How difficult it is to change an objects motion. Think of a tablecloth; it takes energy to move it from underneath objects on the table, and it takes energy to move the objects themselves on the table.
B. base
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The halogens are five non-metallic elements found in group 17 of the periodic table. The term "halogen" means "salt-former" and compounds containing halogens are called "salts". All halogens have 7 electrons in their outer shells, giving them an oxidation number of -1.
