Atomic Structure | AQA GCSE Physics Notes | Topic 4

• Atoms have a small radius of about 1 × 10^-10 metres.
• The radius of a nucleus is less than 1/10 000 of the radius of an atom.
• Atoms have a positively charged nucleus with protons and neutrons, surrounded by negative electrons.
• Most of the mass of an atom is in the nucleus.
• The electrons are arranged in shells. They may change arrangement if electromagnetic radiation is absorbed (move further from the nucleus; a higher energy level).
• Atoms have the same number of protons and electrons unless they get charged.

• Atomic number: number of protons (should be the number of electrons too).
• Mass number: number of protons and neutrons added up.
• Ions: charged particles (when an atom has lost or gained an electron).
• Isotopes: atoms with the same number of protons but different number of neutrons.
• Before the discovery of the electron: atoms as tiny spheres that could not be divided.
• Plum Pudding Model: the atom as a ball of positive charge with negative electrons embedded in it.
• Alpha particle scattering experiment: they shot alpha particles at gold foil. Most particles went straight through, some deflected, very few reflected. Conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged.
• Bohr: improved the nuclear model, saying that electrons orbit the nucleus at specific distances (shells). Experiments also found each proton as one unit of positve charge.
• Chadwick: showed the existence of neutrons in the nucleus 20 years after the nucleus was found.
• Radioactive decay: when unstable atomic nuclei give out radiation to become more stable.
• Activity: the rate at which a source of unstable nuclei decays. Becquerel (Bq).
• Count-rate is the number of decays recorded each second by a detector (eg Geiger-Muller tube).
• an alpha particle (α): mass number = 4, proton number = 2 (same as a helium nucleus)
• a beta particle (β): mass number = 0, proton number = -1 (a high speed electron ejected from the nucleus as a neutron turns into a proton)
• a gamma ray (γ): electromagnetic radiation from the nucleus
• neutron decay (n) - no explanation needed
• alpha is the least penetrating and most ionising. gamma is the most penetrating and least ionising.
• alpha is absorbed by a sheet of paper, beta is absorbed by 5mm aluminium, gamma is absorbed by 10cm lead.
• Ionising power: can knock off electrons and damage cells.
• Radioactive decay is random.
• Half-life: time taken for the number of nuclei of the isotope in a sample to half.
• Radioactive contamination: unwanted presence of materials containing radioactive atoms on other materials. The hazard from contamination is due to the decay of the contaminating atoms. The type of radiation emitted affects the level of hazard.
• Irradiation: exposing an object to nuclear radiation. The irradiated object does not become radioactive. This is everywhere e.g. rocks, banana, weapons.
• Radioactive isotopes have a very wide range of half-life values. A short half life is important e.g. for medical equipment.
• Nuclear radiations are used in medicine to explore internal organs, control or destruct unwanted tissue.

AQA GCSE Physics Notes for Triple Science Students Only

• Nuclear fission is the splitting of a large and unstable nucleus (eg uranium or plutonium).
• Spontaneous fission is rare.
• The unstable nucleus absorbs a neutron. The nucleus undergoing fission splits into two smaller nuclei and emits two or three neutrons plus gamma rays.
• Energy is released. All of the fission products have kinetic energy.
• The neutrons may go on to start a chain reaction.
• The chain reaction is controlled in a nuclear reactor to control the energy released.
• Explosions caused by a nuclear weapon is caused by an uncontrolled chain reaction.
• Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. Some of the mass may be converted into energy.