Radioactive emissions — OCR GCSE Physics

Atoms of one element always share the same proton number, but they need not share the same neutron number. Variants of an element with different neutron counts are isotopes, and because neutrons carry mass, isotopes differ in nuclear mass while keeping the same nuclear charge and chemical identity. For example, chlorine-35 has 17 protons and 18 neutrons, while chlorine-37 has 17 protons and 20 neutrons; both are chlorine, but the second is heavier. A reliable method is to subtract the proton number from the mass number to find the neutron count, then compare two atoms of the same element. In a multiple-choice question, accept an option that keeps the proton number fixed and varies the neutron number, and reject options that change the element or move electrons.

Isotopes are atoms of the same element whose nuclei hold the same number of protons but different numbers of neutrons. The conventional nuclear symbol places the mass number A as a superscript before the element symbol and the atomic number Z as a subscript before it, so a nucleus is written with A above Z to the left of the symbol. For carbon-12 the symbol shows A = 12 and Z = 6; carbon-14 shows A = 14 and Z = 6. Because Z fixes the element, both are carbon, so identity is unchanged. The nucleus charge is +Z times the elementary charge, so both carbon isotopes carry a nuclear charge of +6e; the extra neutrons in carbon-14 add mass but no charge. Mass number counts protons plus neutrons, so the mass difference of two units equals two extra neutrons. Use A − Z to find the neutron number and compare isotopes by reading the superscript and subscript directly.

Many nuclei are stable, but some have too much energy or an unsuitable balance of protons and neutrons and are unstable. An unstable nucleus can lose energy by emitting radiation, and this happens randomly rather than at a set time. The emissions named are an alpha particle (two protons and two neutrons, a helium nucleus), a beta particle (a fast electron produced when a neutron changes into a proton), a neutron, and gamma rays, which are electromagnetic radiation of very short wavelength and carry no mass or charge. Emitting an alpha particle lowers the mass number by 4 and the atomic number by 2; beta emission leaves the mass number unchanged and raises the atomic number by 1; gamma emission changes neither. A single nucleus may emit one or more of these, and the type emitted depends on the nucleus, not on temperature or chemical form.

An unstable nucleus can shed energy and particles, and each emission changes the nucleus in a particular way. An alpha particle is a helium nucleus, 2 protons and 2 neutrons, so emitting one lowers the mass number by 4 and the proton (atomic) number by 2, reducing both mass and positive charge. A beta particle is a fast electron created when a neutron changes into a proton, so the mass number stays the same while the proton number rises by 1, increasing the positive charge. Gamma radiation is a high-energy electromagnetic wave with no mass and no charge, so it changes neither mass number nor proton number; it usually accompanies alpha or beta emission as the nucleus loses surplus energy. A neutron emission removes one neutron, lowering the mass number by 1 while the proton number is unchanged, so charge stays the same. Track mass number and proton number separately to predict the change.

A nuclear equation is balanced when the mass numbers and the proton numbers each add up to the same total on both sides. Write the parent nucleus with its mass number as a superscript and its proton number as a subscript, for example uranium-238 as ²³⁸₉₂U. Then write the emitted particle with its own numbers: an alpha particle as ⁴₂He, a beta particle as ⁰₋₁e, and a gamma photon as ⁰₀γ. The daughter nucleus is found by subtraction. For alpha decay of ²³⁸₉₂U, the daughter has mass number 238 − 4 = 234 and proton number 92 − 2 = 90, giving ²³⁴₉₀Th. For beta decay the mass number is unchanged and the proton number rises by 1. Check the totals on each side match before accepting an equation. Use element symbols correctly and keep the superscript and subscript in their proper positions.

An emission equation is balanced when the mass numbers and the atomic (proton) numbers on both sides agree. Write the parent nuclide on the left and the daughter nuclide plus the emitted particle on the right. An alpha particle is a helium nucleus, so it carries mass number 4 and charge 2⁺; emitting it lowers the mass number by 4 and the atomic number by 2. A beta particle is an electron emitted when a neutron changes into a proton, so it carries mass number 0 and charge 1⁻; emitting it leaves the mass number unchanged and raises the atomic number by 1. A gamma photon carries mass number 0 and charge 0, so emitting it changes neither number. For example, uranium-238 emitting an alpha particle gives thorium-234: 238 = 234 + 4 and 92 = 90 + 2. Check both totals before choosing an answer.

Electrons in an atom occupy energy levels at different distances from the nucleus, with inner levels closer and lower in energy. An inner electron can absorb energy from electromagnetic radiation and become excited, rising to a higher energy level. The electron is unstable there, so it drops back and emits the energy difference as electromagnetic radiation, often seen as a characteristic line in a spectrum. Separately, an atom becomes a positive ion when it loses one or more outer electrons; this removal is called ionisation. Keep the two ideas distinct: excitation moves an electron between levels inside the atom, whereas ionisation removes an outer electron entirely. For example, a gas discharge tube shows coloured light because excited electrons emit radiation as they fall back, while a spark or radiation can ionise gas atoms by stripping outer electrons.

Atoms and nuclei are not fixed, silent objects: when their internal arrangement changes, energy can leave as radiation or be taken in from radiation already present. The key idea is that these changes are not limited to one band of the electromagnetic spectrum. A rearrangement of electrons in an atom, for example an electron dropping to a lower energy level, can emit visible or ultraviolet light, while a change inside a nucleus can emit gamma radiation. Absorption works the other way: incoming radiation of a suitable frequency can be taken up and change the atom or nucleus. So the radiation linked to atomic and nuclear changes may come from any part of the electromagnetic spectrum, and gamma rays are one important example. When answering, link the change (atomic or nuclear) to the radiation produced or absorbed, and remember the spectrum is wider than just gamma.

Radioactive decay is random: you cannot predict which particular nucleus will decay next, or exactly when it will happen. Despite that randomness, a large sample shows a predictable statistical pattern. The half-life is the time taken for the number of undecayed nuclei in a sample to halve, or equivalently for the count rate from the sample to fall to half its starting value. Because individual decays are random, half-life is a statistical property of a large group of nuclei, not a timer attached to one nucleus. After one half-life half the nuclei remain, after two half-lives one quarter remain, and so on. A useful method is to halve the count rate repeatedly and add the half-life each time until you reach the measured value. When answering, connect the random nature of decay to why half-life describes a large sample rather than predicting a single nucleus.

After each half-life the number of undecayed nuclei, and so the activity, halves. After n whole half-lives the fraction remaining is (1/2)ⁿ, so the net decline expressed as a ratio is the remaining fraction compared with the original, written as 1 : 2ⁿ. For example, after 3 half-lives the fraction remaining is (1/2)³ = 1/8, so the ratio of remaining to original is 1 : 8, and the decline itself is 7/8 of the original. Read the number of half-lives from a half-life graph by counting successive halvings of the activity on the vertical axis, or divide the elapsed time by the half-life to find n, then apply the power. Keep n a whole number as the statement requires, and state clearly whether the ratio describes what remains or what has decayed.

The three emissions differ in how far they travel through matter because of their charge, mass and nature. Alpha particles are relatively heavy and carry a 2⁺ charge, so they ionise strongly and are stopped by a few centimetres of air or a sheet of paper. Beta particles are lighter and carry a 1⁻ charge, ionise less strongly and are stopped by a few millimetres of aluminium. Gamma rays are electromagnetic waves with no charge, ionise weakly and are reduced but not completely stopped by thick lead or concrete. Recall the order of penetrating power as alpha least, beta intermediate and gamma greatest, and link each to the material that absorbs it.