Development Of The Modern Theory Of Bonding
The discovery of the electron by J. J. Thomson in 1897 was, in the long run, the key needed to solve the problem of bonding. In the short run, however, it was a serious hindrance to resolving that issue. The question that troubled many chemists at first was how two particles with the same electrical charge (as atoms then seemed to be) could combine with each other.
An answer to that dilemma slowly began to evolve, beginning with the work of the young German chemist Richard Abegg. In the early 1900s, Abegg came to the conclusion that inert gases are stable elements because their outermost shell of electrons always contain eight electrons. Perhaps atoms combine with each other, Abegg said, when they exchange electrons in such as way that they all end up with eight electrons in their outer orbit. In a simplistic way, Abegg had laid out the principle of ionic bonding. Ionic bonds are formed when one atom completely gives up one or more electrons, and a second atom takes on those electrons.
Since Abegg was killed in 1910 at the age of 41 in a balloon accident, he was prevented from improving upon his original hypothesis. That work was taken up in the 1910s, however, by a number of other scientists, most prominently the German chemist Walther Kossel and the American chemists Irving Langmuir and Gilbert Newton Lewis.
Working independently, these researchers came up with a second method by which atoms might bond to each other. Rather than completely losing or gaining electrons, they hypothesized, perhaps atoms can share electrons with each other. One might imagine, for example, that in a molecule of methane (CH4), each of the four valence electrons in carbon is shared with the single electron available from each of the four hydrogen atoms. Such an arrangement could provide carbon with a full outer shell of eight electrons and each hydrogen atom with a full outer shell of two. Chemical bonds in which two atoms share pairs of electrons with each other are known as covalent bonds.
In trying to illustrate this concept, Lewis developed another system for representing chemical bonds. In the Lewis system (also known as the electron-dot system), each atom is represented by its chemical symbol with the number of electrons in its outermost orbit, its bonding or valence electrons. The formula of a compound, then, is to be represented by showing how two or more atoms share electrons with each other.