An atom consists of a nucleus containing protons and neutrons surrounded by electrons.

A proton has a relative charge of . The actual charge of a proton is . We use relative charges to keep nuclear and atomic equations neat. The relative charge of an electron is , but its actual charge is . The relative charges enable us to quickly compare the proton and the electron. A neutron has zero charge.

, this is the common notion for representing atoms. The top number represents the mass number of the atom, indicating the total number of protons and neutrons within the atom. It is the relative mass, as protons and neutrons do not have a mass of one but and respectively. The bottom number represents the atomic number of the atom, which equals the number of protons inside the nucleus.

, is another atom of chlorine, it has the same atomic number as , but has a a different mass number. These are Isotopes. Isotopes are different atoms of the same element, with the same number of protons but different number of neutrons.

The specific charge of an atom is the total charge of the atom by the mass of the atom.

The specific charge of the chlorine-35 atom is .

*Note, this equation is not given to you on your formula sheet

Inside the nucleus, the electrostatic force of repulsion acts between the positively charged protons. To keep the nucleus together, another force acts, known as the strong nuclear force.

The strong nuclear force is an attractive short-range force, acting on nucleons between . Less than it is repulsive.

The N-Z diagram below shows us that for small nuclei, up to around an atomic number of the number of protons inside the nucleus equals the number of neutrons. However, for large nuclei, the stability line curves to the left, indicating that large nuclei have more neutrons than protons. The electrostatic force of repulsion increases as the number of protons increases, so to maintain stability, large nuclei have more neutrons than protons to increase the strong nuclear force.

If an atom falls to the left of the stability line, decay occurs as there are too many neutrons inside the nucleus.

If an atom falls to the right of the stability line decay occurs as there are too many protons inside the nucleus.

If an atom is unstable, either:

• It contains too many protons, in which case Beta-positive decay occurs, , where a positron (antiparticle of an electron), relative charge , is emitted from the nucleus. Or electron-capture occurs, where a proton captures an inner shell electron.

• It contains too many neutrons, in which case Beta-minus decay occurs, , where a fast-moving electron, relative charge , is emitted from the nucleus.

• It contains too many nucleons, in which case alpha decay occurs, . An alpha particle is a helium nucleus, consisting of protons and neutrons. Relative charge .

• It is excited (usually as the result of another decay), in which case gamma decay occurs, , when the excited nucleus de-excites, emitting a high-energy electromagnetic wave.

Alpha and beta decay are known as ionising radiation, as they knock electrons off atoms when they collide. Beta is less ionising than alpha, as beta particles are smaller; therefore, fewer collisions occur per metre, resulting in less energy loss, which explains why beta has a longer range.

Penetration And Range For Different Materials

Decay Equations

When writing nuclear decay equations, we must ensure that the total number of nucleons is conserved, charge, lepton and baryon numbers are conserved (more on this later).

For alpha decay, as a helium nucleus is emitted, the decay product will have two less protons and neutrons. The mass number will thus drop by four, and the atomic number will drop by two.

For beta-minus decay, as the nucleus has too many neutrons, one of the neutrons in the nucleus changes into a proton; thus, the mass number stays the same, but the atomic number will increase by one.

We need to balance the lepton number (more on conservation rules later). A beta-minus particle is emitted, which is an electron. An electron is a lepton (a type of particle), as there were no leptons to begin with, and we end with a lepton, we also need an anti-lepton to conserve total lepton number. This anti-lepton is known as an anti-neutrino. It has zero mass and zero charge. If we examine what happens inside the nucleus, we can see that the charges are balanced, with a total charge of zero before and after the decay. .

For beta-plus decay, as the nucleus has too many protons, one of the protons in the nucleus decays into a neutron, either by beta-positive decay or by electron capture.

In beta-positive decay, as a proton changes into a neutron, the mass number remains the same, but the atomic number decreases by one.

We need to balance the lepton number. A beta-positive particle is emitted, which is a positron. A positron is an anti-lepton. As there were no leptons to begin with, and we end with an anti-lepton, we also need a lepton to conserve total lepton number. This lepton is known as a neutrino. It has zero mass and zero charge. If we examine what happens inside the nucleus, we can see that the charges are balanced, with a total charge of before and after the decay. .

In electron capture, a proton captures an orbiting electron, decaying into a neutron; the mass number stays the same, but the atomic number decreases by one.

We need to balance the lepton number. An electron is a lepton. As there is one lepton to begin with, we need one lepton after the decay to conserve total lepton number. This lepton is known as a neutrino. It has zero mass and zero charge. If we examine what happens inside the nucleus, we can see that the charges are balanced, with a total charge of zero before and after the decay.
. An EM wave is also emitted in this decay, as when the inner shell electron is captured by the nucleus, an outer shell electron falls down to this now vacant lower energy level, releasing a gamma ray.