The modern atomic theory we have today came from contributions of many different scientists. Democritus, Dalton, Thompson, Millikan and Rutherford all had their specific contributions to the development of the Atomic theory we have come to know today.
Democritus, for example, developed and named the building blocks of matter atomos, meaning literally “indivisible,” about 430 BC. Democritus believed that atoms were uniform, solid, hard, incompressible, and indestructible and that they moved in infinite numbers through empty space until stopped.
Differences in atomic shape and size determined the various properties of matter. In Democritus’ philosophy, atoms existed not only for matter but also for such qualities as perception and the human soul. For example, sourness was caused by needle-shaped atoms, while the colour white was composed of smooth-surfaced atoms. The atoms of the soul were considered to be particularly fine. Democritus developed his atomic philosophy as a middle ground between two opposing Greek theories about reality and the illusion of change.
He argued that matter was subdivided into indivisible and immutable particles that created the appearance of change when they joined and separated from others.
The British chemist and physicist John Dalton extended former chemists work and converted the atomic philosophy of the Greeks into a scientific theory between 1803 and 1808. His book New System of Chemical Philosophy (part I, 1808; part II, 1810) was the first application of atomic theory to chemistry. It provided a physical picture of how elements combine to form compounds and a phenomenological reason for believing that atoms exist.
His work, together with that of Joseph-Louis Gay-Lussac of France and Amedeo Avogadro of Italy, provided the experimental foundation of atomic chemistry.
On the basis of the law of definite proportions, Dalton deduced the law of multiple proportions, which stated that when two elements form more than one compound by combining in more than one proportion by weight, the weight of one element in one of the compounds is in simple, integer ratios to its weights in the other compounds.
During the 1880s and ’90s, scientists searched cathode rays for the carrier of the electrical properties in matter. Their work culminated in J.J. Thomson’s discovery of the electron in 1897. The existence of the electron showed that the 2,000-year-old conception of the atom as a homogeneous particle was wrong and that in fact the atom has a complex structure.
In 1909 the American physicist Robert Andrews Millikin greatly improved a method employed by Thomson for measuring the electron charge directly. Millikan produced microscopic oil droplets and observed them falling in the space between two electrically charged plates. Some of the droplets became charged and could be suspended by a delicate adjustment of the electric field. Millikan knew the weight of the droplets from their rate of fall when the electric field was turned off. From the balance of the gravitational and electrical forces, he could determine the charge on the droplets. He could find charges only in integral multiples of a quantity that in contemporary units is 1.602 10-19 coulomb. Millikan’s electron charge experiment was the first to detect and measure the effect of an individual subatomic particle. Besides confirming the particulate nature of electricity, his experiment also supported previous determinations of Avogadro’s number. Avogadro’s number times the unit of charge gives Faraday’s constant, the amount of charge required to electrolyze one mole of a chemical ion.
Experiments conducted by the British physicist Ernest Rutherford in 1899 showed that radioactive substances emit more than one kind of ray. It was determined that part of the radiation is 100 times more penetrating than the rest and can pass through aluminum foil 1/50 of a millimetre thick. Rutherford named the less penetrating emanations alpha rays and the more powerful ones beta rays, after the first two letters of the Greek alphabet. Investigators who, in 1899, found that beta rays were deflected by a magnetic field concluded that they are negatively charged particles similar to cathode rays. In 1903 Rutherford found that alpha rays were deflected slightly in the opposite direction, showing that they are massive, positively charged particles. Much later, Rutherford proved that alpha rays are nuclei of helium atoms by collecting the rays in an evacuated tube and detecting the buildup of helium gas over several days.
The discoveries and contributions of Democritus, Dalton, Thompson, Millikan and Rutherford led to the modern atomic theory we have today. It is, however, simply a theory.