To describe nuclear decay reactions, chemists have extended the notation for nuclides to include radioactive emissions. Table 26.1 "Nuclear Decay Emissions and Their Symbols" lists the name and symbol for each type of emitted radiation. 

Table 26.1 Nuclear Decay Emissions and Their Symbols

Like the notation used to indicate isotopes, the upper left superscript in the symbol for a particle gives the mass number, which is the total number of protons and neutrons. For a proton or a neutron, A = 1. Because neither an electron nor a positron contains protons or neutrons, its mass number is 0. The numbers should not be taken literally, however, as meaning that these particles have zero mass; ejection of a beta particle (an electron) simply has a negligible effect on the mass of a nucleus.

Similarly, the lower left subscript gives the charge of the particle. Because protons carry a positive charge, Z = +1 for a proton. In contrast, a neutron contains no protons and is electrically neutral, so Z = 0. In the case of an electron, Z = ?1, and for a positron, Z = +1. Because ? rays are high-energy photons, both A and Z are 0. In some cases, two different symbols are used for particles that are identical but produced in different ways. For example, the symbol

which is usually simplified to e^?, represents a free electron or an electron associated with an atom, whereas the symbol

which is often simplified to ?^?, denotes an electron that originates from within the nucleus, which is a ? particle. Similarly,
refers to the nucleus of a helium atom, and

denotes an identical particle that has been ejected from a heavier nucleus.There are six fundamentally different kinds of nuclear decay reactions, and each releases a different kind of particle or energy. The essential features of each reaction are shown in Figure 26.2 "Common Modes of Nuclear Decay". The most common are alpha and beta decay and gamma emission, but the others are essential to an understanding of nuclear decay reactions.

Figure 20.4 Common Modes of Nuclear Decay