ETYM Latin atomus, Greek, uncut, indivisible; a priv. + temnein to cut: cf. French atome. Related to Tome.
(formerly) ultimate unit of matter; smallest particle of element that can exist alone or as a constituent of molecule; any very small thing or quantity.
Smallest unit of matter that can take part in a chemical reaction, and which cannot be broken down chemically into anything simpler. An atom is made up of protons and neutrons in a central nucleus surrounded by electrons (see atomic structure). The atoms of the various elements differ in atomic number, atomic weight, and chemical behavior. There are 109 different types of atom, corresponding with the 109 known elements as listed in the periodic table of the elements.
Atoms are much too small to be seen even by even the most powerful optical microscope (the largest, cesium, has a diameter of 0.0000005 mm/0.00000002 in), and they are in constant motion. However, modern electron microscopes, such as the scanning tunneling microscope (STM) and the atomic force microscope (AFM), can produce images of individual atoms and molecules.
1. (Nontechnical usage) A tiny piece of anything; SYN. molecule, particle, mote, speck.
2. (Physics and chemistry) The smallest component of an element having the chemical properties of the element.