Elmhurst College
Ionic Compounds Review Covalent/Ionic Cpds    Chemistry Department
Covalent Compounds  Virtual ChemBook


 What are Compounds and Molecules?

INTRODUCTION: Since there are only some ninety elements, but over two million known compounds, most substances are not found as independent atoms but as groups of atoms known as molecules in compounds. The compounds may be divided into two groups. Those molecules that consist of charged ions with opposite charges are called IONIC. These ionic compounds are generally solids with high melting points and conduct electrical current. The other type of molecules are called COVALENT and do not consist of ions. Covalent compounds have low melting points and do not conduct electric current.

COMPOUNDS contain two or more different elements in a chemically combined form.

The simplest form of most matter, other than simple elements which still retains specific properties, is in the form of compounds. The simplest form of a compound is a molecule.

A MOLECULE contains two or more atoms.

Since a compound contains two or more different elements, it also contains two or more different atoms, hence the simplest form of a compound is a molecule. As an example, a molecule of water contains two hydrogen atoms and one oxygen atom, (written as H2O).

When an element consists of two identical atoms it is called DIATOMIC molecule. The simplest form of some elements is a molecule. For example oxygen in air consists of molecules containing two atoms of oxygen, (written as O2).



Compounds contain atoms of different elements combined in whole number ratios as stated as the Law of Definite Composition. The Law of Definite Composition states that two or more elements combine to form a compound in a fixed proportion by weight without regard to method of preparation.

For example water is always 88.9% by weight oxygen and 11.1% by weight hydrogen. Through a fairly complicated process the percentages by weight are ultimately converted into simple whole number ratios. Water molecules contain a ratio of two hydrogen for every one oxygen atom . This is expressed in the form of a formula.

Water is always H2O and not HO or H3O.

 A good contrast to the compound of water is the compound hydrogen peroxide, a very similar, but very distinct compound with one extra oxygen atom. The ratio is two hydrogen atoms to two oxygen atoms. The formula for hydrogen peroxide is H2O2.

The composition of compounds is indicated by a formula using symbols for the elements. The number of atoms in a formula is given by a subscript. If no subscript is given, then the number of atoms is assumed to be one.

The composition of compounds is also indicated by the names of compounds. For example carbon monoxide (CO) consists of one carbon and one oxygen atom ("mono" = one) . While carbon dioxide (CO2) consists of one carbon and two oxygen atoms ("di" = two). As you study chemistry, there will be constant references to both formulas and names of compounds. You must learn these as they are the language of chemistry.