Man's physical universe

xanabras

THE NATURE OF MOLECULES 581

orbit tend to gain electrons and are said to exhibit a nonmetallic, or

negative, valence.

The larger atoms show a greater tendency to lose electrons, inasmuch

as the outer electrons are held less firmly because of the greater

distance that separates them from the nucleus. Atoms of light nonmetals

tend to gain electrons more readily than heavy nonmetallic

atoms for the same reason.

In many chemical reactions there is no gain or loss but rather a

The compounds thus produced are called covalent

sharing of electrons.

compounds, and the valence represented is called covalence.

of elements, for example, are covalent in nature.

All Chemical Reactions Are Accompanied by Energy Changes.

Molecules

Inasmuch as the energy which holds the atoms together in molecules

differs in each compound, every chemical reaction involves either an

absorption or evolution of energy. In some cases the energy evolved

may be in the form of electrical energy, as in the case of chemical cells.

In other cases the energy is evolved as light, but in the majority of

cases it is evolved as heat. All reactions in which heat is evolved are

called exothermic, and those reactions which absorb heat are called

endothermic.

It is a matter of conjecture as to the actual causes of this

absorption or evolution of energy in chemical reactions, but it is certain

that the explanation will be based on the study of atomic structures.

Electrovalent Reactions Do Not Need to Be Activated, but Covalent

Reactions Require Activation.

In the case of reactions between electrovalent compounds, there is

simply a new combination of charged atoms. These charged atoms,

or ions, as they are called, must be separated from each other before

they can combine in new ways. In order to free the ions in electrovalent

compounds, it is only necessary to dissolve them in a solvent

which acts as an electrical insulator in so far as the ions are concerned.

By evaporating the solvent, the ions combine once more.

If any two or more ions may combine in such a way as to be removed

from such a solution, they will do so, and thus new compounds are

formed; for example, it will be shown in a later section that ions may

be removed from solution by forming insoluble solid or gaseous compounds.

In the case of covalent compounds, however, it is difficult to bring

about recombination of the atoms to form new molecules.

In general,

it is thought that molecules must collide with each other in order to

react, and that these collisions produce no chemical reactions except

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