atom
n. countablen. the smallest possible piece of a chemical element. Everything in the universe is built from these tiny, invisible blocks.
n. the smallest unit of a chemical element that retains the properties of that element. It consists of a dense central nucleus surrounded by a cloud of negatively charged electrons.
A single drop of water contains billions of atoms.
In chemistry class, the students learned how oxygen and hydrogen atoms bond together to form water.
When a radioactive atom decays, its unstable nucleus releases energy and particles, eventually transforming into an entirely different element over time.
From Middle English attome, from Middle French athome, from Latin atomus (“smallest particle”), from Ancient Greek ἄτομος (átomos, “indivisible”), from ἀ- (a-, “not”) + τέμνω (témnō, “to cut”, o-grade in τομ-) + -ος (-os). Atoms are so named because they were historically thought up as to be the smallest unit of matter, and thus indivisible. Doublet of atomus.