6.1 Measuring Interatomic Bonds

Bonds between atoms vary in their strength, depending on the types of atoms involved and the type of bond. We can measure bonds by the energy it takes to tear them apart. Atoms and molecules do everything they do because of energy; certain arrangements have more energy than others, and the default tendency is for everything to change to a lower energy state.

Recall that a mole is Avogadro's number (about 600 thousand million million million) of atoms or molecules of a substance.

Some chemists prefer the joule (J) as the unit of energy. It is equal to a force of one newton moving an object one meter; a newton is, funnily enough, about equal to the weight of half an apple in Earth's gravity. (For electronics enthusiasts, one joule is also the heat generated by one amp passing through one ohm for one second.) We can measure bond strengths in kilojoules per mole (kJ/mol).

Other chemists prefer the calorie (cal), defined as the amount of energy used when heating one ml of water one degree C. Dietary calories are actually kilocalories (kcal), but that's okay because we'll be using kilocalories per mole (kcal/mol) anyway.

Both units are metric units and it doesn't really matter which one you prefer. In these lessons I will be giving values in both units. The conversion is one kcal = 4.184 kJ.

The strongest bonds between atoms are covalent bonds, ranging from 140 kJ/mol or 33 kcal/mol for the O-O bond of hydrogen peroxide (H2O2 or HO-OH) to 1080 kJ/mol or 260 kcal/mol for the C≡O bond.

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