Simple Compounds Covalent Compounds  Elmhurst College
Positive Ions Sodium Fluoride  Magnesium Oxide  Chemistry Department
Negative Ions Iron Oxide Review Covalent/Ionic Cpds  Virtual ChemBook


 Formation of Ionic Iron Oxide

How do atoms of iron react with molecules of oxygen in air to form a compound?

Steel wool burns in air with a very bright white light, giving off a large amount of energy, and white smoke with is iron oxide in very fine particles. The steel wool changes color to a more gray appearance which is the iron oxide. It is proven that the steel wool has changed into a compound because you can no longer ignite the steel wool after it has burned.

Graphic of Burning Steel Wool.

A different reaction occurs more slowly to give the orange/brown color of rust that you may see on various iron objects.

Iron Oxide: Determine the formula of a compound formed by the reaction of iron and oxygen.


First examine the electron arrangement of the iron and oxygen atoms.


 Atomic No.
 Bohr diagram

 Group No.
 Lewis Dots



 2 - 8 - 14 - 2

 8 B

 3 *



 2 - 6



*Iron does not follow the rules in determining the Lewis diagram. This is a special case and unless you are an expert, you would not be expected to figure this out by yourself.

Write the Lewis symbols for each atom. See graphic on the left.

Determine the numbers of electrons which the atoms will lose and gain by applying the Octet Rule. Fe loses three electrons**. Oxygen gains two electrons to have an octet.
**Octet rule does not apply to iron.

 Fe Atom

Fe Ion

 O Atom

O Ion

 26 p+

 26 p+

 8 p+

 8 p+

 26 e-
 lose 3 e-

 23 e -

 8 e-
 gain 2 e-

 10 e-

 0 charge

 +3 charge

 0 charge

 -2 charge

The ionic bond between ions results from the electrostatic attraction of opposite charges.

The final formula of iron oxide is Fe2O3.

 Why does iron oxide have subscripts in the formulas?

The Octet Rule is the basis for the predictions about the charges on ions. A compound is not stable unless the number of electrons which are lost equals those gained. The electrons gained or lost on each atom may be different. The exchange of electrons continues until there are no left over electrons and no empty spots. This sometimes results in several atoms being required. The subscripts are written to show how many of each kind of atom is involved.


An ionic compound is formed by the complete transfer of electrons from a metal to a nonmetal and the resulting ions have achieved an octet. The protons do not change. Metal atoms in Groups 1-3 lose electrons to non-metal atoms with 5-7 electrons missing in the outer level. Non-metals gain 1-4 electrons to complete an octet.


Elemental atoms generally lose, gain, or share electrons with other atoms in order to achieve the same electron structure as the nearest rare gas with eight electrons in the outer level.

The proper application of the Octet Rule provides valuable assistance in predicting and explaining various aspects of chemical formulas.