Atom: The smallest part of an element; composed of neutrons, protons and electrons.
Molecule: Persistent ensemble of atoms with a unique connectivity between the constituent atoms; smallest unit of a compound that can exist independently.
Compound: Pure substance made up of different elements combined in a definite way. Compounds can be further broken down into elements.
Element: A substance that cannot be further divided by chemical means. An element is defined by its atomic number (nuclear charge and number of electrons in the free atom).
Mole: Number of carbon atoms in exactly 12 grams of carbon-12. One mole of any substance contains 6.022 x 1023 (Avogadro’s Number) molecules, atoms or ions.
Nucleus: Small but massive internal core of the atom in which is concentrated the mass and positive charge. The nucleus is composed of neutrons (zero charge) and protons(+1 charge).
Electron: Smallest particle of negative charge; forms an integral part of every atom.
Matter: Anything that has mass and occupies space. The three most important phases of matter that earthbound chemists deal with are solid, liquid and gas. Ion:  An atom or group of atoms that has gained or lost one or more electrons and thus carries a negative or positive charge.

Democritus c. 400 BC

Democritus was one of the early Atomists. He reasoned that things were composed of tiny, indivisible particles.

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