An ionic bond forms when one atom transfers valence electrons to another atom, resulting in the creation of two oppositely charged particles called ions. The atom that loses electrons becomes a positively charged cation, while the atom that gains them becomes a negatively charged anion. The resulting bond is a powerful electrostatic attraction between these opposing charges, which holds the compound together. This fundamental electron transfer explains why ionic compounds do not form individual, isolatable units known as molecules, unlike compounds joined by covalent bonds.
The Continuous Structure of Ionic Compounds
Ionic compounds do not exist as discrete, isolated pairs of ions in the way that water or carbon dioxide exist as individual molecules. Instead, the strong electrostatic forces holding the ions together extend uniformly in all directions. This arrangement leads to the formation of a crystal lattice, which is a continuous, repeating, three-dimensional array of alternating positive and negative ions.
The structure maximizes the attractive forces between oppositely charged ions while minimizing the repulsive forces between ions of the same charge. For instance, in sodium chloride (NaCl), every sodium ion is surrounded by six chloride ions, and every chloride ion is surrounded by six sodium ions. This continuous structure means no single ion is specifically partnered with only one other ion, making the term “molecule” inappropriate for describing ionic compounds.
Defining the Formula Unit
Since a molecule does not exist, chemists use the term formula unit to represent the smallest representative unit of an ionic compound. The formula unit is defined as the simplest whole-number ratio of cations and anions required to maintain the electrical neutrality of the entire crystal lattice. It is a conceptual tool used for chemical calculations and nomenclature, not a physical particle that can be isolated.
All ionic compounds must have an overall net charge of zero, meaning the total positive charge from cations must exactly cancel the total negative charge from anions. For example, in sodium chloride, the +1 sodium ion and the -1 chloride ion result in the formula unit NaCl, representing a simple 1:1 ratio. The formula unit thus serves as the empirical formula for the substance, showing the lowest possible ratio of ions present.
Determining the Simplest Ratio
The specific ratio of ions in a formula unit is determined by the charges of the constituent ions, a principle known as charge balance. When a metal and non-metal combine, the number of electrons lost by the metal must equal the number of electrons gained by the non-metal. This requirement dictates the subscripts used in the final formula unit.
Consider magnesium chloride, where magnesium forms a +2 cation (Mg²⁺) and chlorine forms a -1 anion (Cl⁻). To achieve electrical neutrality, one magnesium ion requires two chloride ions to balance the charges (+2 + 2(-1) = 0). This results in the formula unit MgCl₂, representing a 1:2 ratio.
The same principle applies to compounds with higher charges, such as aluminum oxide. Aluminum forms a +3 cation (Al³⁺), and oxygen forms a -2 anion (O²⁻). To balance these charges, the lowest common multiple between the charges is 6. Therefore, two aluminum ions (2 x +3 = +6) and three oxide ions (3 x -2 = -6) are required, yielding the formula unit Al₂O₃.