Some Properties of Matter. Грекова О.А. - 4 стр.

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             Some Properties of Matter
                                                    II

        All matter consists of very minute particles called atoms. In the world around us there
are over 90 kinds of atoms which have different chemical properties and so can be separated
and prepared by chemical means; these are called elements. All the countless thousands of
different kinds of substances around us are made up of combinations of elements.
        The commonest element is oxygen, which in the free state is a gas that makes up, by
weight, nearly a quarter of the earth’s atmosphere, but which exists in combinations in so
many solids that it constitutes about half the weight of the earth’s crust.
        The next commonest element is silicon, which, combined with oxygen, makes up rocks
and sands; and the next aluminium, which also exists in combination in common kinds of
rocks and clay. Neither of these two elements exists in a free form in nature, and 150 years ago
they were unknown.
        Atoms are very, very small. The atoms of different elements vary somewhat in size, but
they are all about one hundred-millionth of an inch across, the smallest being about half this
and the largest about twice this.
        Gases are the simplest form of matter and the best understood. Gases belonging to a
certain group, the so-called rare or noble gases, are elements consisting of single separate
atoms: we may mention as typical rare gases neon and argon, which are used in certain electric
discharge lamps. these gases are also called monatomic (that is, one-atomic), to emphasize
their nature. Most gases, however, consist of particles made up of two or more atoms. Such
groups of two or more atoms closely bound together are called molecules.
        Most gases, however, are not elementary, that is, do not consist of one kind of atom
only, but consist of molecules in which different sorts of atoms are combined. Thus in carbon
dioxide, the gas used in aerated drinks, the molecule consists of one atom of carbon and two
atoms of oxygen, tightly bound together. We may say that all gases are made up of molecules
if in the case of the rare gases we consider a single atom as a particularly simple form of
molecule.
        In the gases the molecules are separated by, on the average, distances large compared to
their size.
        In liquids and solids conditions are quite different, the atoms and molecules being very
close to one another, so that the space between them is small compared to their size. In the case
of solids the molecules and their atoms keep to the same place: they are like a tree shaking in
the wind, which, although it moves somewhat to and fro, stays in the same spot. In the case of
liquids the molecules slowly move, like a shaking man shouldering his way through a crowd,
not as they do in gases, with long free runs. It is the difficulties caused by the closeness of the
molecules that make the behaviour of solids and liquids less simple than that of gases.
        Great many solids have a crystalline form: common examples are salt and sugar, less
common examples are diamonds and rubies. In a crystal the atoms are all arranged in a regular
pattern.
        Pure metals are simple solids, for they are elements, each consisting of one kind of atom
only.
Copper consists of copper atoms only, zinc of zinc atoms only. Many familiar metals are,
however, melted together. Brass, for instance, consists of copper and zinc. A surprisingly large
number of substances have a regular crystalline structure, even wool. There are, however,
some substances, such as dough, which do not, and they are very troublesome to handle
scientifically.