It takes a considerably larger amount of energy to pull apart a water molecule into separate atoms than to pull apart either hydrogen or oxygen molecules. Naturally, that greater energy is also returned once the hydrogen and oxygen atoms are allowed to come back together into water molecules.
Next, imagine pulling apart hydrogen and oxygen molecules into hydrogen and oxygen atoms and then having those atoms come together to form water molecules. A certain amount of energy is put into the system to break up the hydrogen and oxygen molecules, but then a much greater amount of energy is given off when the water molecules form.
It is for that reason that a great deal of energy (mostly in the form of heat) is given off if a jet of hydrogen gas and a jet of oxygen gas are allowed to mix in such a way as to form water.
Just mixing the hydrogen and oxygen isn’t enough. The molecules of hydrogen and oxygen must be separated and that takes a little energy. The energy in a match flame is enough to raise the temperature of the mixture and to make the hydrogen and oxygen molecules move about more rapidly and more energetically. This increases the chance that some molecules will be broken up into separate atoms (though the actual process is rather complicated). An oxygen atom might then strike a hydrogen molecule to form water (O + H₂ → H₂O) and more energy is given off than was absorbed from the match flame. The temperature goes up still higher so that further breakup among the oxygen and hydrogen molecules is encouraged.
The formation of a sodium chloride molecule.
This happens over and over again so that in very little time, the temperature is very high and the hydrogen and oxygen are combining to form water at an enormous rate. If a great deal of hydrogen and oxygen are well-mixed to begin with, the rate of reaction is so great that an explosion occurs.
Such a situation, in which each reacting bit of the system adds energy to the system by its reaction and brings about more reactions like itself, is called a “chain reaction”. Thus, a match flame put to one corner of a large sheet of paper will set that corner burning. The heat of the burning will ignite a neighboring portion of the sheet and so on till the entire sheet is burned. For that matter a single smoldering cigarette end can serve to burn down an entire forest in a vastly destructive chain reaction.
Electrons and Energy
The discovery of the structure of the atom sharpened the understanding of chemical energy.