exhibit identical chemical behavior. Induced nuclear reactions can produce both stable
and radioactive nuclei. If the nucleus of the atom is unbalanced during the
bombardment reaction the atom is called a radioisotope. Radioisotopes, such as
cobalt66 for treatment of cancer and iodine131 for diagnosing of thyroid tumors, are of
vital importance in the medical field. The presence of isotopes helps to explain why
many atomic weights in the periodic table are not whole numbers since all of the
isotopes must be considered when computing the average atomic weight of the
element.
1-6.
We have now developed the concept that matter was built from a basic unit
called the atom, and we have discussed the nature of the atom. We know, however,
that very little matter exists as free elements. Most of the things around us are
combinations of elements. Logically, the next step is to consider how things combine.
a. Valence. The valence of an element can be defined as a measure of its
combining power or the number of electrons an atom must gain, lose, or share to have
a full or stable outer electron shell. The reason atoms combine is contained in this
definition. There are certain electron configurations in nature that are unusually stable
(unreactive). The elements that have these configurations are in Group VIII A of the
periodic table. They are sometimes referred to as the inert or noble gasses because
they are found in very few combinations in nature. Other elements, by gaining, losing,
or sharing electrons, can try to make their outer electron shells resemble the shells of
the noble gases and hence become very stable. We can see how this works by
considering the two simplest elements, hydrogen, and helium. Hydrogen has one
electron in the K shell since it has only one proton. Therefore, hydrogen is a very
reactive element, occurring naturally in many compounds. Helium, a noble gas, has two
electrons in the K shell since it has two protons. Helium is very unreactive. Note that
helium, by having two electrons, has a completed outer shell, since the K shell can hold
only two electrons. Hydrogen would like to be as stable as helium and could be if it
could gain or share one more electron to give it a completed outer shell. Hydrogen
seeks this electron in nature by combining with other elements.
b. Octet Rule. If you examine the noble (inert) gases (like helium), you will see
that not all have a completed (full) electron shell. Except for helium, the noble gases
have eight electrons in their outer shell, yet they are still very stable. Chemists have
observed that other elements sometimes gain, lose, or share electrons in order to have
eight electrons in their outer shell. This observation led to the development of the octet
rule, which states that outer electron shells prefer to have eight electrons even though
the shell may not be full. (Octet means a group of eight.) On the next page are some
examples of the electron configurations for various elements which indicate to us how
many electrons they can gain, lose, or share to fit the octet rule or have a completed
outer shell.
MD0803
1-13