COMPOUNDS
A compound is a substance in which the atoms of two or more elements are joined together by
chemical bonds. Because of these bonds, it can be very difficult to split a compound into its individual
elements. The properties of a compound can be very different from those of the elements it contains. For
example, the salt we use in our food is a harmless compound, yet it is made up of two dangerous elements
-- sodium, which reacts violently with air; and chlorine, which is a poisonous gas.
Another important characteristic of a compound is that the weight of each element in the compound
always has a fixed, definite ratio to the weight of the other elements in the compound. For example, water
always breaks down into 2.016 parts of hydrogen by weight to 16.000 parts of oxygen by weight, which is a
ratio of about 1 to 8, regardless of whether the water came from the Mississippi River or the ice of
Antarctica. In other words, a compound has a definite, invariable composition, always containing the same
elements in the same proportions by weight; this is the law of definite proportions.
Water is an example of a compound. A water molecule consists of
an oxygen atom and two hydrogen atoms.
Many elements combine in more than one ratio, giving different
compounds. In addition to forming water, hydrogen and oxygen also form
hydrogen peroxide. Hydrogen peroxide has 2.016 parts of hydrogen to 32
parts of oxygen; that is, 1.008 parts of hydrogen to 16 parts of oxygen.
Water, as stated above, has 2.016 parts of hydrogen to 16 parts of oxygen.
The figure 2.016 is twice 1.008. This example illustrates the law of multiple proportions. When two elements
combine to form more than one compound, the element whose mass varies combines with a fixed mass of
the second element weights in a simple whole- number ratio such as 2:1, 3:1, or 3:2.
BINARY COMPOUNDS
A binary compound is one that is formed from two types of elements. Three possibilities exist.
metal + metal → metallic compound
metal + non-metal →ionic compound
non-metal + non-metal → covalent compound
Metallic compounds
The metallic elements are found on the left-hand side of the Periodic
Table. The proposed structure for samples of metals is that of the "band
theory" of metallic bonding. Metal samples are considered to be composed of
metal atoms that are held together by delocalized bonds formed by all of the
atoms in the sample. It is proposed that electrons can move easily within the
sample.
Ionic Compounds
Ionic compounds are crystalline compounds in which the forces
providing the structure of the material are ionic bonds. The crystals are made
up of positive and negative ions, but the compounds themselves possess no
net charge since the oppositely charged ions exist in a perfect balance with
each other. The common table salt, is a representative of an important class
of compounds called salts. A salt consists of positive and negative ions, Na
and Cl in NaCl.
Covalent Compounds
Covalent compounds are compounds in which the atoms are held
together by covalent bonds. It has lower melting and boiling points than
ionic compounds. Covalent compounds are usually gases or liquids,
although some are solids with giant molecular structures. The gases have
no significant forces of attraction between molecules.Covalent compounds
are formed as a result of the reaction of nonmetallic elements. They
cannot dissociate into ions, so they cannot function as electrolytes. Many
covalent compounds are insoluble in water, but may dissolve in a number of organic solvents.
NAME:_____________________________ GRADE&SECTION:______________________
SCIENCE 7: COMPOUNDS
