bond energy
/ bond EN-er-jee /
Think of two pieces of strong tape stuck together. To peel them apart you have to pull, and the harder the stick, the more effort it takes. A chemical bond is similar: to break it you must supply energy to overcome the attraction. The bond energy is precisely that cost — the energy needed to pull apart a single bond between two atoms.
Because the bonded pair sits at lower energy than the separated atoms, it takes a definite amount of energy to lift them back out and break the bond. That amount is the bond energy, usually quoted in electron-volts per bond or in kilojoules for a large batch of bonds. The exact same amount of energy is released when the bond first forms, so bond energy is the two-way ledger of how much making and breaking a bond costs or pays.
Bond energy matters because it is the heartbeat of chemistry: a reaction releases energy when the new bonds it forms are stronger than the old ones it breaks, which is why burning fuel gives off heat. The honest caveat is that the quoted figure is usually an average — the energy to break a given bond can shift a little depending on what else is attached to the atoms — so tabulated bond energies are reliable guides rather than exact values for every situation.
When natural gas burns, oxygen and methane molecules break their bonds and recombine into water and carbon dioxide. The new bonds in those products are stronger than the ones torn apart, so the leftover bond energy escapes as heat and light — the flame on your stove.
Burning gas forms stronger bonds than it breaks, releasing the difference as heat.
Bond energy is about breaking one bond between two atoms, while cohesive energy is about pulling a whole solid apart into free atoms. In a solid each atom feels many bonds at once, so the two quantities are related but not interchangeable.