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 right)

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


 10 n

 9 e-

 add 1 electron

 10 e-

 0 charge


 - 1 charge

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


A negative ion is formed by the addition of negatively charged electrons. The protons do not change. Only non-metal atoms with l-3 electrons missing in the outer level add electrons to complete that level like the nearest rare gas. Metals never form negative ions.

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.