IONIC COMPOUNDS
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 Negative Ions

How will fluorine complete its octet?

First examine the electron arrangement of the atom. The atomic number is nine, therefore, there are nine electrons and nine protons on the neutral fluorine atom. Write the Bohr diagram and Lewis symbol for fluorine: (see Graphic on the left)

This analysis shows that fluorine already has seven electrons in its outer level. The nearest rare gas is neon with 8 electron in the outer energy level. Therefore only one additional electron is needed to complete the octet in the fluorine atom to make the fluoride ion. If the one electron is added, the Bohr diagrams and Lewis symbols for fluorine and neon are identical. The octet rule is satisfied.

 Ion Charge?

What is the charge on fluorine as a result of adding one electron? A comparison of the atom and the ion will yield this answer.

 Fluorine Atom
 

 Fluoride Ion *
 

 9 p+

 to complete

 9 p +

 Protons are identical in the atom and ion. Negative charge is caused by excess electrons

 10 n

 octet

 10 n

 9 e-

 add 1 electron

 10 e-

 0 charge
 

 - 1 charge

 * The "ide" ending in the name signifies a simple negative ion.

SUMMARY PRINCIPLE OF IONIC COMPOUNDS

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.

OCTET RULE:

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.