Historical Concepts of Atoms
The Greek philosopher Democritus was the first to propose the idea of atoms. He thought that matter was inherently grainy; you could divide a sample of matter only so many times until you came upon a particle that could not be split any further. He called this particle, the smallest unit of matter, the atom, Greek for "indivisible." Democritus believed that atoms were held together by "hooks and eyes," a simplistic but plausible explanation. Other philosophers, such as Aristotle, believed that you could divide matter infinitely, reaching ever smaller particles. However, the argument was purely philosophical; no experimental data was available.
The controversy over the divisibility of matter raged for almost two thousand years, until the science of chemistry began to develop after the Renaissance. In the late 1700's, two significant breakthroughs occurred: Antoine Lavoisier formulated the law of conservation of matter in 1774, which states that matter is neither created nor destroyed in chemical reactions; and Joseph Proust developed the law of constant composition in 1799, in which he observed that a given compound always has the same percentage by weight of its component elements (for example, water is always 11.11% hydrogen and 88.89% oxygen by weight). The second theory hinted that there was some small particle that joined with other small particles in fixed ratios. Using these propositions, John Dalton developed the first experiment-based atomic theory in the first decade of the 19th century. He made several predictions in his theory:
- Matter is made up of tiny, indestructible particles called atoms
- All the atoms of each element have identical properties, and the atoms of different atoms have different properties
- Compounds are made up of atoms in small, whole-number ratios; chemical reactions involve atoms shifting from one configuration to another
Dalton's atomic theory was a good observation in that it matched experimental results and made predictions that could be confirmed by more experiments. Dalton later formulated a law of multiple proportions, in which he stated that when two elements form two compounds, the mass ratios of the elements in the two compounds will themselves be in small, whole-number ratios. For example, CO has a carbon-oxygen mass ratio of 12:16, or 3:4 (.75). CO2 has a C:O mass ratio of 3:8 (0.375). In this case, the first ratio is 2 times (a small, whole number) the mass ratio of the second. The same principle holds for all molecules, such as NO and NO2, and FeO and Fe2O3.
Additional research in the late nineteenth century led to the discovery of radioactivity by Henri Becquerel, using the element uranium. Marie Curie and others discovered additional radioactive elements, including radium, and she postulated that radioactive particles are released when atoms disintegrate. This required a new atomic theory, both because the atoms were no longer indestructible and because some particle smaller particles must exist within the atom. Research into electricity and radioactivity by 1932 had established three sets of subatomic particles: the proton, a massive positively-charged particle; the neutron, a neutrally-charged particle with about the same mass as the proton; and the electron, a negatively-charged particle with a very low mass.
In 1909, before the discovery of the neutron, scientists believed that protons and electrons were simply lumped together into a mass, known as the "plum-pudding model" for its resemblance to the raisins (protons) stuck in the English pudding. However, Ernest Rutherford designed an experiment in which heavy radioactive particles (alpha particles: two neutrons and two protons) were shot at a thin gold sheet. Most often, the alpha particles passed through the foil, but sometimes a particle shot out at an angle or even back towards the particle gun. Rutherford postulated, in his nuclear model, that a very small, massive, positively-charged lump (called the nucleus) occupied the center of the atom; the rest of the atom was mostly empty space. In fact, the radius of the nucleus is about 100,000 times smaller than the radius of an atom.
By 1910, Neils Bohr had developed the most popular version of the structure of an atom had been developed: the protons make up the atom's nucleus (the neutrons were added to the nucleus on their discovery), while the electrons orbit the nucleus in concentric shells, like planets around the sun; the similarity caused this model to be called the solar-system model. While discoveries in physics would rapidly make this theory obsolete, its simplicity still makes it attractive to the public. The image to the right shows this view of the atom, which you have probably seen many times.
As we shall see, however, this model of the atom was surpassed less than twenty years after its discovery; to do this, we'll explore the relationship between electromagnetic radiation and the atom.