e. Important Symbols and Valences. Since it is very tedious to continually
write complete names for elements, chemists developed the symbols for the elements
which you observed on the periodic table. It will not be necessary for you to know all
the symbols for your work but a number of them appear frequently enough that they
should be memorized. Table 1-3 lists important elements with their symbols and
valences. These should be committed to memory. (Please note that most, but not all,
valences conform either to the completed shell or octet rules.)
f. Ions. Any atom that gains or loses electrons becomes charged (electrical
charge) and is called an ion. An ion can be defined as any charged atom or group of
atoms. If the ion is positively charged, it is called a cation. If it is negatively charged, it
is called an anion. A group of atoms that has a charge and goes through a reaction
unchanged is called a radical. Whenever we write the symbol for an element and wish
to indicate it is an ion, we write the charge as a superscript to the symbol, for example,
Cl 1 or Na 1.
g. Chemical Bonding. When elements combine to form chemical compounds,
the electrons in the outer shell may be transferred from one atom to another or there
may be a mutual sharing of the electrons. In either case, a chemical bond is produced.
This means the two atoms do not travel or react independently of one another but are
held together by the exchange or sharing of the electrons. Both atoms involved in the
reaction attain a completed outer orbit, and stability results. There are three types of
chemical bonds--electrovalent, covalent, and coordinate covalent.
(1) Electrovalent (ionic) bonding. A transfer of one electron from one atom
to another resulting in opposite charges on the two atoms that holds them together by
electrostatic (opposite charges attract) attraction is called an electrovalent or ionic bond.
A good example of this is the bond formed between a Na (sodium) and a Cl (chlorine)
1 e in M shell
7 e in M Shell
Sodium has a complete outer shell
and a charge of +1. Chlorine has
met the octet rule in the M shell and
has a charge of 1.