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Particle

Charge

Size

Found

Neutron

None

1

In the nucleus

Proton

+

1

Electron

-

Tiny

Orbits the nucleus

Atom

Same number of protons and electrons

Ion

Unequal number of electrons to protons

Mass number

Number of protons and neutrons

Atomic number

Number of protons

Democritus

Suggested idea of atoms as small spheres that cannot be cut.

J J Thomson

(1897)

Discovered electrons– emitted from surface of hot metal. Showed electrons are negatively charged and that they are much less massive than atoms.

Thomson (1904)

Proposed ‘plum pudding’ model – atoms are a ball of positive charge with negative electrons embedded in it.

Geiger and Marsden (1909)

Directed beam of alpha particles (He²⁺)at a thin sheet of gold foil. Found some travelled through, some were deflected, some bounced back.

Rutherford (1911)

Used above evidence to suggest alpha particles deflected due to electrostatic interaction between the very small charged nucleus, nucleus was massive. Proposed mass and positive charge contained in nucleus while electrons found outside the nucleus which cancel the positive charge exactly.

Bohr (1913)

Suggested modern model of atom – electrons in circular orbits around nucleus, electrons can change orbits by emitting or absorbing electromagnetic radiation. His research led to the idea of some particles within the nucleus having positive charge; these were named protons.

Chadwick (1932)

Discovered neutrons in nucleus – enabling other scientists to account for mass of atom.

Nuclear fission

One large unstable nucleus splits to make two smaller nuclei

Neutron hits U-235 nucleus, nucleus absorbs neutron, splits emitting two or three neutrons and two smaller nuclei. Process also releases energy.

Process repeats, chain reaction formed

Used in nuclear power stations

Nuclear fusion

Two small nuclei join to make one larger nucleus

Difficult to do on Earth – huge amounts of pressure and temperature needed.

Occurs in stars

Fuel rods

Made of U-238, ‘enriched’ with U-235 (3%). Long and thin to allow neutrons to escape, hitting nuclei.

Control rods

Made of Boron. Controls the rate of reaction. Boron absorbs excess neutrons.

Concrete

Neutrons hazardous to humans – thick concreate shield protects workers.

Decay

Range in air

Ionising power

Penetration power

Alpha

Few cm

Very strong

Stopped by paper

Beta

Few m

Medium

Stopped by Aluminium

Gamma

Great distances

Weak

Stopped by thick lead

Radioactive decay

Unstable atoms randomly emit radiation to become stable

Detecting

Use Geiger Muller tube

Unit

Becquerel

Ionisation

All radiation ionises

Isotope

​

​

​

​

Different forms of an element with the same number of protons but different number of neutrons

Contamination

Unwanted presence of radioactive atoms

Irradiation

Person is in exposed to radioactive source

Sievert

Unit measuring dose of radiation

Background

Constant low level environmental radiation, e.g. from nuclear testing, nuclear power, waste

Half life

The time taken to lose half of its initial radioactivity

Uses

Different isotopes have different half lives

Short half-lives used in high doses, long half lives used in low doses.

Tracers

Used within body

Isotope with short half life injected, allowed to circulate and collect in damaged areas. PET scanner used to detect emitting radiation. Must be beta or gamma as alpha does not penetrate the body.

Radiation therapy

Used to treat illnesses e.g. cancer

Cancer cells killed by gamma rays. High dose used to kill cells. Damage to healthy cells prevented by focussed gamma ray gun.

Decay

Emitted from nucleus

Changes in mass number and atomic number

Alpha (α)

​

-4

-2

Beta (β)

​

0

+1

Gamma (γ)

Electromagnetic wave

0

0

Neutron

Neutron

-1

0