Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Chemical Foundations: Elements, Atoms, and Ions
Dalton’s Atomic Theory

Objectives

· To learn about Dalton’s theory of atoms.

· To understand and illustrate the law of constant composition.

As scientists of the eighteenth century studied the nature of materials, several things became clear:

1. Most natural materials are mixtures of pure substances.

2. Pure substances are either elements or combinations of elements called compounds.

3. A given compound always contains the same proportions (by mass) of the elements. For example, water always contains g of oxygen for every g of hydrogen, and carbon dioxide always contains g of oxygen for every g of carbon. This principle became known as the law of constant composition . It means that a given compound always has the same composition, regardless of where it comes from.

John Dalton (Fig. 4.1), an English scientist and teacher, was aware of these observations, and in about 1808 he offered an explanation for them that became known as Dalton’s atomic theory . The main ideas of this theory (model) can be stated as follows:

Dalton’s Atomic Theory

1. Elements are made of tiny particles called atoms .

2. All atoms of a given element are identical.

3. The atoms of a given element are different from those of any other element.

4. Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms.

5. Atoms are indivisible in chemical processes. That is, atoms are not created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together.

Figure 4.1.Dalton’s Atomic Theory

Reproduced by permission, Manchester Literary and Philosophical Society

John Dalton (1766—1844) was an English scientist who made his living as a teacher in Manchester. Although Dalton is best known for his atomic theory, he made contributions in many other areas, including meteorology (he recorded daily weather conditions for years, producing a total of data entries). A rather shy man, Dalton was colorblind to red (a special handicap for a chemist) and suffered from lead poisoning contracted from drinking stout (strong beer or ale) that had been drawn through lead pipes.

Dalton’s model successfully explained important observations such as the law of constant composition. This law makes sense because if a compound always contains the same relative numbers of atoms, it will always contain the same proportions by mass of the various elements.

Like most new ideas, Dalton’s model was not accepted immediately. However, Dalton was convinced he was right and used his model to predict how a given pair of elements might combine to form more than one compound. For example, nitrogen and oxygen might form a compound containing one atom of nitrogen and one atom of oxygen (written ), a compound containing two atoms of nitrogen and one atom of oxygen (written ), a compound containing one atom of nitrogen and two atoms of oxygen (written ), and so on (Fig. 4.2). When the existence of these substances was verified, it was a triumph for Dalton’s model. Because Dalton was able to predict correctly the formation of multiple compounds between two elements, his atomic theory became widely accepted.

Figure 4.2.An illustration shows space filling models of the molecules NO, NO subscript 2, and N subscript 2 O. The space filling model of NO shows a nitrogen atom, represented by a blue sphere, attached to an oxygen atom, represented by a red sphere. The space filling model of NO subscript 2 shows a central nitrogen atom represented by a blue sphere, attached to two oxygen atoms on its either side, each of which is represented by a red sphere. The space filling model of NO shows two nitrogen atoms attached together, represented by a blue sphere, which is further attached to an oxygen atom represented by a red sphere.

Dalton pictured compounds as collections of atoms. Here , , and are represented. Note that the number of atoms of each type in a molecule is given by a subscript, except that the number is always assumed and never written.