Chapter 4, Section 1
Early Models of the Atom
Have you ever been asked to believe in something you couldn’t see? Using your unaided eyes, you cannot see the tiny fundamental particles that make up matter. Yet all matter is composed of such particles, which are called atoms. An atom is the smallest particle of an element that retains its identity in a chemical reaction.
Atom comes from the Greek word atomos, meaning “indivisible.” If the suffix-ize means “to become like,” what do you think the word atomize means?
The concept of the atom intrigued a number of early scholars. Although these philosophers and scientists could not observe individual atoms, they still were able to propose ideas on the structure of atoms.
Democritus’s Atomic Philosophy
The Greek philosopher Democritus (460 B.C.–370 B.C.) was among the first to suggest the existence of atoms. Democritus believed that atoms were indivisible and indestructible. Although Democritus’s ideas agreed with later scientific theory, they did not explain chemical behavior. They also lacked experimental support because Democritus’s approach was not based on the scientific method.
Figure 4.1 Democritus believed that matter consisted of tiny, indivisible, unchangeable particles called atoms. His ideas were later challenged by the Greek philosophers Plato and Aristotle.
Dalton’s Atomic Theory
The real nature of atoms and the connection between observable changes and events at the atomic level were not established for more than 2000 years after Democritus. The modern process of discovery regarding atoms began with John Dalton (1766–1844), an English chemist and schoolteacher. By using experimental methods, Dalton transformed Democritus’s ideas on atoms into a scientific theory. Dalton studied the ratios in which elements combine in chemical reactions. Based on the results of his experiments, Dalton formulated hypotheses and theories to explain his observations. The result was Dalton’s atomic theory, which includes the ideas illustrated in Figure 4.2 and listed below.
All elements are composed of tiny indivisible particles called atoms.
Atoms of the same element are identical. The atoms of any one element are different from those of any other element.
Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.
Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.
