An atom is the smallest possible quantity of matter that can't be broken into smaller bits of matter by normal means. There are 109 distinct known distinct types of atoms, each type corresponding with an element in the periodic table (e.g. hydrogen, iron, carbon).

Atoms and molecules are not the same. Atoms are the building blocks that make molecules, and matter isn't normally of practical use to humans unless it consists of molecules. Atoms and electrons are not the same, either. Electrons are particles which orbit around the group of particles clustered at the center (nucleus) of the atom, much the same way that planets orbit around a star. An atom is the combination of both the nucleus and all orbiting electrons.

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