Atoms - Part 2 - Ancient Greece
About 400 B.C. the Greek philosopher Democritus proposed that matter consisted of various types of tiny discrete particles and that the properties of matter were determined by the properties of these particles. Liquids were made of smooth and round atoms that could slide by each other, metals of rough and barbed atoms that clung together. This theory was later elaborated in a work by Lucretius. These philosophers had no way to verify the theory and it was not pursued for many centuries. The theory reappeared in the early 19th century as an explanation for several laws that had been established in the previous century when chemists had begun carefully measuring the mass of reactants and products.
One of these was the law of conservation of mass, first stated by the French chemist Lavoisier, which says that there is no change in mass with a chemical reaction. The total mass of products is the same as the total mass of reactants. Another was the observation that compounds always contain the same elements in the same proportions, the law of constant composition. Table salt NaCl always has the same ratio of Na and Cl.
Note: Today it is known that some compounds, particularly metal oxides and sulfides, exist in ratios that vary slightly from simple whole number ratios. Some of these have the property of superconductivity. These are known as nonstoichiometric compounds.
A third law is the law of multiple proportions which states that a given mass of one element can combine with various masses of another element (or elements) but always in small whole number ratios. An example is the two combinations of carbon and oxygen, CO2 and CO.
John Dalton revived the atomic theory in order to explain these observations. In 1808 he proposed that a chemical element consisted of tiny particles (atoms), all of which had the same chemical properties. (A chemical element could not be decomposed into two or more components.) Further, the atoms of a given element have different properties than the atoms of other elements and that these atoms are not changed during ordinary chemical reactions. It took many years for the concept to become widely accepted. Of course nowadays the atomic theory is fundamental to the physical sciences.
When white light is passed though a prism it is differentiated into the colors of the spectrum as shown in Figure 1. The colors have different wavelengths. Visible light is a small part of the range of wavelengths of electromagnetic radiation. Wavelengths longer than the visible range are called infrared, shorter wavelengths are called ultraviolet.
Figure 1.
During the 1880's it was discovered that various substances absorbed electromagnetic radiation only at certain characteristic wavelenths. The absorption spectrum of hydrogen in the visible range has a pattern of sharp lines (Figure 2). These are the wavelengths where absorption of radiation takes place.
Figure 2.
When heated to very high temperatures substances emit electromagnetic radiation at certain frequencies. The emission spectrum includes the lines of the absorption spectrum of a substance and contains more lines at longer wavelengths.
When hydrogen gas is heated it emits electromagnetic radiation in the ultraviolet, visible and infrared regions (Figure 3). The highest energy series of the hydrogen emission spectrum matches the hydrogen absorption spectrum and is called the Lyman series. The Lyman series is in the (high energy) ultraviolet region. The next series, at lower energy, is in the visible and ultraviolet region and is called the Balmer series. The Paschen series is at even lower energy in the infrared region.
Figure 3.