Man's physical universe

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THE ATOM IS THE UNIT OF CHEMICAL CHANGE 563

All chemical reactions were shown to be changes in which definite

weights of substances react with each other to produce definite weights

of products.

If an excess of one of the reactants is present, it will not

enter into the reaction but will

reaction.

The Atomic Theory Was Advanced by Dalton.

remain unchanged at the end of the

In 1807 John Dalton (1766-1844), an English schoolteacher, advanced

his atomic theory, one of the three or four great theories of

physical science. This theory may be summarized as follows:

1. The atom is the smallest division of an element which can exist

within its molecules or the molecules of any of its compounds.

2. Atoms are indivisible, eternal, and indestructible.

3. Atoms of different elements differ in chemical nature and have

different masses and volumes, but all atoms of the same element

are alike in properties and mass.

4. Compounds are formed by combining definite whole numbers of

atoms.

The laws of chemical combination are readily explained by Dalton's

atomic theory. Inasmuch as the atoms are assumed to be eternal and

indestructible, it follows that all matter which is made up of these

atoms is therefore indestructible; this is the law of conservation of

mass. Inasmuch as molecules are formed by the combination of definite

numbers of atoms, each of which represents a definite weight, it follows

that molecules have a definite composition by weight; this is the

law of definite composition.

The atom is sometimes called the chemical unit of matter, while the

molecule is called the physical unit. Matter rarely occurs in the form

of atoms, but rather in the form of molecules, which usually consist of

two or more atoms. Molecules of elements contain usually two or

more atoms of the same element, while molecules of compounds necessarily

contain two or more atoms of different elements. Molecules may

be defined as the smallest particles of a substance that can continue to

exist in a free state. Some elements, such as hydrogen, oxygen, and

chlorine, may be obtained in the form of atoms, but unless something

else with which these atoms may react is present, they will react with

other atoms of their own kind to form molecules of these elements.

A few elements — the rare gases, helium, argon, neon, krypton, xenon,

and radon — which do not react, exist in the form of atoms. In this

case the molecule is said to be composed of just one atom; i.e., the atom

and molecule are identical.

In a previous Unit the actual existence of molecules was discussed.

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