Even in ancient times there were many scientists who did not doubt the existence of atoms. But how are these atoms linked to one another in substances? On this matter philosophical thought was either silent or sailed high and low over the sea of fantasy.

    For example, the famous French naturalist Descartes believed that some atoms were furnished with hook-like projections, and others, with eye-like ones. He held that two atoms combined when the hook of one got caught in the eye of the other.

    As long as people knew little or nothing about atomic structure, all their ideas about the linkage between atoms, about chemical bonding, remained groundless. The electron helped scientists to discover the truth. 

    This did not happen all at once. The electron was discovered in 1895, but the first attempts to use it to account for chemical bonding were made only some twenty years later, after the arrangement of electrons around the atomic nucleus became clear.

    Not all atomic electrons participate in chemical bonds, but only those situated in their outer or at the most, in their last and second-last shells. 

    Suppose an atom of sodium meets an atom of fluorine. The former has one electron revolving on. its outside shell, and the latter, seven. The encounter instantly results in a very stable molecule of sodium fluoride. But how? By a rearrangement of electrons.
 

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