adding the results.
0.6917 × 62.919 598 amu + 0.3083 × 64.927 793 amu = 63.55 amu
Therefore, the calculated average atomic mass of naturally occurring copper is
63.55 amu. Average atomic masses of the elements listed in the Periodic Table,
rounded to one decimal place for use in calculations and also in full to four
decimal places, are given in the Chemical Elements table in the Tables for
Reference section at the back of the book.
Valence Electrons
Each atom attempts to have its outer energy level complete and accomplishes this
by borrowing, lending, or sharing its electrons. The electrons found in the
outermost energy level are called valence electrons. The remainder of the
electrons are called core electrons. The absolute number of electrons gained, lost,
or borrowed is referred to as the valence of the atom.
EXAMPLE:
This picture can be simplified to , showing only the valence electrons as
dots in an electron dot notation. This is called the Lewis structure of the atom.
To complete its outer orbit to eight electrons, chlorine must borrow an electron
from another atom. Its valence number then is 1. As stated above, when electrons
are gained, we assign a − sign to this number, so the oxidation number of chlorine
is –1.
TIP
A Lewis structure shows the atomic symbol to represent the nucleus and inner shell
electrons. It shows dots to represent the valence electrons.
ANOTHER EXAMPLE:
Since sodium tends to lose this electron, its oxidation number is +1.