Atomic models, ionising radiation, nuclear decay, half-life and safe exposure.
Revise the key ideas
Atomic models and particle numbers
Thomson’s plum-pudding model described negative electrons embedded in spread-out positive charge. Later evidence changed this model.
Rutherford’s team directed positive alpha particles at thin gold foil. Most passed through; some were deflected; very few were deflected through large angles or back towards the source.
Most passing through showed the atom is mostly empty space. Large deflections showed positive charge and much of the mass concentrated in a tiny nucleus; the plum-pudding model could not explain them.A small concentrated nucleus explains rare large deflections.
The nuclear model has positive protons and neutral neutrons in the nucleus, with negative electrons outside it. Rutherford’s original model did not yet include the later discovery of neutrons.
Protons have relative charge +1 and mass about 1; neutrons charge 0 and mass about 1; electrons charge −1 and much smaller mass, about 1/1840 of a proton.
Atomic number Z counts protons; mass/nucleon number A counts protons plus neutrons. Neutrons = A − Z. In a neutral atom, electrons = protons; ions have unequal numbers.A neutral sodium-23 atom also has 11 electrons.
Isotopes have the same proton number but different neutron numbers. Carbon-12, carbon-13 and carbon-14 are isotopes of carbon.
Bohr’s model uses fixed electron energy levels/shells. Electrons absorb energy to move to higher levels and emit electromagnetic radiation when returning to lower levels.
When electrons move between energy levels, different energy changes produce radiation of different frequencies. The separate lines in emission and absorption spectra show that the energy levels have fixed, distinct values. Different elements produce different line patterns.
Ionisation is removal or addition of electrons to form ions. Radiation that removes an electron leaves a positive ion; it does not change the number of protons.
Background radiation and detection
Background radiation is radiation present around us even without a nearby experimental source. Natural sources include rocks, radon gas, cosmic radiation and naturally radioactive materials in food.
Artificial sources include medical uses and smaller contributions from nuclear industry and past activities. Contributions vary with location and occupation.
A Geiger–Müller tube and counter detect ionising radiation as pulses. Count rate is counts per second or minute; not every decay in the source is detected.
Source activity is the number of nuclear decays per second, measured in becquerels (Bq). One Bq means one decay per second; activity and detector count rate are different quantities.
Measure background without the source for a suitable time. Divide counts by time, then subtract this background count rate from the source-plus-background rate measured consistently.
Radioactive counts fluctuate randomly. Count for longer and repeat to reduce relative random uncertainty; keep distance, detector position and counting time controlled.
Photographic film darkens on exposure. Film badges and other dosimeters monitor exposure; they do not shield the wearer or replace limiting exposure time.
Types of nuclear radiation
An unstable nucleus can decay spontaneously, emitting radiation. Decay is random: it is impossible to predict when one particular nucleus will decay.
An alpha particle is a helium nucleus: two protons and two neutrons, charge +2. Alpha is strongly ionising, short-range in air and stopped by paper or the outer skin layer.
Beta minus is a fast electron emitted from the nucleus when a neutron changes into a proton. It is not an electron ejected from an outer shell.
Beta plus is a positron, the positive counterpart of an electron, emitted when a proton changes into a neutron. Both beta types are moderately penetrating and ionising compared with alpha and gamma.
A thin aluminium sheet can substantially stop beta radiation; required thickness depends on beta energy. Beta travels further in air than alpha.
Gamma is high-frequency electromagnetic radiation emitted when a nucleus loses excess energy. It has no charge or rest mass and does not alter A or Z.
Gamma is highly penetrating and weakly ionising compared with alpha. Thick lead or concrete reduces intensity substantially, rather than guaranteeing every gamma photon is stopped.These are qualitative comparisons; shielding reduces the radiation reaching a detector.
A neutron can also be emitted from a nucleus: mass number falls by 1 and proton number stays unchanged. Different radiation types need suitable shielding.
Balancing nuclear changes
In alpha decay, A decreases by 4 and Z decreases by 2. For example, radium-226 (Z = 88) becomes radon-222 (Z = 86) plus helium-4 (Z = 2).
In beta-minus decay, A stays unchanged and Z increases by 1. The emitted electron has nucleon number 0 and charge number −1, keeping equation totals balanced.
In beta-plus decay, A stays unchanged and Z decreases by 1. The emitted positron has nucleon number 0 and charge number +1.
In gamma emission, A and Z stay the same. The nucleus has less energy, but it still has the same numbers of protons and neutrons: it is the same nuclide.
Check nuclear equations by balancing total nucleon numbers and total charge numbers on both sides. The identity of the element is set by Z, not by A alone.Balance nucleon numbers and charge numbers in each equation.
Half-life and decay calculations
Half-life is the time for half the undecayed nuclei in a large sample to decay, or for the activity to halve. It does not mean half the total material vanishes.
The activity decreases as fewer undecayed nuclei remain. After 1, 2 and 3 half-lives, the fraction remaining is ½, ¼ and ⅛.
After n half-lives, remaining activity = initial activity ÷ 2ⁿ. Time elapsed = number of half-lives × half-life duration.
If activity starts at 80 Bq and half-life is 5 years, activities after 5, 10 and 15 years are 40, 20 and 10 Bq.Equal time intervals halve the activity; the curve is exponential, not linear.
To read half-life from a graph, choose an activity, find when it falls to half, and subtract the times. Check several pairs and subtract background from measured count-rate data first.
Half-life lets us predict how a large radioactive sample will behave overall, even though individual decays are random. Smaller samples show more variation. As fewer undecayed nuclei remain, fewer decay each second: the decrease is not a straight line.
Hazards and precautions
Ionising radiation can damage cells and DNA, causing mutations and increasing cancer risk. Effects depend on dose and exposure; not every exposure causes a tumour.
Keep sources at a distance using tongs, minimise exposure time, use appropriate shielding and store them securely. Follow supervised handling procedures and never point a source towards someone.
Irradiation means exposure to radiation. Contamination means radioactive material gets onto or into an object or person, so exposure can continue while it remains there.Contamination can continue irradiating the person until the material is removed or decays.
Ordinary irradiation does not mean radioactive material has been deposited. Contamination may need removal and containment; merely moving away from the original source is insufficient.
Alpha is particularly dangerous if an alpha-emitting material enters the body: strong ionisation affects nearby tissue without the skin barrier. Penetrating gamma is also an external-exposure hazard.
Medical staff monitor dose and limit time near sources, use shielding and distance, and balance treatment or diagnostic benefit against radiation risk.
Revise radioactivity with this narrated video. Use the player controls to pause, seek, adjust the volume or mute. Turn English captions on or off using the captions menu.
Mr Kidman's Classroom