Fundamentals of Chemistry
DOE-HDBK-1015/1-93
THE PERIODIC TABLE
Rev. 0
CH-01
Page 21
A more specific statement can now be made about which electrons are involved in chemical
reactions. Chemical reactions involve primarily the electrons in the outermost shell of an atom.
The term outermost shell refers to the shell farthest from the nucleus that has some or all of its
allotted number of electrons. Some atoms have more than one partially-filled shell. All of the
partially-filled shells have some effect on chemical behavior, but the outermost one has the
greatest effect. The outermost shell is called the valence shell, and the electrons in that shell
are called valence electrons. The term valence (of an atom) is defined as the number of
electrons an element gains or loses, or the number of pairs of electrons it shares when it
interacts with other elements.
The periodic chart is arranged so that the valence of an atom can be easily determined. For the
elements in the A groups of the periodic chart, the number of valence electrons is the same as
the group number; that is, carbon (C) is in Group IVA and has four valence electrons. The
noble gases (Group 0) have eight in their valence shell with the exception of helium, which has
two.
The arrangement in which the outermost shell is either completely filled (as with He and Ne)
or contains eight electrons (as with Ne, Ar, Kr, Xe, Rn) is called the inert gas configuration.
The inert gas configuration is exceptionally stable energetically because these inert gases are
the least reactive of all the elements.
The first element in the periodic table, hydrogen, does not have properties that satisfactorily
place it in any group. Hydrogen has two unique features: (a) the highest energy shell of a
hydrogen atom can hold only two electrons, in contrast to all others (except helium) that can
hold eight or more; and (b) when hydrogen loses its electron, the ion formed, H , is a bare
+
nucleus. The hydrogen ion is very small in comparison with a positive ion of any other element,
which must still have some electrons surrounding the nucleus. Hydrogen can either gain or lose
an electron. It has some properties similar to Group IA elements, and some similar to
Group VIIA elements.
The number of electrons in the outer, or valence, shell determines the relative activity of the
element. The elements are arranged in the periodic table so that elements of the same group
have the same number of electrons in the outer shell (except for the Transition Groups). The
arrangement of electrons in the outer shell explains why some elements are chemically very
active, some are not very active, and others are inert. In general, the fewer electrons an element
must lose, gain, or share to reach a stable shell structure, the more chemically active the element
is. The likelihood of elements forming compounds is strongly influenced by this valence shell
and on the stability of the resulting molecule. The more stable the molecules are, the more
likely these molecules are to form.
