Atomic structure & radioactivity flashcards

Key points

• A tiny, positive nucleus of protons and neutrons, with electrons in shells around it.
• Almost all the mass is in the nucleus, which is less than 1/10,000 of the atom's width.
• An atom's radius is about 1 × 10⁻¹⁰ m. It has no overall charge: it has as many electrons as protons.
• Atomic number = protons. Mass number = protons + neutrons.

Key points

• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
• Carbon-12 and carbon-14 both have 6 protons, but 6 and 8 neutrons.
• Some isotopes have unstable nuclei, which give out radiation to become more stable: they are radioactive.
• Radioactive decay is random. Nothing, not heating or squeezing, changes when a nucleus decays.

Neutrons in carbon-14

Key points

• The plum pudding model was a ball of positive charge with electrons stuck in it.
• Alpha particles were fired at thin gold foil. Most went straight through, so the atom is mostly empty space.
• A very few bounced back, so the mass and positive charge must be packed into a tiny nucleus.
• This gave the nuclear model. Bohr then put the electrons in shells, and Chadwick found the neutron.

Key points

• Alpha: two protons and two neutrons, the same as a helium nucleus. Charge +2.
• Beta: a high-speed electron, made in the nucleus when a neutron turns into a proton.
• Gamma: electromagnetic radiation from the nucleus, with no mass and no charge.

Key points

• Alpha: stopped by a sheet of paper, skin or a few centimetres of air. The most ionising.
• Beta: passes through paper, but stopped by a few millimetres of aluminium.
• Gamma: very penetrating. It takes thick lead or metres of concrete to absorb most of it. The least ionising.
• Gamma has no charge or mass, so it rarely interacts with atoms and travels a long way.

Key points

• The alpha particle takes away 2 protons and 2 neutrons.
• So the mass number falls by 4 and the atomic number falls by 2: a new element forms.
• The mass numbers and the atomic numbers must balance on both sides.

Radium-226 (88 protons) emits alpha

Key points

• Beta decay: a neutron turns into a proton and an electron is shot out.
• So the atomic number rises by 1, and the mass number stays the same.
• Gamma emission changes neither number: it only carries away energy.

Strontium-90 (38 protons) emits beta

Key points

• Half-life is the time for the number of unstable nuclei in a sample, or its activity, to halve.
• Activity is the rate of decay, in becquerels (Bq): 1 Bq is one decay per second.
• Decay is random, but a sample has so many nuclei that the half-life is very predictable.

800 Bq, half-life 2 h: left after 6 h

Key points

• Pick an activity on the vertical axis, then halve it.
• Read off the time for each value from the curve: the gap between them is the half-life.
• It is the same from any starting point, 800 to 400 Bq or 400 to 200 Bq.
• Each half-life halves what is left: ½, then ¼, then ⅛.

The fraction left after 4 half-lives

Key points

• Irradiation: being exposed to radiation from an outside source. It stops when the source is removed.
• Contamination: unwanted radioactive atoms getting onto or into an object or a person.
• Contamination is usually worse: the atoms keep decaying until they are removed or decay away.
• Alpha is fairly safe outside the body, stopped by skin, but very harmful inside it, where it ionises nearby cells.

Key points

• Distance: hold sources with long tongs, at arm's length.
• Shielding: keep sources in lead-lined boxes and work behind screens.
• Time: spend as little time near the source as possible.
• Workers wear badges that measure the dose they have received.