Page 49 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 3

Atoms and Atomic

Masses

3.1. INTRODUCTION
In this chapter we will discuss
1. Dalton’s postulates regarding the existence of the atom and the laws on which those postulates are based
2. Atomic mass—its uses and limitations
3. The structure of the atom
4. The existence of isotopes
5. The periodic table, which for now is presented only enough to introduce the concepts of periodic groups or
families and the numbers of electrons in the outermost electron shells

3.2. ATOMIC THEORY
In 1804, John Dalton proposed the existence of atoms. He not only postulated that atoms exist, as had ancient
Greek philosophers, but he also attributed to the atom certain properties. His postulates were as follows:
1. Elements are composed of indivisible particles, called atoms.
2. All atoms of a given element have the same mass, and the mass of an atom of a given element is different
from the mass of an atom of any other element.
3. When elements combine to form a given compound, the atoms of one element combine with those of the
other element(s) in a definite ratio to form molecules. Atoms are not destroyed in this process.
4. Atoms of two or more elements may combine in different ratios to form different compounds.
5. The most common ratio of atoms is 1:1, and where more than one compound of two or more elements exists,
the most stable is the one with 1:1 ratio of atoms. (This postulate is incorrect.)

Dalton’s postulates stimulated great activity among chemists, who sought to prove or disprove them. The
fifth postulate was very quickly shown to be incorrect, and the first three have had to be modified in light of later
knowledge; however, the first four postulates were close enough to the truth to lay the foundations for a basic
understanding of mass relationships in chemical compounds and chemical reactions.
Dalton’s postulates can be used to explain three quantitative laws that had been developed shortly before he
proposed his theory.
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