Compounds are pure substances that differ from elements because they contain more than one kind of atom. The Law of Definite Composition (or the Law of Definite Proportions) applies to compounds.

For example, sodium chloride contains one atom of sodium for each atom of chlorine. Water contains two atoms of hydrogen for each atom of oxygen.

It is known that two elements can combine with one another in different proportions to form more than one compound. This obeys the Law of Multiple Proportions (or Composition).

For example, the elements carbon and oxygen can form two different compounds: carbon monoxide (CO) and carbon dioxide (CO2)

Molecules and Molecular Compounds

A molecule is an uncharged particle made up of more than one atom Hydrogen gas (H2) and water (H2O) are examples of molecules.

A compound that is also a molecule is called a molecular compound. Water, H20, is an example of a molecular compound.

Ions and Ionic Compounds

We know that an atom is electrically neutral when it has an equal number of protons and electrons. When an atom or group of atoms gains or loses electrons, and the number of electrons is no longer equal to the number of protons, then the atom becomes an electrically charged ion.

Positive ions are called cations, and negative ions are called anions. When forming ions, the following principles usually apply:

 – Atoms of the metallic elements tend to lose one or more electrons and become positive.

For example, magnesium loses two electrons and becomes the ion Mg2+.

 – Atoms of the nonmetallic elements tend to gain one or more electrons and become negative.

For example, oxygen gains two electrons and becomes the ion O2-.

Compounds consisting of ions are known as ionic compounds. An ionic compound exists as a group of charged atoms. For, example, magnesium oxide (MgO) is made up of an ion of Mg2+ and an ion of O2

Another example of an ionic compound is sodium chloride (NaCl), which is made up of an ion of Na+ and an ion of Cl-:

The following is a list of the chemical formulas for several ionic compounds:

Name                             Chemical Formula

barium chloride                              BaCl2

magnesium hydroxide                Mg(OH)2

ammonium chloride                      NH4Cl

lithium cyanide                               LiCN

calcium carbonate                         CaCO3

potassium permanganate               KMNO4

sodium hydride                                 NaH

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