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.