An atom is the main component of matter. All atoms have a certain number of protons, electrons, and neutrons in its nucleus. Every element has a different type of atom, with a certain number of protons and electrons. The number of protons and electrons are almost always equal, but atoms can transfer electrons with each other, making them charged (these charged atoms are called Ions.) Atoms are very small; their typical sizes are around 100 picometers (a ten-billionth of a meter).
Structure of an atom
History of the Concept
The concept that matter is made out of atoms first appeared in Greece. The word «atom» (Greek for «indivisible») was coined by the ancient Greek philosophers Leucippus and his student Democritus, who thought of atoms as solid spheres.  Near the end of the 18th century, British scientist John Dalton improved atomic theory, by claiming that each element has its own type of atom, with different characteristics like mass and reactivity. Atoms were later found to be mostly empty space, with the electrons orbiting the nucleus.
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