2a. What is Radioactivity? and why does it happen?

• The nucleus is composed of protons and neutrons and glued together by a strong nuclear attractive forces BUT only certain combinations of the neutron/proton (n/p) ratio in the nucleus seem to be more stable than others (see stability curve graph on right).
• Any isotope of any element that does not lie in the stability band with a stable n/p ratio is more likely to be radioactive!
• Radioactivity results from the random and spontaneous breakdown of the unstable nucleus of an atom.
• This breakdown is called radioactive decay (nuclear decay) of the unstable nucleus of the atom-isotope (radioisotope).
• In the breakdown of the unstable nucleus, energy is released by the emission (usually) of three types of ionising radiation (nuclear radiation) called  alpha particle radiation, beta particle radiation and gamma radiation (see diagram below).
  • These emissions are ALL high velocity and high energy particles or radiation.
  • Alpha particles are positive (2+), mass 4, 2 protons (+) combined with 2 neutrons (0).
    • The expelled alpha particle is the equivalent of the nucleus of a helium atom (which is what it is!)

• alpha particle emission on left
• more on properties of alpha particles & uses of alpha radiation & alpha decay nuclear equations

• Beta particles are negative, mass 1/1850, a negative electron (charge -, minus), ⁰_-1e.
  • The expelled beta minus particle from the unstable nucleus is a high speed electron.
    • (you should state that this electron is a beta minus, because there is beta plus particle called a positron!

• beta particle emission on right
• more on properties of beta particles & uses of beta radiation & beta decay nuclear equations

• Gamma photon, mass 0 (~zero), radiation is electrically neutral (charge 0 or zero)
  • Gamma radiation emission often accompanies alpha and beta particle emission - its a way that a newly formed and temporarily unstable nucleus gets rid of its excess energy to become more stable.

• gamma photon emission at the bottom
• more on the properties of gamma photons and uses of gamma radiation

• When an unstable nucleus splits a different nucleus with a different number of protons is formed, as well as the emission of radiation, and so a different element is formed (NOT possible in chemical changes, but this is a nuclear reaction!).
• The breakdown of an unstable atom is referred to as radioactive decay or radioactive disintegration.
  • It is a totally random process meaning it is a matter of chance which particular nucleus decays.

• For more details about radiations, see detailed table of information in 4a. on the three types of radioactive emission, alpha, beta and gamma ionising radiations.

• Because it involves high energy nuclear changes, radioactivity is completely unaffected by the temperature, pressure, physical state or chemical state of the atoms.
  • You cannot alter the rate of decay at all!
  • The energy changes involved with nuclear changes like radioactivity, are much greater, than those for physical or chemical changes of materials like boiling or fuel combustion etc.
  • I've some calculations in section 4. to prove this point (but NOT required by GCSE students!).

2b. How did they find out there were three types of atomic ionising radiation?

Basic experimental technique for separating beams of a mixture of particles

• A radium source was used, which also contains other radio-isotopes, and a lead casing was the only safety precaution used!
  • How people like Marie Curie and Ernest Rutherford etc. survived beyond the age of 60 is a mystery!
• Anyway! it was found that the original 'emission' from the radioactive radium source was split into three beams by an electric or magnetic field (diagram on the right).
  • (i) The alpha beam was attracted/deflected towards a negative plate, showing it was positive, since opposite charges attract/like charges repel - the rules of electric charge interaction.
  • (ii) The beta (negative electrons) beam bent towards a positive plate (showing it was negative).
  • (iii) The gamma beam passed through un-deflected (showing it had no charge).
• Note: Although the alpha particles have the biggest charge of +2, the beta beam (smaller charge of -1) is more easily deflected because of its much smaller mass (nearly 8000 x smaller, mass ratio for He²⁺ : e^- is 4 : ¹/₁₈₅₀).
• Note that in other nuclear particle separation experiments, a proton (+), or any other positive particle beam, bends towards the negative plate.
• Other experiments have detected positron (positive electron) emission from beta plus radioactive decay.

Key revision points about What is Radioactivity? Why does it happen? How do we know there 3 types of emitted radiation?

See the main index for all the detailed notes on radioactivity

1. What is Radioactivity?

• Definition: The spontaneous emission of particles or energy from unstable atomic nuclei.
• Key idea: Atoms with too many protons or neutrons (or too much energy) are unstable and decay to become more stable.
• Random process: Individual decays cannot be predicted, but overall rates follow statistical patterns (half-life).

2. Why Does Radioactivity Happen?

• Nuclear instability: Balance between strong nuclear force (binding protons & neutrons) and electrostatic repulsion of protons.
• Too many neutrons/protons → nucleus unstable.
• Decay pathways:
  • Emit particles (α, β) or energy (γ) to reduce instability.
  • Transform into a different element (transmutation).

3. How Do We Know There Are 3 Types of Radiation?

• Historical experiments (Rutherford, early 20th century):
  • Radiation passed through electric/magnetic fields → separated into three distinct paths.
  • Alpha (α): Deflected slightly, positive charge.
  • Beta (β−): Deflected strongly, negative charge.
  • Gamma (γ): No deflection, neutral, electromagnetic wave.

4. Typical Misconceptions

• Forgetting gamma has no mass/charge.
• Misbalancing nuclear equations (e.g., omitting electron in β decay).
• Assuming all isotopes are radioactive (many are stable).
• Believing radiation makes atoms “explode” — decay is subtle, not explosive.

5. Final key point

Radioactivity is the natural consequence of nuclear instability.

Experiments with electric/magnetic fields revealed three distinct types of radiation, each with unique properties.

See the main index for all the detailed notes on radioactivity

1. Atomic structure and fundamental particle knowledge needed to understand radioactivity

2. What is Radioactivity? Why does it happen? Three types of atomic-nuclear-ionising radiation

3. Detection of radioactivity, its measurement and radiation dose units, ionising radiation sources - radioactive materials, background radiation

4. Alpha, beta & gamma radiation - properties of 3 types of radioactive nuclear emission & symbols ,dangers of radioactive emissions - health and safety issues and ionising radiation

5. Uses of radioactive isotopes emitting alpha, beta (+/–) or gamma radiation in industry and medicine

6. The half-life of a radioisotope - how long does material remain radioactive? implications!, uses of decay data and half-life values - archaeological radiocarbon dating, dating ancient rocks

7. What actually happens to the nucleus in alpha and beta radioactive decay and why? nuclear equations!, the production of radioisotopes - artificial sources of radioactive-isotopes, cyclotron

8. Nuclear fusion reactions and the formation of 'heavy elements' by bombardment techniques

9. Nuclear Fission Reactions, nuclear power as an energy resource

Easier Foundation Tier Radioactivity multiple choice QUIZ

Harder Higher Tier Radioactivity multiple choice QUIZ

Worksheet QUIZ Question 1 on RADIOACTIVITY - absorption of alpha, beta and gamma radiation

Worksheet QUIZ Question 2 on RADIOACTIVITY - dangers & monitoring ionising radiation levels

Worksheet QUIZ Question 3 on RADIOACTIVITY - revision of atomic structure

Worksheet QUIZ Question 4 on RADIOACTIVITY - what happens to atoms in radioactive decay?

Worksheet QUIZ Question 5 on RADIOACTIVITY - uses of radioisotope and half-life data

ANSWERS to the WORD-FILL WORKSHEET QUIZZES

Crossword puzzle on radioactivity and ANSWERS!