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| A substance that cannot be broken down to other substance by chemical means. Scientist recognize 92 chemical elements occuring in nature. |
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| An element that is essential for the survival of an organism but is needed in only minute quantities. |
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| A substance containing two or more elements in a fixed ratio; for example, table salt (NaCl) consists of one atom of the element sodium (Na) for every atom of Chlorine. (Cl) |
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| The smallest unit of matter that retains that properties of an element |
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| A variant form of an atom. Isotopes of an element have the same number of protons and electrons but different numebers of neutrons. |
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| An isotope whose nucleus decays spontaniously, giving off particles and energy. |
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An attraction between two atoms resulting from a sharing of outer-shell electrons or the presence of opposite charges on te atoms. The bonded atoms gain complete outer elctron shells. |
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| An attraction between two ions with opposition electrical charges. The electrical attraction of the opposite charges holds the ions together. |
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| An attraction between atoms that share one or more pairs of outer-shell electrons; symbolized by a single line between the atoms. |
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