Chemistry Reference
In-Depth Information
Molecules
Molecules
, the smallest units of matter that have the properties of a sub-
stance, are made up of two or more atoms. The molecules of some chem-
ical elements, such as oxygen and nitrogen, mentioned above, for example,
are made up of two identical atoms. The molecules of compounds, that
consist of two or more combined elements are made up two or more atoms
of different elements bonded together (see Textbox 2).
The chemical and physical properties of single, uncombined atoms and
those of the same atoms bonded (chemically linked) to other atoms in
the molecules are basically different: single atoms are usually electrically
uncharged, whereas when they are part of molecules, they bear a positive
or negative electric charge of some measure. Atoms bearing an electric
charge are known as
ions
. Positively charged ions, formed when atoms
lose electrons, are known as
cations
. Negatively charged ions, formed
when atoms gain electrons, are known as
anions
. Most atoms of such
metals as copper and iron easily lose electrons when reacting chemically,
and form cations. When the atoms of nonmetallic elements react, on
the other hand, they generally gain electrons, and form
negative
anions.
The anions formed by chlorine, known as
chloride
, and by sulfur, known
as
sulfide
, are examples of common anions. Some nonmetallic elements -
for example, nitrogen, carbon, phosphorus, silicon, and sulfur - form
complex anions
, which are made up of atomic groups of two, three,
or more atoms. The
nitrate
anion, which is made up of one atom of nitro-
gen and three atoms of oxygen; the
carbonate
anion, made up of one
atom of carbon and three of oxygen, and the
sulfate
ion, made up of one
atom of sulfur and four of oxygen, are common examples of complex
anions.
A few general rules are helpful for understanding the electric charges
acquired by single as well as complex ions:
•
Metals and hydrogen (a nonmetal) form positively charged cations.
•
Nonmetals generally form negatively charged anions.
•
Some nonmetals form complex (polyatomic) anions, which consist
of a group of three or more atoms bearing a negative charge.
•
Cations combine with anions to form compounds known as
salts
;
when dissolved in water, the salts form solutions that conduct
electricity.
•
Solutions of nonionic compounds in water do not conduct
electricity.
•
Nonmetals can also combine with other nonmetals to form nonionic
compounds (most biological substances are nonionic).
