Ionic Compounds Do Not Contain Molecules

What students typically write

“A molecule of sodium chloride has the formula NaCl.” Or: “In NaCl, each molecule contains one sodium and one chlorine.”

Students see the formula NaCl and assume it represents a molecule in the same way H2O represents a water molecule. This transfers the molecular model inappropriately to ionic compounds.

Why it is wrong

In sodium chloride, there are no molecules. The structure is a giant ionic lattice: a regular three-dimensional arrangement where every Na+ ion is surrounded by six Cl- ions, and every Cl- ion is surrounded by six Na+ ions. The electrostatic attraction extends in all directions, not just between one specific pair.

The formula NaCl is an empirical formula. It tells you the simplest ratio of sodium ions to chloride ions is 1:1. It does not mean there is a discrete unit of one Na bonded to one Cl floating around independently.

This distinction matters for explaining properties. Ionic compounds have high melting points because breaking the lattice means overcoming many strong electrostatic attractions, not just one bond. If NaCl were molecular, you would expect a low melting point (like most simple covalent molecules). The fact that NaCl melts at 801 degrees C is evidence of a giant lattice structure.

What the mark scheme actually wants

A typical question: “Describe the structure of sodium chloride and explain why it has a high melting point.” [3]

Marking points:

1. Giant ionic lattice / regular arrangement of ions (1 mark)
2. Strong electrostatic forces of attraction between oppositely charged ions (1 mark)
3. A lot of energy is needed to overcome the many/strong attractions (1 mark)

An answer that describes NaCl as molecular would contradict marking point 1 and likely score 0 for structure.

Worked example: wrong vs right

Question: Sodium chloride has a high melting point. Explain why. [2]

Wrong answer: “The NaCl molecules are held together by strong bonds, so lots of energy is needed to separate them.” This scores 0-1/2. Calling them molecules is wrong and “strong bonds” is vague. The examiner may give partial credit for the energy idea, but the structural description undermines the answer.

Right answer: “Sodium chloride has a giant ionic lattice with strong electrostatic forces of attraction between Na+ and Cl- ions (1). A large amount of energy is needed to overcome these many strong attractions (1).” This scores 2/2.

How to avoid this mistake

Apply this rule: if a compound is ionic (metal + non-metal, or contains a polyatomic ion like NH4+ or NO3-), never use the word “molecule” to describe it. Use “formula unit” for the ratio, “lattice” for the structure, and “ions” for the particles.

Reserve “molecule” for covalent substances: H2O, CO2, CH4 and so on, where atoms are genuinely joined in discrete groups by shared pairs of electrons.

Substance	Structure type	Say	Do not say
NaCl	Giant ionic lattice	”formula unit of NaCl"	"molecule of NaCl”
H2O	Simple molecular	”molecule of H2O"	"lattice of H2O”
SiO2	Giant covalent	”structure of SiO2"	"molecule of SiO2”

For the full treatment of ionic lattices, see ions and ionic bonds. The contrast with molecular structures is covered in simple molecules and covalent bonds.