IGCSE §7: Radioactivity and Particles · IGCSE
The Structure of the Atom
Revision notes on The Structure of the Atom for the OCR A-level Physics specification (H556). Free to read, with 4 practice questions in the app.
Atom — a central nucleus containing protons and neutrons, surrounded by electrons in orbits.
The three particles:
| Particle | Relative charge | Relative mass | Where |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | 1/1836, effectively 0 | Orbiting the nucleus |
Nucleon — a proton or a neutron; that is, anything found in the nucleus.
Atomic (proton) number, Z — the number of protons in the nucleus. It determines which element the atom is: every carbon atom has 6 protons, and anything with 6 protons is carbon.
Mass (nucleon) number, A — the total number of protons and neutrons.
So the number of neutrons is found by subtraction:
neutrons = A − Z
Atoms are neutral overall, because the number of electrons equals the number of protons and their charges are equal and opposite.
Ion — an atom that has lost or gained electrons, so it carries a net charge. Losing an electron leaves a positive ion; gaining one makes a negative ion. Note that the nucleus is unchanged — it is still the same element.
The scale of the thing. The nucleus is roughly 10 000 times smaller in radius than the atom, yet it holds almost all the mass. If an atom were the size of a football stadium, the nucleus would be a pea on the centre spot, and everything between would be empty. Nearly all of what feels solid is empty space held apart by electric forces.
Rutherford's alpha scattering experiment established this. Alpha particles were fired at thin gold foil, and most passed straight through — showing the atom is mostly empty. A very few were deflected through large angles, and some bounced straight back, which could only happen if the positive charge and nearly all the mass were concentrated in a tiny, dense nucleus. The plum pudding model, in which charge was spread evenly, predicted no such deflections.
Example: an atom with A = 23 and Z = 11 is sodium. It has 11 protons, 23 − 11 = 12 neutrons, and 11 electrons when neutral.
4 questions on The Structure of the Atom
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