period, a horizontal (side-to-side) row on the periodic table, group, a vertical (up-and-down) column on the periodic table, family, another word for group, atomic number, shows the number of protons in an element's atoms, atomic mass, shows the weight of one atom of an element, similar properties, elements in the same group have ___, reactive, describes an element likes to combine with other elements - like friends, stable, describes an element that does NOT like to combine with other elements - happy alone, 18, the number of groups on the periodic table, 7, the number of periods on the periodic table.

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