Ionic compounds and formula units Knowledge-Based Study

Study notes

1. What is an Ionic Compound?

   An ionic compound is a neutral chemical substance composed of positively charged ions (cations) and negatively charged ions (anions) held together by strong electrostatic attractions called ionic bonds.

   Unlike covalent substances, which consist of discrete, individual molecules, ionic compounds form vast, continuous three-dimensional networks known as crystal lattices. In a crystal lattice, each cation is surrounded by multiple anions, and each anion is surrounded by multiple cations, maximizing attractive forces while minimizing repulsive forces.

   Key Properties of Ionic Compounds

   • High melting and boiling points: Large amounts of thermal energy are required to overcome the strong electrostatic attractions throughout the lattice.
   • Brittleness: Applying a mechanical force shifts layers of ions, aligning like charges next to each other, which causes strong repulsive forces that shatter the crystal.
   • Electrical conductivity: Solid ionic compounds do not conduct electricity because the ions are fixed in place. However, when melted (molten) or dissolved in water (aqueous), the ions become mobile and can carry an electric current.

   Why Not "Molecules"?

   Because ionic compounds do not exist as distinct individual molecules, it is chemically incorrect to refer to a "molecule of sodium chloride" ($NaCl$). Instead, chemists use the term formula unit.

   A formula unit is the lowest whole-number ratio of ions represented in an ionic compound that maintains electrical neutrality. It serves as the empirical representation of the extended ionic lattice.

   Example: Sodium Chloride

   In a solid crystal of table salt, millions of sodium ions ($Na^+$) and chloride ions ($Cl^-$) are packed in an alternating pattern where each ion has six nearest neighbors. The ratio of sodium ions to chloride ions is strictly 1:1. Therefore, the formula unit is written simply as $NaCl$.

   Example: Calcium Fluoride

   In calcium fluoride, calcium forms $Ca^{2+}$ cations and fluorine forms $F^-$ anions. To maintain an electrically neutral network, there must be two fluoride ions for every calcium ion. Thus, the formula unit is $CaF_2$.

   Principle of Electroneutrality

   All ionic compounds are electrically neutral overall. The total positive charge contributed by the cations must exactly balance the total negative charge contributed by the anions:

   $$\text{Total Positive Charge} + \text{Total Negative Charge} = 0$$

   Step-by-Step Method for Writing Formula Units

   1. Identify the ions and their charges: Determine the cation and anion from their group numbers or oxidation states (e.g., $Al^{3+}$ and $O^{2-}$).
   2. Find the lowest common multiple: Determine the smallest common multiple of the charges to find the number of each ion needed for neutrality. For charges of $+3$ and $-2$, the lowest common multiple is 6.
   3. Determine the subscripts: Divide the lowest common multiple by each ion's charge magnitude to obtain the lowest whole-number subscripts. Two $Al^{3+}$ ions provide a total charge of $+6$, and three $O^{2-}$ ions provide a total charge of $-6$.
   4. Write the formula: Place the cation first, followed by the anion, using subscripts to indicate the ratio: $Al_2O_3$.

   Note: Subscripts of 1 are never written. For polyatomic ions, use parentheses if more than one polyatomic unit is required, such as in $Mg(NO_3)_2$.