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To the modern chemist, the atom is the smallest particle of an element that can enter into a chemical reaction. Thus, each element has atoms that are peculiar to itself and different from those of each of the other elements. Chemical reactions occur when atoms of different kinds unite to form groups in which they bear definite relationships to each other or when these groups undergo disruption or rearrangement. Chemical unions are of two general types.

In one type of union, atoms become bonded together to form definite aggregates that exist as independent, electrically neutral particles and are known as molecules (Latin "little mass"). Some elements have atoms that unite with others of their own kind to form molecules. These are ! known as elemental molecules and are exemplified by the chlorine , molecule which is made up of two chlorine atoms. Compound molecules are composed of two or more kinds of atoms and are exemplified by the water molecule, which contains two atoms of hydrogen and one of i oxygen.

To give a short definition of a molecule is not to give a more or less full account of properties.

Molecules are regarded as the smallest particles or elementary substances that can have independent existence. They account for the chemical properties and at least some of the physical properties of the substance they constitute. A single molecule does not exhibit in full the physical properties commonly associated with its particular variety of matter. These properties arise both within the molecule itself and within the aggregates of like molecules that constitute a sample of the given I substance. The density of water depends not only on the mass and volume of individual molecules but also on the manner in which the molecules are packed together. Since the chemist works with the aggregates, their j properties are of great practical importance.

A molecule of a compound contains, of necessity, at least two different atoms. An element molecule may contain only one atom, or it may contain two or more. Helium has monoatomic molecules; chlorine and hydrogen each exist as diatomic molecules; and sulphur molecules contain eight atoms. During reactions the atoms of elemental molecules usually are separated and individually redistributed in new combinations.


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