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Anything that occupies space and has mass.
Matter
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A substance that cannon be broken down into other substances by ordinary means.
Element
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A substance containing two or more elements in a fixed ratio.
Compound
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A subatomic particle with a single positive electrical charge, found in the nucleus of an atom.
Proton
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A subatomic particle with a single negative charge.
Electron
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A subatomic particle that is electrically neutral.
Neurtron
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An atom's central core, containing protons and neutrons.
Nucleus
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The number of protons in each atom of a particular element.
Atomic Number
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The approximate total mass of an atom; alos called atomic weight. Given as a whole number, the atomic mass approximately equals the mass number.
Atomic Mass
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The sum of the number of protons and neutrons in an atom's nucleus.
Mass Number
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A variant form of an atom. Isotopes of an element have the same number of protons, but different numbers of neutrons.
Isotope
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An isotope whose nucleus decays spontaneously, giving off particles and energy.
Radioactive Isotope
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An energy level representing the distance of an electron from the nucleus of an atom.
Electron Shell
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An attraction between two atoms resulting from a sharing outer-shell electrons or the presence of opposite charges on the atoms. The bonded atoms gain complete outer electron shells.
Chemical Bond
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An atom or molecule that has gained or lost one or more electrons, thus acquiring an electrical charge.
Ion
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An attraction between two ions with opposite electrical charges. The electrical attraction of the opposite charges holds the ions together.
Ionic Bond
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A compound resulting from the formation of ionic bonds; also called an ionic compound.
Salt
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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.
Covalent Bond
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A group of two or more atoms held together by covalent bonds.
Molecule
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A type of covalent bond in which two atoms share pairs of electrons; symbolized by a pair of lines between the bonded atoms.
Double Bond
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