salts, which passed through the bag

Question 4

What is radioactivity?

Answer 4

The tendency of unstable atoms to eject particles from their nuclei to become more stable. This is a random process that cannot be predicted

Question 5

Describe an alpha particle

Answer 5

Helium nucleus (2p, 2n), powerful ionising effects, stopped by paper, short range (a few cm)

Question 6

Describe a beta particle

Answer 6

An electron created when a neutron splits into an electron and a proton. Medium ionising effects, stopped by aluminium, medium range (a few metres)

Question 7

Describe a gamma ray

Answer 7

An electromagnetic wave (like light) with no charge and no mass, that carries energy away from an unstable nucleus. Long range, can only be stopped by thick lead, weak ionising effects.

Question 8

Describe neutron emission

Answer 8

An unstable nucleus emits a neutron to reduce its mass number

Question 9

What is ionisation?

Answer 9

When high-speed radioactive particles remove electrons from atoms, turning them into ions

Question 10

What is a becquerel?

Answer 10

an emission of 1 particle per second (measure of activity)

Question 11

How can radiation be detected?

Answer 11

Geiger-Muller tube

Question 12

What is a half-life?

Answer 12

The amount of time taken for half the nuclei in a sample of radioactive material to decay

Question 13

What factor do you have to consider when measuring radiation with a GM tube?

Answer 13

Background radiation

Question 14

What is carbon dating?

Answer 14

The process in which the age of the remains of a living organism can be identified. The amount of carbon-14 in the sample is measured and used to calculate the age (details of how this works are not in the AQA specification)

Question 15

How does radiation affect the body?

Answer 15

Alpha particles behave like miniature bullets. Alpha radiation can also create strong acids that can destroy cells and cause mutations in genes

Question 16

Which type of radiation is the most damaging inside the body?

Answer 16

Alpha (hardly any of it escapes)

Question 17

What is the unit of radiation exposure?

Answer 17

Sieverts (Sv)

Question 18

What is a radioactive tracer?

Answer 18

A radioactive isotope (usually technetium-99) which is used to help doctors examine internal organs. The isotope is attached to a biochemical agent, allowing it to be absorbed by the body. The radiation it emits is then used to build up an image. Technetium-99 is usually used because it only emits gamma rays, does minimal damage and has a short half life (6 hours).

Question 19

How can radiation be used to treat cancer?

Answer 19

Radiation is used to destroy tumours. Either a strong burst of gamma radiation from outside the body is directed at the tumour, or the tumour is destroyed from within using chemicals that emit alpha or beta radiation

Question 20

What is irradiation?

Answer 20

When alpha or beta particles disperse into a substance

Question 21

What is contamination?

Answer 21

When actively decaying radioactive isotopes disperse into a substance

Question 22

Why is it dangerous if radioactive isotopes disperse into the atmosphere?

Answer 22

They enter the food chain and cause cancer

Question 23

What is nuclear fission?

Answer 23

The process in which an unstable nucleus splits into 2 smaller nuclei. A neutron hits the nucleus, making it so unstable that it splits, releasing energy and neutrons, and forming 2 smaller nonidentical nuclei which are often unstable

Question 24

Describe the process in which nuclear energy is transferred to usable energy

Answer 24

After the fission completes, the 2 positive nuclei repel each other and gain kinetic energy. They hit other atoms, causing their internal energy (and temperature) to rise. This can be used to generate energy in power stations. The rate of fission can also be controlled by changing the temperature

Question 25

Describe the process of a nuclear chain reaction

Answer 25

Each neutron emitted in nuclear fission strikes another nucleus and causes it to split. In a controlled chain reaction, only one neutron is released per fission (this is what happens in a power stations). In an uncontrolled chain reaction, multiple neutrons are released, causing a massive release of energy (this is what happens in atomic bombs)

Question 26

What is nuclear fission?

Answer 26

The process in which two small nuclei join to make a single larger nucleus, releasing energy. This process takes place inside stars

Question 27

How was the nuclear model of the atom discovered?

Answer 27

Geiger and Marsden directed a beam of alpha particles at a thin sheet of gold foil. They thought the gold atoms were like Professor Thompson’s plum pudding model (weak positive charge all throughout), so they expected the alpha particles to pass straight through the foil. They realised that a small number actually bounced back. Rutherford produced a theory to explain this

Question 28

What was Rutherford’s theory?

Answer 28

He suggested that the deflection of an alpha particle was caused by the electrostatic interaction between it and a small, charged nucleus. He suggested that the nucleus could either be positively (would repel an alpha particle backwards) or negatively charged (would pull the alpha particle around it)

Question 29

What is the Bohr model of the atom?

Answer 29

A model of the atom suggested by Niels Bohr in which elections move around the nucleus in circular orbits, and can change their orbits by releasing or absorbing electromagnetic radiation

Question 30

Who discovered neutrons?

Answer 30

James Chadwick in 1932. His discovery allowed scientists to account for the mass of the atom

Question 31

Why does alpha scattering happen?

Answer 31

Most of the atom is empty space (diameter of an atom is 20000x the diameter of the nucleus), so the majority of alpha particles pass straight through without getting near the nucleus. Some approach the nucleus head-on and deflect the way they came, and some pass near the nucleus and their direction changes. Later experiments showed that the nucleus could be further subdivided into smaller particles (protons)

Question 32

How can the reaction in a nuclear power station be controlled?

Answer 32

By lowering boron control rods into the reactor, causing the reaction to slow down due to the fact that boron absorbs neutrons. In the case of an emergency, the rods are pushed all the way into the core of the reactor and the reaction stops completely

Question 33

How are workers protected in a nuclear power station?

Answer 33

There’s a thick concrete shield around the reactor to prevent workers from being hit by hazardous neutrons