b. Notice that the total charge of the oxygen plus the hydrogens is zero. A
molecule of water is quite stable; the hydrogen and oxygen do not separate very easily.
Sometimes this molecule is called H20 and now the reason is obvious. When the atoms
of different elements combine in this way, the substance formed is called a compound.
Here the compound, water, is made up of H20 molecules. A compound is defined as
the chemical combination of two or more elements.
c. Often in the process of forming compounds, large amounts of energy are
given off. Hydrogen gas may explode violently and, when it does, it is simply combining
with oxygen to form water.
d. Another type of molecule encountered can be formed from hydrogen. There
is no reason why one hydrogen atom cannot unite with another. In this case, there
would be two electrons orbiting two hydrogen nuclei. Here the K-shell would be
complete at least part of the time for each hydrogen atom (see Figure 1-5). This sort of
molecule is called a diatomic molecule. Hydrogen gas is composed of diatomic
molecules of hydrogen. Thus, a molecule is the smallest particle of any substance,
element, or compound as it normally exists in nature.
e. It is therefore apparent that the electron structures of atoms are important
things. They determine the way in which compounds are made and the world we live in
is composed mostly of compounds. Also, the electronic structure is intimately involved
in the production of visible light and other electromagnetic radiation.
1-7. ATOMIC NOTATION
a. In order to simplify discussions concerning elements and atoms, a standard
notational form is used to talk about atoms. It is based upon the primary characteristics
of the atom. The first of these characteristics is the number of protons in the nucleus of
the atom, which in a neutral atom is also the number of electrons in shells around the
nucleus. This number, which determines the element to which the atom belongs, is
called the atomic number. It is usually symbolized by the capital letter "Z." The second
primary characteristic is the number of neutrons in the nucleus, a factor which, to some
degree, determines the nuclear characteristics of the atom. The number of neutrons in
the nucleus is described in terms of the total number of major particles in the nucleus;
the total number of nucleons (sum of neutrons and protons) is called the atomic mass
number. It is usually symbolized by the capital letter "A." Therefore, the number of
neutrons is given by A - Z. In addition, each element has its own symbol or
abbreviation, such as "H" for hydrogen or "Fe" for iron. The standard notation takes the