Double bonds

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How do you know when atoms are supposed to have double or triple bonds just given the formula? For example, how would I know that C2H2 (acetylene) has a triple bond just based on the formula?

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Knowing whether a bond is single, double, or triple comes from counting the electrons in each atom and making bonds that fulfill the octet rule. Each hydrogen can form a single covalent bond and each carbon can form 4 covalent bonds. So to make each carbon atom have 8 valence electrons, a triple bond between carbons is required.