Stories About Chemistry
55. Two Kinds of Chemical Fetters
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.
The sodium atom easily gets rid of its outer electron. In doing so it becomes a positively charged ion and unveils its second-last electron shell. This shell contains eight electrons, an octet configuration which is very difficult to break up.
On the other hand, the fluorine atom readily accepts an additional electron on its outer shell; this makes the latter also an eight-electron shell.
And a negatively charged fluorine ion appears.
Positive attracts negative. Electrical forces draw the oppositely charged sodium and fluorine ions strongly together. A chemical bond appears between them. This bond is called ionic, and it is one of the principal types of chemical bonds.
The second is as follows.
How can such a compound as, say, the fluorine molecule F2 exist? Fluorine atoms cannot discard electrons from their outer shells. Differently charged ions cannot form in this case.
The chemical linkage between the fluorine atoms is accomplished by means of a pair of electrons. Each of the atoms deals out one electron to be pooled for common use. As a result, both of the atoms now have eight electrons, as it were, in their outer shells, six of their own and two pooled ones. Such a bond is called covalent.
The majority of chemical compounds known to us are formed by means of chemical bonds of the first or second type.