ELEMENTS AND ATOMIC MASSES
INTRODUCTION
An element is a fundamental substance that cannot be broken down by chemical means into
simpler substances. Elements are the building blocks of all matter. Currently 106 different elements
are known. Of these only 88 are found naturally. Some of the other elements may have existed in
earlier times; all have been produced, usually in extremely small quantities, in the laboratory.
If one could take a small piece of an element and divide it and subdivide it into smaller and
smaller particles, one would finally come to a single unit that can no longer be subdivided and still
show the characteristics of the element. This ultimate particle, the smallest particle of an element
that can exist, is called an atom. An atom is also the smallest unit of an element that can enter into a
chemical reaction. Atoms can be decomposed into smaller parts; however, these subunits no longer
have the properties of elements.
Chemists have assigned specific abbreviations to each element. These are called the
chemical symbols of the elements. Fourteen of the elements have a single letter as their symbol and
all the rest have two letters. The symbol can stand for the element itself, for one atom of that
element or for a particular quantity of that element. When using elemental symbols it is important
that certain rules be followed in the writing of these rules:
1.Symbols consist of one or two letters.
2. If one letter is used, it is capitalized.
3. If two letters are used, the first is capitalized and the second is a lower-case letter.
4. Always print the capital letters.
Pure substances composed of two or more elements are called compounds. Because they
consist of two or more elements, compounds unlike elements are capable of being decomposed into
simpler substances by chemical changes. The atoms of the elements in a compounds are combined
in whole number ratios, not in fractional parts of atoms. For a specific compound these atom ratios
are always the same. Another way of saying the same thing is that a given compound always
contains the same elements in a fixed proportion by weight. This is the Law of Constant
Composition, which basically states that the composition of a substance will always be the same
regardless of origin. It represents one of the cornerstones of chemistry.
Experimentally we find that 1 g of hydrogen gas combines with 8 g of oxygen gas to form 9
g of water. The mass ratio of hydrogen to oxygen is 1/8. In terms of percent we can say that 1 g out
of 9 g or (1/9)(100)% = 11% represents hydrogen. The rest, or 89%, is oxygen. If we also know
that two atoms of hydrogen combine with one atom of oxygen then the mass ratio of one hydrogen
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