The Big Picture: Chemistry Gets Organized

One day down, four to go. Let’s rock on.

The major topic for the next couple of days is MODERN ATOMIC THEORY. Chemistry did not begin until this principle had gained general acceptance in the early 1800s . The first recorded hypothesis regarding the basic unit of material things is attributed to the pre-Socratic Greek philosopher, Democritus (c. 460–c. 370 B.C.). After his teacher Leucippus had noted that a beach looks smooth from afar but really consists of individual sand grains, Democritus said that the concept could be extended to all matter because material things were made of indivisible particles. Democritus called his particles atomos, meaning "cannot be cut." His ideas were largely ignored until the scientific revolution of the western Enlightenment (16^th to 18^th centuries) due to widespread acceptance of Aristotle’s (c. 384–c. 322 B.C.) view that all matter was comprised of earth, air, water, and fire in varying proportions, and that matter could be transmuted into gold by adjusting the ratios of these four elements. (Aristotle also limited matter to four essential properties: hot, cold, dry, and wet!)

In modern times, atomic theory was rediscovered by John Dalton (1766-1844), an English physical scientist. In the early 1800s he was a professor of mathematics and natural philosophy, and he dedicated his research efforts to standardizing then known chemical knowledge. The result was a series of principles to explain the structure of matter:

1. Matter consists of tiny particles (atoms).
2. Atoms are indestructible. In chemical reactions they can rearrange but not break apart.
3. All atoms of a given element are identical in mass and other properties (true then, as isotopes had not been discovered).
4. Atoms of different elements differ in mass and other properties.
5. Elements combined into a given compound always react in a fixed ratio.

The atomic theory provided the impetus for later attempts to develop a unifying principle for all chemistry (and physics!) knowledge, the PERIODIC TABLE. Many scientists in the late 1700s and early 1800s contributed to its theoretical underpinnings, but the laurel for the first modern version goes to Dmitri Mendeleev – to give the most common of the spellings for his name). This Russian chemist is accorded the honor because his array not only attempted to systematize existing chemical knowledge but because he used his model to make predictions about the existence and properties of then unknown elements. Mendeleev arranged his table into columns (groups) and rows (periods) according to elemental atomic weights (representing the mass of protons and neutrons), while modern tables use atomic number (proton number). Entities in a given group or period share certain chemical properties based on their atomic structure, and particularly the number and arrangement of electrons in the outer (or valence) orbital. But more on that tomorrow….