Ionic and Molecular Compounds: A First Look

Introduction

Table salt and sugar are both white crystalline compounds found in every kitchen, yet they behave very differently. Salt melts at around 800 °C and conducts electricity when molten; sugar melts and chars at under 200 °C and never conducts. The difference lies in the kinds of elements they contain and the way their atoms are held together. This page gives a first look at the two great families of compounds: ionic and molecular.

Core explanation

A quick rule of thumb. Look at the elements in a compound and where they sit on the periodic table:

- metal + non-metal → usually ionic (for example NaCl, MgO, CaF₂) - non-metal + non-metal → usually molecular (for example H₂O, CO₂, CH₄, NH₃)

Ionic compounds. Metal atoms tend to lose electrons, becoming positive ions ; non-metal atoms tend to gain electrons, becoming negative ions . In sodium chloride, each sodium atom gives one electron to a chlorine atom, forming Na⁺ and Cl⁻. The oppositely charged ions attract strongly and pack together in a huge, regular three-dimensional lattice. There are no separate molecules; the formula NaCl simply tells us there is one sodium ion for every chloride ion.

Molecular compounds. When two non-metals combine, neither atom gives up electrons easily. Instead they share pairs of electrons, forming covalent bonds . The atoms are grouped into small, separate molecules. In water, each molecule is one oxygen atom sharing electrons with two hydrogen atoms. The bonds inside a molecule are strong, but the forces between neighbouring molecules are weak.

Typical properties compared:

Property Ionic (e.g. NaCl) Molecular (e.g. CO₂, H₂O) --- --- --- State at room temperature solid often gas or liquid Melting point high low Conducts when solid? no no Conducts when molten or dissolved? yes usually no

Why the difference? Melting an ionic compound means pulling apart ions held by strong attractions throughout the lattice, which takes a lot of energy. Melting a molecular substance only needs the weak forces between molecules to be overcome; the molecules themselves stay intact.

Exceptions exist. Some non-metal compounds, such as silicon dioxide, form giant covalent structures with very high melting points, and ammonium salts such as NH₄Cl are ionic without containing a metal. The rule of thumb is a starting point, not a law.

Step-by-step reasoning

To predict the type of a compound:

1. Write down the elements in its formula. 2. Classify each as a metal or non-metal using the periodic table. 3. If a metal is combined with a non-metal, predict ionic. 4. If all elements are non-metals, predict molecular. 5. Check against properties such as melting point where data are given.

Visual explanation

Picture an ionic lattice as a chessboard extended in three dimensions: small purple spheres (Na⁺) alternate with large green spheres (Cl⁻) in every direction. Contrast this with a box of separate water molecules, each a red sphere with two white spheres attached, tumbling past one another.

Real-world analogy

An ionic lattice is like a crowd of people all holding hands with everyone around them: pulling one person free means breaking many grips. A molecular substance is like pairs of dancers on a dance floor: each pair holds tight, but separating one pair from another is easy.

Real-world example

Road gritters spread rock salt, an ionic compound, which dissolves in water on the road and lowers its freezing point. Meanwhile the carbon dioxide in fizzy drinks is a molecular compound, a gas at room temperature because its small molecules are held to one another only very weakly.

Why?

Why do metals and non-metals form ions with each other? Metal atoms hold their outer electrons loosely, while non-metal atoms attract extra electrons strongly. Transferring electrons from metal to non-metal gives both a more stable arrangement, and the resulting opposite charges hold the compound together.

Common misconception

"Sodium chloride is made of NaCl molecules." Solid sodium chloride contains no molecules at all, only a continuous lattice of Na⁺ and Cl⁻ ions. The formula NaCl gives the ratio of ions, 1 : 1, not a molecule.

Worked example

Question: Predict whether magnesium oxide (MgO) and methane (CH₄) are ionic or molecular, and which has the higher melting point.

Reasoning: Magnesium is a metal and oxygen a non-metal, so MgO is ionic. Carbon and hydrogen are both non-metals, so CH₄ is molecular. Ionic lattices have strong attractions throughout.

Answer: MgO is ionic and CH₄ is molecular; MgO has the far higher melting point (about 2850 °C, compared with about −182 °C for methane).

Quick check

1. Is potassium bromide, KBr, likely to be ionic or molecular? Answer: Ionic, because potassium is a metal and bromine is a non-metal.

Exam focus

Use the metal/non-metal rule to classify compounds, and link structure to properties: ionic compounds have high melting points and conduct when molten or dissolved; simple molecular substances have low melting points and do not conduct. Always say "ions" for ionic and "molecules" for molecular.

Advanced insight

Bonding is really a spectrum. When two atoms attract electrons about equally, they share them evenly; when one attracts much more strongly, the sharing becomes lopsided, and at the extreme the electron is effectively transferred. Chemists measure this pulling power with electronegativity, and the difference between two atoms helps predict where a bond lies between covalent and ionic.

Summary

Compounds of a metal with a non-metal are usually ionic, made of positive and negative ions in a giant lattice with high melting points. Compounds of non-metals only are usually molecular, made of separate molecules held by covalent bonds, with low melting points. Ionic formulae give a ratio of ions; molecular formulae describe a molecule.

Practice questions

1. Classify each as ionic or molecular: CaCl₂, SO₂, Li₂O, NH₃. Answer: CaCl₂ ionic, SO₂ molecular, Li₂O ionic, NH₃ molecular. 2. What is an ion? Answer: An atom or group of atoms that carries an electric charge because it has lost or gained electrons. 3. Explain why sodium chloride has a much higher melting point than water. Answer: Melting sodium chloride means overcoming strong attractions between oppositely charged ions throughout the lattice, whereas melting ice only needs the weaker forces between water molecules to be overcome. 4. Why does molten sodium chloride conduct electricity but solid sodium chloride does not? Answer: In the molten state the ions are free to move and carry charge; in the solid they are held in fixed positions.