Atomic Theory (Encyclopedia of Science)
An atomic theory is a model developed to explain the properties and behaviors of atoms. As with any scientific theory, an atomic theory is based on scientific evidence available at any given time and serves to suggest future lines of research about atoms.
The concept of an atom can be traced to debates among Greek philosophers that took place around the sixth century B.C. One of the questions that interested these thinkers was the nature of matter. Is matter, they asked, continuous or discontinuous? That is, if you could break apart a piece of chalk as long as you wanted, would you ever reach some ultimate particle beyond which further division was impossible? Or could you keep up that process of division forever? A proponent of the ultimate particle concept was the philosopher Democritus (c. 470. 380 B.C.), who named those particles atomos. In Greek, atomos means "indivisible."
The debate over ultimate particles was never resolved. Greek philosophers had no interest in testing their ideas with experiments. They preferred to choose those concepts that were most sound logically. For more than 2,000 years, the Democritus concept of atoms languished as kind of a secondary interest among scientists.
Then, in the first decade of the 1800s, the idea was revived. English chemist John Dalton...
(The entire section is 1700 words.)
Want to Read More?
Subscribe now to read the rest of this article. Plus get complete access to 30,000+ study guides!
Atomic Theory (World of Earth Science)
One of the points of dispute among early Greek philosophers was the ultimate nature of matter. The question was whether the characteristics of matter that can be observed with the five senses are a true representation of matter at its most basic level. Some philosophers thought that they were. Anaxagoras of Klazomenai (c. 49828 B.C.), for example, taught that matter can be sub-divided without limit and that it retains its characteristics no matter how it is divided.
An alternative view was that of Leucippus of Miletus (about 490 B.C.) and his pupil, Democritus of Abdera (c. 46070 B.C.). The views of these scholars are preserved in a few fragments of their writings and of commentaries on their teachings. Some writers doubt that Leucippus even existed. In any case, the ideas attributed to them are widely known. They thought that all matter consists of tiny, indivisible particles moving randomly about in a void (a vacuum). The particles were described as hard, with form and size, but no color, taste, or smell. They became known by the Greek word atomos, meaning "indivisible." Democritus suggested that, from time to time, atoms collide and combine with each other by means of hook-and-eye attachments on their surfaces.
Perhaps the most effective popularizer of the atomic theory was the Roman poet and naturalist, Lucretius. In his poem De Rerum Natura (On the nature of things) Lucretius states that only two realities exist, solid, everlasting particles and the void. This atomistic philosophy was in competition with other ideas about the fundamental nature of matter. Aristotle, for example, rejected Democritus' ideas because he could not accept the concept of a vacuum nor the idea that particles could move about on their own.
In addition, debates between atomists and anti-atomists quickly developed religious overtones. As the natural philosophy of Aristotle was adopted by and incorporated into early Christian theology, anti-atomism became acceptable and "correct," atomism, heretical. In fact, one objective of Lucretius' poem was to provide a materialistic explanation of the world designed to counteract religious superstition rampant at the time.
In spite of official disapproval, the idea of fundamental particles held a strong appeal for at least some philosophers through the ages. The French philosopher Pierre Gassendi (1592655) was especially influential in reviving and promoting the idea of atomism. Robert Boyle and Isaac Newton were both enthusiastic supporters of the theory.
Credit for the first modern atomic theory goes to the English chemist, John Dalton. In his 1808 book, A New System of Chemical Philosophy, Dalton outlined five fundamental postulates about atoms: 1. All matter consists of tiny, indivisible particles, which Dalton called atoms. 2. All atoms of a particular element are exactly alike, but atoms of different elements are different. 3. All atoms are unchangeable. 4. Atoms of elements combine to form "compound atoms" (i.e., molecules) of compounds. 5. In chemical reactions, atoms are neither created nor destroyed, but are only rearranged.
A key distinguishing feature of Dalton's theory was his emphasis on the weights of atoms. He argued that every atom had a specific weight that could be determined by experimental analysis. Although the specific details of Dalton's proposed mechanism for determining atomic weights were flawed, his proposal stimulated other chemists to begin research on atomic weights.
Dalton's theory was widely accepted because it explained so many existing experimental observations and because it was so fruitful in suggesting new lines of research. But the theory proved to be wrong in many of its particulars. For example, in 1897, the English physicist Joseph J. Thomson showed that particles even smaller than the atom, electrons, could be extracted from atoms. Atoms could not, therefore, be indivisible. The discovery of radioactivity at about the same time showed that at least some atoms are not unchangeable, but instead, spontaneously decay into other kinds of atoms.
By 1913, the main features of the modern atomic theory had been worked out. The work of Ernest Rutherford, Niels Bohr and others, suggested that an atom consists of a central core, the nucleus, surrounded by one or more electrons, arranged in energy levels each of which can hold some specific number of electrons.
Bohr's atomic model marked the beginning of a new approach in constructing atomic theory. His work, along with that of Erwin Schrödinger, Louis Victor de Broglie, Werner Karl Heisenberg, Paul Adrien Maurice Dirac, and others showed that atoms could be understood and represented better through mathematics than through physical models. Instead of drawing pictures that show the location and movement of particles within the atoms, modern scientists tend to write mathematical equations that describe the behavior of observed atomic phenomena.
See also Atomic mass and weight; Atomic number; Chemical bonds and physical properties