Writing Formula Units of Ionic Compounds

Writing Formula Units of Ionic Compounds

Introduction to Formula Units

A formula unit represents the empirical (simplest) ratio of ions in an ionic solid that combine to give a neutral charge. In a solid, ions are locked together in a crystal lattice, not as discrete molecules.

• Crystal Lattice: A three-dimensional arrangement of several cations and anions together that form a stable pattern.

The Formula Unit

Ions	Crystal Lattice	Formula Unit
Sodium Ion (Na+) Chloride Ion (Cl-)	Repeating pattern of Na+ and Cl- ions	Sodium Chloride (NaCl)

Rules for Writing Ionic Compounds

1. Write the ions involved in the compound from the provided names.

2. Use the charges as subscripts in the formula of the ionic compound:

   • When numbers in charges are the same, they cancel out to combine the elements in a 1:1 ratio.

   • When numbers in charges are different, they cross over as subscripts to combine the elements.

Examples

• Aluminum Nitride: Al3+ and N3- → AlN (charges cancel, 1:1 ratio)

• Barium Phosphate: Ba2+ and PO43- → Ba3(PO4)2 (cross charges: 3 Ba2+ for every 2 PO43-)

General Formula for Ionic Compounds

For ions Am+ and Bn-:

$A_nB_m$

where the subscripts n and m are the absolute values of the charges on the opposite ions.

Practice Problem

• Provide the formula unit for the compound formed by the following ions: Mg2+ and SO42- Answer: MgSO4

• Write the molecular formula for Sodium dichromate: Answer: Na2Cr2O7

Key Terms

• Ion: An atom or molecule with a net electric charge due to the loss or gain of one or more electrons.

• Empirical Formula: The simplest whole-number ratio of atoms of each element in a compound.

• Crystal Lattice: The regular, repeating arrangement of ions in an ionic solid.

Additional info:

• Polyatomic ions (such as SO42- and PO43-) are treated as single units when writing formulas. Parentheses are used if more than one polyatomic ion is needed (e.g., Ba3(PO4)2).