Atom: definition in chemistry
Atom: The smallest particle of an element that still has the chemical properties of that element. It consists of a tiny nucleus of protons and neutrons surrounded by electrons.
Atoms are extremely small, mostly between 0.1 and 0.5 nanometers across, and are mostly empty space. An atom has no overall charge because its number of electrons equals its number of protons. Atoms join through chemical bonds to form molecules and compounds, and they are rearranged, not destroyed, in chemical reactions. The idea that matter is made of atoms goes back to ancient Greece, but John Dalton turned it into a scientific theory in the early 1800s.
Examples
- Hydrogen atom — one proton and one electron
- Carbon-12 atom — 6 protons, 6 neutrons and 6 electrons
- Gold atom — about 0.29 nm across
- A single drop of water contains more than 10²¹ atoms
On the periodic table
Each box on the periodic table describes one kind of atom, identified by its atomic number, the number of protons in its nucleus.