Particle model and atomic structure
Physics · GCSE · The School
Particles and density
DENSITY = mass ÷ volume. A change of state does not change the number of particles or their mass — only their arrangement and energy — so mass is conserved while density usually changes. Internal energy rises during melting and boiling without the temperature rising, because the energy is going into breaking bonds rather than into speed. That flat section on a heating curve is the whole idea.
Inside the atom
A nucleus of protons and neutrons, with electrons in shells around it — and almost all of it empty space. The model was not obvious: Thomson's 'plum pudding' was replaced when Rutherford's team fired alpha particles at gold foil and a few bounced straight back, which cannot happen unless the positive charge is concentrated in a tiny nucleus. Bohr then placed electrons at set distances.
Decay and half-life
Some nuclei are unstable and emit radiation: ALPHA (a helium nucleus, stopped by paper, strongly ionising), BETA (an electron, stopped by aluminium), GAMMA (a wave, needs thick lead). HALF-LIFE is the time for half the unstable nuclei to decay — it is a statistical property of the substance and cannot be altered by heating, pressure or chemistry.
A sample has an activity of 800 counts per minute and a half-life of 6 hours. After 6 hours: 400. After 12 hours: 200. After 18: 100. After 24: 50. That is four half-lives in 24 hours, so the activity has fallen to 800 ÷ 2⁴ = 50. Note it never reaches zero by halving — which is why waste with a half-life of thousands of years is a storage problem rather than a waiting one.
Isotopes: same element, different mass
Every atom of an element has the same number of protons, which is its ATOMIC NUMBER, but the number of neutrons can vary — atoms of the same element with different neutron counts are ISOTOPES. Carbon-12 and carbon-14 both have 6 protons but 6 and 8 neutrons; both are carbon, but carbon-14 is unstable and decays, which is what makes radiocarbon dating possible.
Balancing a nuclear equation
An alpha decay drops the mass number by 4 and the atomic number by 2, because an alpha particle IS a helium nucleus, written ⁴₂He. A beta decay leaves the mass number unchanged but raises the atomic number by 1, because a neutron has become a proton and an electron has been emitted. Either way, the mass numbers on both sides must add up, and so must the atomic numbers — that balance is what the exam is checking.