Relating to particles or objects that are smaller than an atom, such as electrons, protons, or neutrons.
From Latin 'sub-' (under, below) + 'atomicus' (atomic). The term was coined in the late 1800s as physicists discovered particles smaller than what they once thought was the smallest unit of matter.
For 2,000 years, atoms were thought to be indivisible—the word itself means 'uncuttable' in Greek—but when scientists discovered subatomic particles in the 1890s, they had to invent new words to describe a whole hidden world of tinier things.
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