The densest region of an atom is the nucleus, containing protons and neutrons—collectively called nucleons.

The type of atom defined by a specific number of protons and neutrons is referred to as a nuclide. The notation for a nuclide comprises an element symbol, atomic number, and mass number.

One of several shorthand notations for nuclides uses the element name, a hyphen, and the mass number.

Nuclides with the same atomic number but different mass numbers are called isotopes of each other. Thus, carbon has three isotopes shown here.

Nuclides are also characterized by their energy state. For example, the single isotope technetium-99 exists in two different states: the lower energy ground state and a long-lived excited state called a metastable state. These two species, even though they have the same numbers of protons and neutrons, are different nuclides.

Interestingly, some elements in the periodic table have stable nuclides, which remain intact indefinitely. In contrast, some elements have only unstable nuclides, called radionuclides. For example, the spontaneous nuclear disintegration of uranium-238 produces thorium-234. The process is termed radioactive decay.

The daughter nuclide produced during the decay may be stable or radioactive. The process is accompanied by the emission of small fragments or electromagnetic radiation. 

Alpha particles are equivalent to helium nuclei. Their emission reduces the atomic number by 2 and the mass number by 4.

Beta particles are equivalent to electrons; when emitted, the atomic number of the daughter nuclide increases by 1. As they carry a negative charge, it is called beta-minus radiation.

The emission of a positron, which has the same mass as an electron but opposite charge, decreases the atomic number by 1. It is often termed beta-plus emission.

Gamma rays are high-energy electromagnetic radiation, the emission of which changes neither atomic nor mass number.

Proton emission decreases the mass number and atomic number by 1 each, while neutron emission reduces the mass number by 1.

Nuclear equations map the difference between parent and daughter nuclides and indicate the nature of decay.

The radioactive decay of uranium-238 to thorium-234 is accompanied by the emission of alpha particles.

Nuclear equations are balanced just like chemical equations. The sum of the mass numbers is the same on each side of the equation. As this is alpha decay, so is the sum of the atomic numbers.