The Nuclear Atom

Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons.
The nucleus at the centre of the atom contains both the protons and neutrons, collectively known as nucleons.
The nucleus is incredibly dense and holds the majority of the atom’s mass, despite being much smaller than the atom overall.
Electrons are located in the space outside the nucleus, called the electron cloud. This region is much less dense but makes up the majority of the atom’s physical size.

Protons are positively charged, with each one contributing a relative charge of +1 and a relative mass of 1.
Neutrons carry no charge (they are neutral) but contribute a relative mass of 1, same as the proton.
Electrons are negatively charged, contributing a relative charge of -1. However, their mass is quite negligible (around 1/2000th of a proton or neutron) and is often approximated as zero in atomic structure discussions.

The atomic number (also called proton number) of an atom is the number of protons in its nucleus, which determines the type of element it is. Elements of the periodic table are arranged in increasing order of atomic number.
The mass number (also called nucleon number) of an atom is the total number of protons and neutrons in its nucleus.

Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons, and therefore different mass numbers.
Isotopes of an element share similar chemical properties, as these are determined largely by the number of electrons (which is the same for all isotopes of a given element).

Some atomic nuclei are unstable — typically those with a high number of protons. These atoms can become more stable by releasing energy through a process called radioactive decay.
There are three main types of radioactive decay: alpha decay (release of a helium-4 nucleus), beta decay (transformation of a neutron into a proton and an electron, with the electron emitted), and gamma decay (release of energy in the form of gamma rays).