Atom
An atom is the smallest unit of a chemical element that retains the chemical properties of that element, consisting of a central nucleus surrounded by electrons. Atoms are the fundamental building blocks of all ordinary matter, and their arrangement and bonding determine the physical and chemical properties of substances. Understanding atomic structure is central to chemistry, physics, and materials science.
Subatomic Particles of an Atom
| Particle | Symbol | Charge | Relative Mass | Location |
|---|---|---|---|---|
| Proton | p⁺ | +1 | 1 | Nucleus |
| Neutron | n⁰ | 0 | 1 | Nucleus |
| Electron | e⁻ | -1 | 1/1836 | Electron shells |
| Nucleus | — | Positive | ~Total atomic mass | Center of atom |
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Related Terms
Proton
A proton is a positively charged subatomic particle found in the nucleus of every atom, with a charge of +1 elementary charge and a mass of approximately 1.673 × 10⁻²⁷ kg (about 1 atomic mass unit). The number of protons in an atom's nucleus defines the element and is called the atomic number, which determines the chemical identity of the atom. Protons are composed of two up quarks and one down quark held together by the strong nuclear force.
Electron
An electron is a negatively charged subatomic particle that occupies the space around an atom's nucleus in regions called orbitals or electron shells, with a charge of -1 elementary charge and a mass of 9.109 × 10⁻³¹ kg (about 1/1836 the mass of a proton). Electrons govern chemical bonding, electrical conductivity, and the optical properties of matter by determining how atoms interact with one another. In a neutral atom, the number of electrons equals the number of protons.
Atomic Nucleus
The atomic nucleus is the small, dense, positively charged core at the center of an atom, containing protons and neutrons (collectively called nucleons) held together by the strong nuclear force. Although the nucleus is approximately 10⁻¹⁵ m (1 femtometre) in diameter — about 100,000 times smaller than the atom itself — it contains nearly all the atom's mass. Nuclear reactions, such as fission and fusion, involve changes to the nucleus and release enormous amounts of energy.
From Greek "atomos" meaning "indivisible" or "uncuttable", from "a-" (not) + "temnein" (to cut). The concept was proposed by ancient Greek philosophers Leucippus and Democritus (~450 BCE), though the modern scientific atom was established by John Dalton in 1803.