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1.0 The Atom
Until the end of the nineteenth century it was assumed that matter was composed of
small, indivisible particles called atoms. The work of J.J. Thompson, Daniel
Rutheford, and Neils Bohr proved that atoms were complex structures that contained
both positive and negative particles. The positive ones were called protons and the
negative ones electrons.
Several models of the atom were proposed: the one by Thompson assumed that
there were equal numbers of protons and electrons inside the atom and that these
elements were scattered at random, as in the leftmost drawing in Figure 1-1. Later,
in 1913, Daniel Rutheford's experiments led him to believe that atoms contained a
heavy central positive nucleus with the electrons scattered randomly. So he modified
Thompson's model as shown in the center drawing. Finally, Neils Bohr
theorized that electrons had different energy levels, as if they moved around the nucleus
in different orbits, like planets around a sun. The rightmost drawing represents
this orbital model.
Investigations also showed that the normal atom is electrically neutral. Protons
(positively charged particles) have a mass of 1.673 X 10-24 grams. Electrons (negatively
charged particles) have a mass of 9.109 X 10-28 grams. Furthermore, the orbital
model of the atom is not actually valid since orbits have little meaning at the atomic
level. A more accurate representation is based on concentric spherical shells about
the nucleus. An active area of research deals with atomic and sub-atomic structures.
The number of protons in an atom determines its atomic number; for example,
the hydrogen atom has a single proton and an atomic number of 1, helium has 2 protons,
carbon has 6, and uranium has 92. But when we compare the ratio of mass to
electrical charge in different atoms we find that the nucleus must be made up of
more than protons. For example, the helium nucleus has twice the charge of the hydrogen
nucleus, but four times the mass. The additional mass is explained by assuming
that there is another particle in the nucleus, called a neutron, which has the
same mass as the proton but no electrical charge. Figure 1-2 shows a model of the
helium atom with two protons, two electrons, and two neutrons.
Figure 1-2 Model of the Helium Atom
1.1 Isotopes and Ions
But nature is not always consistent with such neat models. Whereas in a neutral atom,
the number of protons in the atomic nucleus exactly matches the number of electrons,
the number of protons need not match the number of neutrons. For example, most hydrogen
atoms have a single proton, but no neutrons, while a small percentage have one
neutron, and an even smaller one have two neutrons. In this sense, atoms of an element
that contains different number of neutrons are isotopes of the element; for example
water (H2O) containing hydrogen atoms with two neutrons (deuterium) is called
"heavy water."
An atom that is electrically charged due to an excess or deficiency of electrons is
called an ion. When the dislodged elements are one or more electrons the atom
takes a positive charge. In this case it is called a positive ion. When a stray electron
combines with a normal atom the result is called a negative ion