Why do some substances react with each other, while others don't?

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Atomic theory states that all substances are comprised of atoms.  Something that has all the same atoms is an element; something with a mixture of different atoms is a compound.  Each of these (elements and compounds) have electrons zipping around them; In general, its these electrons that determine if a reaction takes place between two substances.  If conditions are right, a transfer of electrons from one substance to another occurs, and a reaction happens.

Whether or not a reaction takes place is contingent upon the relative electron arrangements of the substances being combined, and how tightly or loosely those electrons are held.

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