Atom

An atom is the smallest unit of matter that retains the characteristics of a chemical element. All tangible matter—air, water, metal—is composed of large numbers of atoms bonded together.

An atom consists of a nucleus and orbiting electrons. The nucleus is made up of protons (positively charged particles) and neutrons (uncharged particles). Electrons carry a negative charge. A neutral atom is electrically balanced, meaning the number of protons and electrons are equal.

Structure

An atom measures approximately a few tenths of a nanometer across, while the nucleus is thousands of times smaller. Most of an atom's volume is empty space through which electrons move. The atom's mass, by contrast, is concentrated in the nucleus, because protons and neutrons are far heavier than electrons.

Elements and Isotopes

The number of protons in the nucleus determines the element (for example, hydrogen has one proton, carbon has six). This number is called the atomic number. Atoms of the same element with different numbers of neutrons are called isotopes. Some isotopes are stable, while others decay by emitting radiation (radioactivity).

Chemical Bonding

Atoms form molecules and crystals by sharing or transferring outer electrons. When electrons are shared, a covalent bond forms; when an electron transfers, an ionic bond forms. The chemical behavior of an element is largely determined by how its outer electrons are arranged.

History of the Concept

The word derives from the Greek 'atomos', meaning 'indivisible'. In the 19th and 20th centuries, experiments measuring electrical charge and particle scattering demonstrated that atoms have internal structure. Today the atom is explained through quantum mechanics, in which electrons are described not as fixed orbits but as probability regions called orbitals.