Atom
Quick Definition
An atom is the smallest unit of a chemical element that retains all the chemical properties of that element, consisting of a nucleus of protons and neutrons surrounded by electrons.
Atom explained
An atom is the fundamental building block of matter. Every substance around you — the air you breathe, the water you drink, the screen you are reading — is made of atoms. Each atom consists of three types of subatomic particles: protons (positively charged) and neutrons (neutral) packed into a dense central nucleus, surrounded by electrons (negatively charged) that occupy regions of space called orbitals or electron shells.
The number of protons in an atom's nucleus is called the atomic number, and it uniquely defines which chemical element the atom belongs to. An atom of hydrogen has 1 proton; carbon has 6; oxygen has 8; gold has 79. When the number of protons equals the number of electrons, the atom is electrically neutral. If electrons are gained or lost, the atom becomes a charged ion. The number of neutrons can vary within the same element — these variants are called isotopes.
Atoms are almost unimaginably small. A human hair is roughly one million carbon atoms wide. Despite their tiny size, their internal structure determines everything about how elements behave chemically — how they bond with other atoms, what compounds they can form, and what physical and chemical properties those compounds have. Modern atomic theory, developed through the work of Dalton, Thomson, Rutherford, Bohr, and quantum physicists, has made chemistry and materials science precise quantitative disciplines.
Synonyms & Related Terms
Real-World Example
A gold ring contains approximately 1.5 × 10²⁴ gold atoms — 1.5 septillion atoms — each with 79 protons, 79 electrons, and typically 118 neutrons. The way gold atoms bond to each other (metallic bonding through shared electron clouds) gives gold its characteristic malleability, ductility, and resistance to corrosion. Changing even the ratio of gold to other elements — adding silver or copper to make an alloy — alters these properties, which is exactly how different carats of gold are made.
Why It Matters
Understanding atoms is foundational to chemistry, materials science, physics, and biology. Every chemical reaction is, at its core, atoms rearranging their bonds. The properties of materials — strength, conductivity, colour, reactivity — all stem from atomic structure. In medicine, nuclear imaging (PET, MRI) and radiotherapy rely on nuclear properties of atoms. In energy, nuclear power plants split uranium atoms (fission), and experimental fusion reactors fuse hydrogen atoms to release enormous energy.
Industry Applications
Related Standards
Frequently Asked Questions
What is an atom?
An atom is the smallest unit of a chemical element that retains the element's chemical properties. It consists of a nucleus (protons and neutrons) surrounded by electrons. The number of protons determines which element the atom is.
What are the three subatomic particles?
Protons (positive charge, in the nucleus), neutrons (no charge, in the nucleus), and electrons (negative charge, orbiting the nucleus). Protons determine the element, neutrons determine the isotope, and electrons determine chemical bonding and reactivity.
What is an atomic number?
The atomic number is the number of protons in an atom's nucleus. It uniquely identifies the element — hydrogen has atomic number 1, carbon 6, oxygen 8, iron 26, gold 79. The Periodic Table arranges elements in order of increasing atomic number.
What is the difference between an atom and a molecule?
An atom is the smallest unit of a single element. A molecule is formed when two or more atoms bond together chemically. Water (H₂O) is a molecule made of two hydrogen atoms and one oxygen atom. A molecule can consist of atoms of the same element (O₂, nitrogen N₂) or different elements (H₂O, CO₂).
What is an isotope?
Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons. Carbon-12 has 6 protons and 6 neutrons; carbon-14 has 6 protons and 8 neutrons. Some isotopes are stable; others are radioactive and decay over time. Carbon-14 is used in radiocarbon dating because of its predictable decay rate.
How small is an atom?
Atoms are extremely small. A typical atom has a diameter of around 0.1–0.5 nanometres (1–5 × 10⁻¹⁰ metres). To put this in perspective, a human hair is about 100,000 nanometres wide — meaning roughly one million carbon atoms could be placed side by side across it.
What is an ion?
An ion is an atom (or molecule) that has gained or lost one or more electrons, giving it a net electrical charge. Atoms that lose electrons become positive ions (cations); atoms that gain electrons become negative ions (anions). Common examples: Na⁺ (sodium ion, lost one electron) and Cl⁻ (chloride ion, gained one electron) — together they form table salt (NaCl).
Can atoms be split?
Yes. Nuclear fission splits a heavy atomic nucleus (such as uranium-235 or plutonium-239) into smaller fragments, releasing enormous amounts of energy. This is the principle behind nuclear power plants and nuclear weapons. The energy released comes from the binding energy that held the nucleus together — described by Einstein's famous equation E = mc².
See Also
- Energy
- Gravity
- Velocity