2. Atoms, Molecules and Ions

Atomic Theory and Atomic Structure

In this chapter we will look at the basic concepts of chemistry. As we noted briefly in Chapter 1, all matter is composed of atoms. In this first part of Chapter 2, we explore the atomic theory of matter and look at the structure of atoms. At the end of this first part, we examine the periodic table, which orders the chemically different kinds of atoms in a way that highlights similarities among the elements.

2.1 Atomic theory of matter

As we noted in Chapter 1, Lavoisier laid the experimental foundation of modern chemistry. But it was the British chemist John Dalton (1766-1844) who provided the basic theory: all matter – whether element, compound, or mixture – is composed of small particles called atoms. The postulates, or basic statements, of Dalton's theory are presented in this section. Note that the terms element, compound, and chemical reaction, which were defined in Chapter 1 in terms of matter as we normally see it, are redefined by these postulates in terms of atoms.

Postulates of Dalton's Atomic Theory

The main points of Dalton's atomic theory, an explanation of the structure of matter in terms of different combinations of very small particles, are given by the following postulates:

1. All matter is composed of indivisible atoms. An atom is an extremely small particle of matter that retains its identity during chemical reactions

2. An element is a type of matter composed of only one kind of atom, each atom of a given kind having the same properties. Mass is one such property. Thus, the atoms of a given element have a characteristic mass. (We will need to revise this definition of the element in Section 2.3 so that it is stated in modern terms.)

3. A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions. The relative numbers of any two kinds of atoms in a compound occur in simple ratios. Water, for example, is a compound of the elements hydrogen and oxygen and consists of hydrogen and oxygen atoms in the ratio of 2 to 1.

4. A chemical reaction consists of the rearrangement of the atoms present in the reacting substances to give new chemical combinations present in the substances formed by the reaction. Atoms are not created, destroyed, or broken into smaller particles by any chemical reaction.

Today we know that atoms are not truly indivisible; they are themselves made up of particles, as we will explain in the next section. Nevertheless, Dalton's postulates are essentially correct.

Deductions from Dalton's Atomic Theory

Note how atomic theory explains the difference between an element and a compound. Atomic theory also explains two laws we considered earlier. One of these is the law of conservation of mass, which states that the total mass remains constant during a chemical reaction. By postulate 2, every atom has a definite mass. Because a chemical reaction only rearranges the chemical combinations of atoms postulate 4, the mass must remain constant. The other law explained by atomic theory is the law of definite proportions (constant composition). Postulate 3 defines a compound as a type of matter containing the atoms of two or more elements n definite proportions. Because the atoms have definite mass, compounds must have the elements in definite proportions by mass.

A good theory should not only explain known facts and laws but also predict new ones. The law of multiple proportions, deduced by Dalton from his atomic theory, is a law stating that when two elements form more than one compound, the masses of one element in these compounds for a fixed mass of the other element are n ratios of small whole numbers. For example, carbon and oxygen form two compounds: carbon monoxide and carbon dioxide. Carbon monoxide contains 1.3321 g of oxygen for each 1.0000 g of carbon, whereas carbon dioxide contains 2.6642 g of oxygen for 1.0000 g of carbon. In other words, carbon dioxide contains twice the mass of oxygen as is contained in carbon monoxide (2.6642 g = 2 x 1.3321 g) for a given mass of carbon. Atomic theory explains this by saying that carbon dioxide contains twice as many oxygen atoms for a given number of carbon atoms as carbon monoxide does. The deduction of the law of multiple proportions from atomic theory was important in convincing chemists of the validity of the theory.

Atomic Symbols

We will find it convenient to use symbols for the atoms of the different elements. An atomic symbol is a one-, two-, or three-letter notation used to represent an atom corresponding to a particular element. Typically, the atomic symbol consists of the first letter, capitalized, from the name of the element, sometimes with an additional letter from the name in lower case. For example, chlorine has the symbol Cl. Other symbols are derived from a foreign (usually Latin) name. Sodium is given the symbol Na from its Latin name, natrium. Symbols of selected elements are listed in the following table.

Name of Element	Atomic Symbol	Physical Appearance of Element*
1	2	3
Aluminum	Al	Silvery-white metal
Barium	Ba	Silvery-white metal
Bromine	Br	Reddish-brown liquid
Calcium	Ca	Silvery-white metal
Carbon	С
Graphite		Soft, black solid
Diamond		Hard, colorless crystal
Chlorine	Cl	Greenish-yellow gas
Chromium	Cr	Silvery-white metal
Cobalt	Co	Silvery-white metal
Copper	Cu (from cuprum)	Reddish metal
Fluorine	F	Pale yellow gas
Gold	Au (from aurum)	Soft, yellow metal
Helium	He	Colorless gas
Hydrogen	H	Colorless gas
Iodine	I	Bluish-black solid
Iron	Fe (from ferrum)	Silvery-white metal
Lead	Pb (from plumbum)	Bluish-white metal
Magnesium	Mg	Silvery-white metal
Manganese	Mn	Gray-white metal
Mercury	Hg (from hydrargyrum)	Silvery-white liquid metal
Neon	Ne	Colorless gas
Nickel	Ni	Silvery-white metal
Nitrogen	N	Colorless gas
Oxygen	О	Colorless gas
Phosphorus (white)	P	Yellowish-white, waxy solid
Potassium	К (from kalium)	Soft, silvery-white metal
Silicon	Si	Gray, lustrous solid
Silver	Ag (from argentum)	Silvery-white metal
Sodium	Na (from natrium)	Soft, silvery-white metal
Sulfur	S	Yellow solid
Tin	Sn (from stannum)	Silvery-white metal
Zinc	Zn	Bluish-white metal

* Common from of the element under normal conditions.