Formation of Ionic Compounds

How do atoms of elements form compounds?

The Octet Rule is the basis for the predictions about the charges on ions. Ionic compounds are formed as the result of the formation of positive and negative ions. Electrons are actually transferred from one atom to another to form rare gas electron structures for each ion. The atom which forms a positive ion loses electrons to the atom which gains electrons to form a negative ion. A compound is not stable unless the number of electrons which are lost and gained are equal .

Example: Determine the formula of a compound formed by the reaction of sodium and fluoride.


First examine the electron arrangement of the sodium and fluorine atoms.


 Atomic No.

 Bohr diagram

 Group No.

 Lewis Dots



 2 - 8 - 1





 2 - 7



Write the Lewis symbol for each atom. See Graphic on the right.

Determine the numbers of electrons which the atoms will lose and gain by applying the Octet Rule. Na loses one electron to have an octet. Fluorine gains one electron to have an octet.

After the electron is lost by sodium, it becomes a positive ion as determined previously (11 p + 10 e- = +1).

After fluorine gains the electron from sodium, it becomes a negative ion as determined previously (9 p + 10 e- = -1).

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

The final formula of sodium fluoride is NaF.


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.

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.