Atom

In chemistry and physics, an atom (Greek άτομον meaning "indivisible") is the smallest possible particle of a chemical element that retains its chemical properties. The word atom may also refer to the smallest possible indivisible fundamental particle. This definition must not be confused with that of chemical atoms, since chemical atoms (hereafter "atoms") are composed of smaller subatomic particles.

Most atoms are composed of three types of subatomic particles which govern their external properties:

• electrons, which have a negative charge and are the least massive of the three;


• protons, which have a positive charge and are about 1836 times more massive than electrons; and


• neutrons, which have no charge and are about 1838 times more massive than electrons.

Protons and neutrons are both nucleons and make up the dense, massive atomic nucleus. The electrons form the much larger electron cloud surrounding the nucleus.

Atoms differ in the number of each of the subatomic particles they contain. The number of protons in an atom (called the atomic number) determines the element of the atom. Within a single element, the number of neutrons may also vary, determining the isotope of that element. Atoms are electrically neutral if they have an equal number of protons and electrons. Electrons that are furthest from the nucleus may be transferred to other nearby atoms or even shared between atoms. Atoms which have either a deficit or a surplus of electrons are called ions. The number of protons and neutrons in the atomic nucleus may also change, via nuclear fusion, nuclear fission or radioactive decay.

Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. Atoms are able to bond into molecules and other types of chemical compounds. Molecules are made up of multiple atoms; for example, a molecule of water is a combination of two hydrogen atoms and one oxygen atom.