Atoms of one element differ in properties from atoms of all other elements.(credit: modification of work by slgc, CC BY 2.0 in Chemistry (OpenStax), CC BY 4.0). A macroscopic sample of an element contains an incredibly large number of atoms, all of which have identical chemical propertiesįigure 5.1a: Macroscopic vs microscopic visual of the element copper: a pre-1982 copper penny (left) contains approximately 3 × 10 22 copper atoms (several dozen are represented as brown spheres at the right), each of which has the same chemical properties. An element consists of only one type of atom, which has a mass that is characteristic of the element and is the same for all atoms of that element (Figure 5.1a ).An atom is the smallest unit of an element that can participate in a chemical change. Matter is composed of exceedingly small, indivisible particles called atoms.Here are the postulates of Dalton’s atomic theory. First published in 1807, many of Dalton’s hypotheses about the microscopic features of matter are still valid in modern atomic theory. The Aristotelian view of the composition of matter held sway for over two thousand years, until English schoolteacher John Dalton helped to revolutionize chemistry with his hypothesis that the behaviour of matter could be explained using an atomic theory. Interestingly, these philosophers thought about atoms and “elements” as philosophical concepts, but apparently never considered performing experiments to test their ideas. Later, Aristotle and others came to the conclusion that matter consisted of various combinations of the four “elements”-fire, earth, air, and water-and could be infinitely divided. In the fifth century BC, Leucippus and Democritus argued that all matter was composed of small, finite particles that they called atomos, a term derived from the Greek word for “indivisible.” They thought of atoms as moving particles that differed in shape and size, and which could join together. The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers, the scientists of their day.

The 2,400-year search for the atom – Theresa Doud. Watch The 2,400-year search for the atom – Theresa Doud (6 mins) Atomic Theory through the Nineteenth Century

The concepts of this foundation include the atomic theory, the composition and mass of an atom, and the variability of the composition of isotopes. This chapter will lay the foundation for our study of the language of chemistry. Chemical equations provide information about the quality and quantity of the changes associated with chemical reactions. Chemical formulas depict molecules as well as the composition of compounds. Chemical symbols are used to represent atoms and elements. The language of chemistry includes its own vocabulary as well as its own form of shorthand. The language used in chemistry is seen and heard in many disciplines, ranging from medicine to engineering to forensics to art. Apply Dalton’s atomic theory to explain the laws of definite and multiple proportions.Summarize the postulates of Dalton’s atomic theory.By the end of this section, you will be able to:

Dalton's atomic theory