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The smallest component of matter that has the properties of a chemical element. All kind of solid, liquid, gas and plasma are composed of atoms. Atoms are very small; typical sizes are around 100 picometers (a ten billionth of a meter). Atom is composed of a nucleus and one or more electrons bound to the nucleus. More than 99.94% of an atom's mass is in the nucleus. The nucleus is made of one or more protons and typically a similar number of neutrons. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. The electrons of an atom are attracted to the protons in an atomic nucleus by this electromagnetic force. The protons and neutrons in the nucleus are attracted to each other by the nuclear force, which is usually stronger than the electromagnetic force. The word "atom" was coined by the ancient Greek philosophers Leucippus and his pupil Democritus. In the early 1800s, John Dalton used the concept of atoms to explain why elements always react in ratios of small whole numbers (the law of multiple proportions).