Chemical Compound

It is difficult for a reader in the 1990s to appreciate the challenge facing a chemist in 1850 who was trying to understand the nature of a compound. Today it is clear that atoms of elements combine with each other to form, in many cases, molecules of a compound. Even the beginning chemistry student can express this concept with facility by using symbols and formulas, as in the formation of iron(II) sulfide from its elements:

The chemist of 1850 was just barely comfortable with the idea of an atom and had not yet heard of the concept of a molecule. Moreover, the connection between ultimate particles (such as atoms and molecules) and materials encountered in the everyday work of a laboratory was not at all clear. As a result, early theories about the nature of compounds were based on empirical data (information collected from experiments), not from theoretical speculation about the behavior of atoms.

One of the earliest theories of compounds was that of the Swedish chemist Jons Jacob Berzelius. Berzelius argued that all compounds consist of two parts, one charged positively and one, negatively. The theory was at least partially based on Berzelius' own studies of electrolysis, studies in which compounds would often be broken apart into two pieces by the passage of an electrical current. Thus, he pictured salts as being composed of a positively charged metal oxide and a negatively charged non-metallic oxide. According this theory, sodium sulfate, Na₂SO₄ could be represented as Na₂O • SO₃.

Other theories followed, many of them developed in an effort to explain the rapidly growing number of organic compounds being discovered and studied. According to the radical theory, for example, compounds were viewed as consisting of two parts, one of which was one of a few standard radicals, or groups of atoms. Organic compounds were explained as being derived from the methyl, ethyl, benzyl, cyanogen, or some other radical.

The type theory, proposed by Charles Gerhardt in the 1840s, said that compounds could be understood as derivatives of one or more basic types, such as water or ammonia. According to this theory, bases such as sodium hydroxide (NaOH) were thought to be derivatives of water (HOH) in which one hydrogen atom is replaced by a metal.