Man's physical universe

xanabras

580 MAN IS MASTERING HIS MATERIAL WORLD

aluminum is 3 because two atoms of aluminum combine with three

atoms of oxygen, each of which has a valence of 2, to form aluminum

oxide, AI2O3 (2 X 3 = 6, 3 X 2 = 6).

It has been found that the maximum valence exhibited by any

element is 8. It was also observed early in the history of modern

chemistry that atoms of metals have a greater tendency to react with

atoms of nonmetals than with atoms of other metals. It was only

natural for the expression metallic and nonmetallic valence to come into

general use. Later these expressions were changed to positive (+) and

negative (— ) valence, respectively. The general conclusion was thus

reached that molecules are formed by the combination of atoms of

positive valence with atoms of negative valence and that the valence

of each atom must be completely satisfied in each reaction.

Valence Is Now Believed to Represent the Number of Electrons Gained

or Lost or the Number of Pairs of Electrons Shared in Chemical

Reactions.

Knowledge of the structure of atoms made possible the electron

theory of valence. According to this theory, the reactions between

atoms are governed by the number of electrons in the outermost levels

or orbits of the atoms. In certain types of reactions the atoms of

metals give up all of their outside electrons to the atoms of nonmetals,

and the nonmetals annex enough electrons to make the sum of eight

in their outermost orbits. As the result of this loss of electrons by the

metals to become positively charged and the gain of electrons by the

nonmetals to become negatively charged, the metals and nonmetals

are attracted to each other to form compounds. Such changes are

called electrovalent reactions, the compounds produced are called

electrovalent compounds, and the valence represented by the gain or

loss of electrons is called electrovalence.

Some atoms exhibit two, three, or more valences.

This is explained

by assuming that in these atoms the number of electrons in the outside

orbit differs in each case, as the result of one or more of the electrons

moving from the outside orbit to an inner orbit or vice versa. Obviously,

energy is used up in changing the orbits of these electrons.

Various factors influence the tendency of elements to gain or lose

electrons. Elements which have fewer than four electrons in the

outer orbit tend to lose their electrons and thus become positively

charged. Such elements are said to have a metallic, or positive, valence.

Elements which have four electrons in the outer orbit tend

either to lose or to gain these electrons and may thus act as metals or

nonmetals. Elements having more than four electrons in. the outer

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