Nuclear Reactions

AZ notation

There are four corners around a chemical symbol

The two slots on the right-hand side are reserved for chemists.

The top left space holds the number of particles, regardless of whether  they are protons or neutrons. This number is called the 'atomic mass number' and is denoted by A

The bottom left space holds the total positive charge. This number is called the 'atomic number' and is denoted by Z. If the symbol is referring to a nucleus, then Z is equal to the number of protons present.

Note the following points:

• If the context is that of a neutral atom, then there will be Z electrons around the nucleus. These do not feature in the notation. If the context is of an ion, this will not be true. There is no way of determining the context from the symbol unless the top right hand box has been filled in, presumably by a chemist.
• Any nucleus containing 82 protons has to be the nucleus of a lead atom. Strictly speaking, the figure 82 and the symbol Pb are not both needed; but both are always given at an elementary level. You will sometimes see this substance called lead-207 or ²⁰⁷Pb.
• The number of neutrons present may be deduced by subtracting Z from A.

In constructing nuclear equations you have to ensure that the A numbers add up to the same on both sides and that the bottom numbers add up on both sides. For each of the equations below, you should check that this is so. (Note that, unfortunately, html does not allow one easily to present equations with the A number directly above the Z number)

Alpha decay

An alpha particle is a bundle of two protons and two neutrons, ejected at high speed from a nucleus. This obviously leaves the nucleus lighter and less positively charged. For example Americium changes to Neptunium:

²⁴¹₉₅Am  ®  ²³⁷₉₃Np + ⁴₂a + energy (carried off as KE by the alpha particle)

Beta decay

A beta particle is produced when a neutron decays into a proton. A beta particle is identical with an electron. An anti-neutrino is produced at the same time:

¹₀n  ®  ¹₁p + ⁰_-1b + ⁰₀n + energy (carried off as KE by the beta particle and anti-neutrino)

For example, Strontium changes to Yttrium in this reaction:

⁹⁰₃₈Sr  ®  ⁹⁰₃₉Y + ⁰_-1b + ⁰₀n + energy (carried off as KE by the beta particle and anti-neutrino)

Gamma decay

Gamma rays are electromagnetic rays rather than charged or uncharged particles, and they are emitted when the particles of a nucleus re-arrange themselves, settling down into a less energetic state. One does not usually bother to write an equation, since no changes take place.

Fission

The most common splitting of the atom is for an alpha particle to break away. But Uranium-236 and Plutonium-240 can split into more equal-sized pairs of particles. For example:

²³⁶₉₂U  ®  ¹⁰⁵₄₂Mo + ¹²⁸₅₀Sn + 3 ¹₀n + energy (carried off as KE mostly by the Molybdenum and Tin nuclei)

The three neutrons can, in principle, combine with any U-235 that happens to be around to make fresh supplies of U-236, and the process can repeat at an escalating rate: see nuclear fission.

An extremely common use of fission reactions of the kind just illustrated is in the fission reactor.