3.7 Nuclear Physics

Question 1

Size of an atom?

Answer 1

Atoms are very small, having a radius of about 10-1 metres.

Question 2

Simple model of an atom?

Answer 2

The simple model of an atom is a small central positively charged nucleus composed of protons and neutrons, surrounded by electrons. The radius of the nucleus is much smaller than that of the atom with almost all of the mass in the nucleus.

Question 3

What provides evidence for the distribution of mass in the atom?

Answer 3

The scattering of alpha particles by thin metal foil provides evidence of the distribution of mass in the atom

Question 4

The relative masses and electric charges of protons, neutrons and electrons are..

Answer 4

Relative mass. Relative charge
proton. 1 1
neutron. 1. 0
electron Very small. -1

Question 5

In an atom the number of electrons is equal to the number of…

Answer 5

In an atom the number of electrons is equal to the number of protons in the nucleus.

Question 6

What charge does an atom have?

Answer 6

The atom has no overall electrical charge.

Question 7

How are electrons arranged?How do atoms form ions?

Answer 7

In each atom its electrons are arranged at various distances from the nucleus. Atoms may lose or gain outer electrons to form charged particles called ions.

Question 8

Isotopes?

Answer 8

The atoms of a particular element always have the same number of protons, but have a different number of neutrons for each isotope.

Question 9

Proton/atomic number?

Answer 9

The total number of protons in an atom is called its proton number or atomic number.

Question 10

Mass number?

Answer 10

The total number of protons and neutrons in an atom is called its mass number.

Question 11

Radioactive decay?

Answer 11

Some atomic nuclei are unstable. The nucleus emits particles or radiation and the nucleus changes to that of a different element and becomes more stable. This is a random process called radioactive decay.

Question 12

How is energy emmited?

Answer 12

Energy is emitted by changes in the nucleus.

Question 13

What do unstable nuclei emit?

Answer 13

Unstable nuclei emit alpha particles, beta particles, or neutrons, and electromagnetic radiation as gamma waves.

Question 14

What affects the behavior of unstable nuclei emitting these products? Is it possible to predict when a random atom may decay?

Answer 14

Neither chemical nor physical processes affect this behaviour. These substances are said to be radioactive and although the general process follows a pattern this radioactive decay is a random process. It is impossible to predict when a particular atom might decay.

Question 15

Background radiation?

Answer 15

Background radiation is around us all of the time. It comes from a range of sources, such as radioactive substances in the environment, from space or from devices such as X-ray machines in hospitals.

Question 16

What does an alpha particle consist of?

Answer 16

An alpha particle consists of two neutrons and two protons (ie a Helium nucleus).

Question 17

What is a beta particle?

Answer 17

A beta particle is a high speed electron ejected from the nucleus as a neutron turns into a proton.

Question 18

Gamma radiation?

Answer 18

Gamma radiation is electromagnetic radiation from the nucleus.

Question 19

What is used to represent radioactive decay?

Answer 19

Nuclear equations are used to represent radioactive decay.

Question 20

What conditions are needed for predicting radioactive decay?

Answer 20

Radioactive decay is random, but with a large enough number of nuclei it is possible to predict how many will decay in a certain amount of time.

Question 21

Half life of a radioactive isotope is?

Answer 21

The half-life of a radioactive isotope is: .
• the average time it takes for the number of nuclei of the isotope in a sample to halve
• the time it takes for the count rate from a sample containing the isotope to fall to half its initial level.

Question 22

What is radioactive contamination?

Answer 22

Radioactive contamination is the unwanted presence of radioactive atoms on other materials.

Question 23

Hazards from contamination?

Answer 23

The hazard from contamination is due to the decay of the contaminating atoms. The type of radiation emitted affects the level of hazard.

Question 24

Irradiation?

Answer 24

Irradiation is the process of exposing an object to ionizing radiation. The irradiated object does not become radioactive. Suitable precautions must be taken to protect against the hazards of the radioactive source used in irradiation.

Question 25

Most unstable nuclei?

Answer 25

Radioactive isotopes have a very wide range of half-life values. The most unstable nuclei have the shortest half-lives; decay is rapid with a lot of radiation emitted in a short time.

Question 26

Least unstable nuclei?

Answer 26

The least unstable nuclei have the longest half-lives; hey emit little radiation each second but emit radiation for a long time. There are uses and dangers associated with each type of nuclear radiation.

Question 27

Nuclear fission?

Answer 27

Nuclear fission is the splitting of a large and unstable nucleus and the release of energy.

Question 28

What the two fissionable substances in common use in nuclear reactors?

Answer 28

There are two fissionable substances in common use in nuclear reactors: uranium-235 and plutonium-239.

Question 29

The majority of nuclear reactors use…

Answer 29

Uranium-235

Question 30

How does fission occur?

Answer 30

For fission to occur a uranium-235 or plutonium-239 nucleus must first absorb a neutron to make the nucleus unstable. The nucleus undergoing fission splits into two smaller nuclei, releasing two or three neutrons and energy. The amount of energy released during nuclear fission is much greater than that released in a chemical reaction involving a similar mass of material.

Question 31

When does a chain reaction occur?

Answer 31

A chain reaction occurs when neutrons from the fission go on to cause further fission. In a nuclear reactor control rods absorb fission neutrons to ensure that on average only one neutron per fission goes on to produce further fission and energy transfer.

Question 32

Do nuclear reactions produce waste? If so what should be done with this waste?

Answer 32

Nuclear reactions produce waste which may be dangerous due to its radioactive nature and may remain so for a long time, depending upon its half life and products. The disposal of such waste needs to be managed with care and is a factor that may influence the use of nuclear power for the generation of electricity.

Question 33

Nuclear fusion?

Answer 33

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.

Question 34

What happens to some of the mass in nuclear fusion?

Answer 34

In this process some of the mass of the smaller nuclei is converted into energy.

Question 35

What happened to the force of repulsion between two positive nuclei?

Answer 35

The force of repulsion between the two positive nuclei must be overcome for them to get close and fuse and this happens at very high temperatures and pressures.

Question 36

Nuclear fusion in stars?

Answer 36

Nuclear fusion is the process by which energy is released in stars.