The Foundations of Chemistry

For Group VIIIA elements, the noble

gases, a molecule contains only one

atom, and so an atom and a molecule

are the same (see Figure 2-1).

You should remember the common

elements that occur as diatomic

molecules: H 2 , N 2 , O 2 , F 2 , Cl 2 , Br 2 , I 2.

2-1 Atoms and Molecules 49

atoms. The masses and charges of the three fundamental particles are shown in Table
2-1. The masses of protons and neutrons are nearly equal, but the mass of an electron is
much smaller. Neutrons carry no charge. The charge on a proton is equal in magnitude,
but opposite in sign, to the charge on an electron. Because atoms are electrically neutral,

an atom contains equal numbers of electrons and protons.

The atomic number(symbol is Z) of an element is defined as the number of protons
in the nucleus. In the periodic table, elements are arranged in order of increasing atomic
numbers. These are the red numbers above the symbols for the elements in the periodic
table on the inside front cover. For example, the atomic number of silver is 47.
A moleculeis the smallest particle of an element or compound that can have a stable
independent existence. In nearly all molecules, two or more atoms are bonded together
in very small, discrete units (particles) that are electrically neutral.
Individual oxygen atoms are not stable at room temperature and atmospheric pressure.
Single atoms of oxygen mixed under these conditions quickly combine to form pairs. The
oxygen with which we are all familiar is made up of two atoms of oxygen; it is a diatomic
molecule, O 2. Hydrogen, nitrogen, fluorine, chlorine, bromine, and iodine are other ex-
amples of diatomic molecules (Figure 2-2).
Some other elements exist as more complex molecules. One form of phosphorus mol-
ecules consists of four atoms, and sulfur exists as eight-atom molecules at ordinary tem-
peratures and pressures. Molecules that contain two or more atoms are called polyatomic
molecules (Figure 2-3).

Figure 2-2 Models of diatomic molecules of some elements, approximately to scale.

TABLE 2-1 Fundamental Particles of Matter

Particle Approximate Mass Charge

(symbol) (amu)* (relative scale)

electron (e) 0.0 1 

proton (por p) 1.0 1 

neutron (nor n^0 ) 1.0 none

*1 amu1.6605
 10 ^24 g

H

H 2

(hydrogen)

O 2

(oxygen)

F 2

(fluorine)

I 2

(iodine)

H OO FF I I

atomic number

symbol

47

Ag