Topic 7: Atomic Physics

Question 1

Nuclear model of atom

Answer 1

Mass and positive charge concentrated in tiny nucleus with negative electrons around it.

Question 2

Limitation of nuclear model

Answer 2

A classical atom would decay quickly since the electrons would spiral into the nucleus.

Question 3

Atomic energy levels

Answer 3

Bohr and Planck proposed distinct energy levels for electrons (quanta).

Question 4

Nuclide

Answer 4

A combination of protons and neutrons that form a nucleus.

Question 5

Isotope

Answer 5

Nuclei with the same number of protons but different numbers of neutrons.

Question 6

Nucleon

Answer 6

The particles in the nucleus (proton or neutron).

Question 7

Natural radioactive decay

Answer 7

A random and spontaneous process in which an unstable nucleus emits a particle (disintegration). The element of the nucleus changes. The emission of an alpha or beta particle or a gamma ray are the three possible processes.

Question 8

Exponential decay

Answer 8

The rate of decay decreases exponentially with time. The number of nuclei remaining (and the activity rate) reduces by half in a constant time.

Question 9

Alpha particle

Answer 9

Consists of two protons and two neutrons (= helium nucleus). Has approximately 5 MeV kinetic energy. Travels at approx 5% speed of light.

Question 10

Beta minus particle

Answer 10

Consists of one electron, often travelling at close to the speed of light. They have a range of speeds and kinetic energies depending on the element and the anti-neutrino.

Question 11

Beta minus decay

Answer 11

In the nucleus, a neutron changes into a proton and an electron (which is emitted as a beta minus particle).

Question 12

Anti-neutrino

Answer 12

Particle emitted with beta minus particle.Carries away some of the kinetic energy.

Question 13

Gamma ray

Answer 13

High energy (also frequency) electromagnetic radiation emitted by nucleus following alpha and beta decay (which left the nucleus in an excited state).

Question 14

Ionisation

Answer 14

Ionisation can be thought of as ‘damage’ to the medium the radiation is passing through. The greater the ionisation, the less the penetration into the medium, since the radiation more rapidly loses energy. Thus the most ionisation is by alpha and the least (almost none) by gamma.

Question 15

Nuclear strong force

Answer 15

The force that holds the particles of a nucleus together. It is strong enough to overcome electrostatic repulsion of protons. It is very short range (less than three nucleon radii), so larger nuclei are less strongly held together by this force and can be unstable..

Question 16

Transmutation

Answer 16

The process by which a nucleus changes by adding or removing nucleons. The process is called ‘artificial’ if an instrument such as a particle accelerator or nuclear reactor is required to provide the initial energy.

Question 17

Binding energy

Answer 17

The energy released when a nucleus assembles from its component nucleons.

Question 18

Mass defect

Answer 18

Mass defect = mass of component nucleons - mass of nucleus

Question 19

Einstein mass-energy equivalence

Answer 19

ΔE = Δmc2. The mass defect is equivalent to a change in energy which is calculated using this equation.

Question 20

MeV

Answer 20

Mega-electron-volt is a convenient unit for measuring nuclear energies. The energy an electron would have if it were accelerated through 1 000 000 V

Question 21

Nuclear fission

Answer 21

A process where a large nucleus splits to make smaller ones. This is permitted when the BE/ nucleon of the products is higher than the initial nucleus. The difference in mass/ energy is released.

Question 22

Nuclear chain reaction

Answer 22

A uranium nucleus can be split by an incident neutron. The fission reaction produces more neutrons. If one of the neutrons produced also causes fission, then a chain reaction will follow. If the number of fissions is greater than one, the reaction will go out of control.

Question 23

Nuclear fusion

Answer 23

A process where small nuclei can combine to make a larger nucleus. This is permitted when the BE/ nucleon of the product is higher. The difference in mass/ energy is released.

Question 24

Radioactive half-life

Answer 24

The time taken for half of the nuclei in a sample to decay. OR The time taken for the activity to reduce to half of its original value

Question 25

Unified atomic mass unit

Answer 25

The mass of one twelfth of the nucleus of a carbon-12 isotope.

Question 26

Half life

Answer 26

Time taken for half the nuclei in the sample to decay