Atoms are held together by bonds, and bonds take energy, so an atom that is bonded has less energy than if it was free floating around on its own. Energy is required to break a bond (bond energy). Energy is released when a bond is created. (We’ll use this idea again later when we talk about Lewis Dot structures.) Each molecule has its own bond energy which you can look up in a table in your chemistry book. For example, C-H bonds take about 100kcal of energy to break 1 mol of C-H bonds, so you’ll find bond energies listed in kcal per mol. If you look up C-C bonds, you’ll find 80 kcal/mol. And a double C-C bond is 145 kcal per mol.

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