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Certain compounds won't form because the constituent atoms won't bond with each other to form it. The reasons for this basically fall into two groups. One is that the charges are not balanced. Table salt is sodium chloride, or NaCl, and we don't see Na2Cl or NaCl2 because the sodium has a +1 electronegativity, and chlorine has a -1 electronegativity. This means for charges to balance, they must combine one-to-one, two-to-two or the like. Sodium and chlorine basically only hook up one way. The other has to do with something a bit less obvious, but not too tough, and we see it here. The "final energy state" of NaCl is something the molecule is "happy" with. We don't see Na2Cl2 or the like. The atoms "decide" to hook up in a one-to-one ratio, and only one on one. Not two on two or something else, because the molecule moves to a lower energy state, and a more stable one, by combining in just that single way. One Na and one Cl yields one Na+ and one Cl- to make one salt molecule. The two atoms get together, and chlorine borrows sodium's lone valence electron making two ions, which are (ionically) bonded together. This latter "variable" that determines what will and will not combine can become extremely complex, particularly as we climb up through the elements in the Periodic Table. An example of an extension of the latter idea is the oxides of iron, which are all iron and oxygen (naturally). The conditions under which the oxide forms determine which ratio of the several possibilities will occur. (Iron has several electronegativities from 1 to 6.) This makes for interesting possibilities. And it's all about whether there is enough energy to form the compound, and to what energy level it can go to by "hooking up" to form that compound.

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16y ago

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