Man's physical universe



Scientists are not absolutely sure that atoms exist, for one cannot be

sure of anything which does not make a direct and unmistakable

impression on at least one of his senses. So useful has the atomic

theory been in

the study of chemical changes and so convincing are

the indications of atoms, however, that one might say scientists are

just as sure of the existence of atoms as they are of the existence of the


The Relative Weights of Atoms Are Called Atomic Weights.

Dalton worked out relative weights which he assigned to different

kinds of atoms. These weights were based upon the weights of different

elements which would react with a fixed weight of some reference

element. Dalton assumed that all binary compounds consisted of only

one atom of each of the two kinds of elements per molecule. At this

time the knowledge of valence had not been developed. The valence

of an element is simply the number of atoms of a reference element which

will combine with, or are equivalent to, one atom of the element in question.

The weight of an element which will combine with eight grams of oxygen

was accepted as the combining weight. Hydrogen was selected as

the reference element, with a combining weight of one. In the compound,

ammonia, NHs, nitrogen has a valence of three, because one

nitrogen atom is combined with three hydrogen atoms.

As quantitative information was gained, it was learned that there

were several combining capacities for a number of elements. It soon

became evident that a simple relation exists between the combining

weights of the elements and their atomic w^eights, namely, that the

combining weight multiplied by the valence is equal to the atomic weight.

Atomic weights are merely numbers which express the relative

weights of the atoms of the different elements, compared with oxygen

with a value of 16 as the standard.

The molecular weight is the sum of the atomic weights. The grammolecular

weight is the molecular weight expressed in grams. In chemical

work symbols and formulas always stand for quantities as well as the

qualitative composition of substances. Thus:

H stands for 1.008 (atomic weight)

H2 stands for 2.016 (molecular weight)

O stands for 16.000 (atomic weight)

H2O stands for 2.016 + 16.000, or 18.016 (molecular weight)

A gram-molecular weight of water, HoO, is, therefore, 18.016 grams.

The number of molecules in the gram-molecular weight of any substance

is called Avogadro's number, and its value is 6.062X10^. This

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