key concept2.1Atomic Structure Explains the Properties of Matter

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All matter is composed of atoms. Atoms are tiny—more than a trillion (1012) of them could make up the period at the end of this sentence. Atoms and their component particles have volume and mass, which are characteristics of all matter. Mass is a measure of the quantity of matter present; the greater the mass, the greater the quantity of matter. Atoms also have electric charges.

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  • Six elements (carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur) make up most of the matter found in living organisms.

  • Reactive atoms tend to gain or lose electrons from their valence (outer) shell in order to achieve a full valence shell, because a full valence shell confers stability.

  • The structure of an atom determines its properties.