Principles of Chemistry Online
ed, by Nivaldo Tro
Chapter 5: Molecules and Compounds
In this chapter we will learn how compounds are formed, the types of chemical bonds in the compounds,
how to write correct formulas and name the two types of compounds.
Molecules and Compounds
A molecule is a particle of matter in which there are two or more atoms combined together chemically.
In Chapter 3, we learned that pure substances could be classified either as elements or compounds. Elements
may exist as individual atoms, or as molecules, depending on the element. Elements that exist as molecules
have more than one atom of the same type chemically joined together.
is made up of
two or more elements (two or more types of atoms)
which have been
chemically combined and therefore exists as molecules.
Examples of compounds are water, H
; carbon monoxide, CO.
Compounds generally have completely different properties than the
elements from which they are formed.
For example, table salt, NaCl (sodium chloride), is commonly used
to make our food taste better, but both sodium metal and chlorine gas can be quite harmful as individual
Law of Constant Composition
In a compound, there are two or more different types of atoms present. However, it is important to realize
that a compound has a
whereas a mixture has variable composition. The Law of Constant
Composition (sometimes called the Law of Definite Proportion) states that a compound will always be made
up of the same elements combined in the same ratios by mass. For example, water will always have eight
parts of oxygen for every part of hydrogen by mass.
Chemical formulas provide us with a shorthand way of describing the makeup of a compound by listing the
type of atoms present as well as the number of atoms in the smallest unit of the compound.
In a chemical formula, the atom types are represented by the element symbol from the periodic table. If more
than one atom of a particular type is present, a subscript numeral to the right of the element symbol specifies
how many are present.
For example, the formula of the compound ammonia is NH
; one unit of ammonia
contains one nitrogen atom and three hydrogen atoms. If there is only one atom of a particular type, the
subscript “1” is not used.
Use parentheses around a repeating group of atoms in a formula.
Elements can be atomic or molecular
As we stated previously, elements can exist as individual atoms (
) or as molecules
Molecular Elements: Rule of Sevens
There are seven common
that occur naturally as 2-atom molecules:
If you locate these elements on the periodic table, you will notice that beginning at nitrogen, six
of these elements form a figure “7”.
You must remember to add hydrogen into this group to account for all
seven elements. This is called the rule of 7’s.