Ionic Bonding Recap

A.  Ionic Bonding Basics

1.  Remember that metals and nonmetals often form bonds which are ionic (have ionic character).  Ionic bonds are characterized by a transfer of electrons from metals to nonmetals.   Many metals and nonmetals have more ionic character than covalent character.  

2.  The degree of ionic character that a bond has can be determined by calculating electronegativity differences between atoms in a bond.  Although not all chemistry texts "agree" on what number "defines" an ionic or covalent bond,  the greater the electronegativity difference between atoms the greater the ionic character of the bond.

In our class, we will use the following definition***

    Electronegativity difference > 1.7, the bond is IONIC (electrons transferred from metal to nonmetal)

    Electronegativity difference < 0.2, the bond is NONPOLAR COVALENT  
(fairly equal sharing of electrons).

    Electronegativity difference from 0.2 to 1.7, the bond is POLAR COVALENT (unequal sharing of electrons)

*** Remember that electronegativity is the attraction that an atom has for the electrons in a bond.

B.  Ways to Represent Ionic Bonding

1.  Metals and nonmetals are represented by Lewis dot diagrams.  

2.  Ionic bonds can be represented in various ways.  Some common forms of showing ionic bonding in the lithium fluoride compound are:

(The graphic above is from an online source.)

C.  Formula Units:  No Molecules Here!

 1.   Ionic substances do not form distinct molecules.  Rather, ionic compounds are characterized by a "formula unit,"  the basic repeating unit.   The formula unit is the chemical formula that is written using an ion chart. 

2.  Even though ionic compounds are represented by a formula unit, such as NaCl or CaCl₂, ionic compounds can be a crystal of any size.  The formula unit just gives the ratio of cations to anions in the crystal. NaCl has one sodium ion for every one chloride ion in the crystal.  CaCl₂ has one calcium ion for every two chloride ions in the crystal.   Different ionic crystal unit cells are shown below:

3.  Different unit cell arrangements (how the ions are packed together) give crystals their shape.  Common unit cells are called cubic, tetragonal, orthorhombic, hexagonal, monoclinic, and triclinic

4.  Each ionic crystal is as beautiful as a diamond!  Too bad ionic crystals dissolve in water!