Molecules of Elements and Molecules of Compounds

Introduction

A molecule is a group of atoms held together by chemical bonds. It is tempting to think that every molecule is a compound, but that is not true. Oxygen gas is made of molecules, yet it is an element. The key question is not whether atoms are joined, but whether the joined atoms are all of the same kind. Particle diagrams, simple pictures made of coloured circles, let us see this difference at a glance and are among the most useful tools in chemistry.

Core explanation

Three types of particle. In particle diagrams we meet:

- Single atoms — for example the noble gases helium and neon, whose atoms exist on their own. - Molecules of elements — two or more atoms of the same element bonded together, such as O₂, H₂, N₂, Cl₂, and S₈ in sulfur. - Molecules of compounds — two or more atoms of different elements bonded together, such as H₂O, CO₂, NH₃ and CH₄.

Elements. A sample of an element contains only one kind of atom. In a diagram, every circle is the same colour, whether the circles appear singly or joined in pairs or larger groups. Metals and carbon in diamond are drawn as regular, touching arrays of identical circles.

Compounds. A sample of a pure compound contains only one kind of particle, and each particle contains at least two different kinds of atom joined together. In a diagram of water, every particle is identical: one large red circle (oxygen) joined to two small white circles (hydrogen).

Mixtures. A mixture contains two or more different kinds of particle that are not chemically bonded to each other. Air is a good example: its diagram shows N₂ molecules, O₂ molecules, single argon atoms and a few CO₂ molecules, all moving independently.

Reading a diagram. Ask two questions:

1. Are all the particles identical? If yes, it is a pure substance; if no, it is a mixture. 2. For a pure substance, does each particle contain only one kind of atom? If yes, it is an element; if no, it is a compound.

Diagrams show states too. Particles touching in a regular pattern represent a solid; touching but disordered, a liquid; widely spaced, a gas. A single diagram can therefore tell you both the type of substance and its state.

Step-by-step reasoning

To classify a particle diagram:

1. Count the different kinds of particle in the box. 2. If there is more than one kind, it is a mixture; stop here. 3. If there is one kind, look inside a particle: one colour of circle means an element. 4. Two or more colours bonded together means a compound.

Visual explanation

Picture four boxes. Box A: separate pairs of blue circles (nitrogen, an element). Box B: identical groups of one black and two red circles (carbon dioxide, a compound). Box C: blue pairs and red pairs moving freely (a mixture of elements). Box D: single grey circles packed in rows (a metal element).

Real-world analogy

Think of a car park. A car park holding only red cars of one model is like an element. One holding only identical red-and-white two-tone cars is like a compound: every unit is the same but made of two colours. A car park with many different cars parked separately is like a mixture.

Real-world example

The air you breathe is a mixture of molecules of elements (N₂ and O₂), single atoms of an element (argon) and molecules of compounds (CO₂ and H₂O vapour). Your lungs absorb oxygen molecules from this mixture without needing to break any chemical bonds, because the gases are not bonded to one another.

Why?

Why do some elements form molecules while others exist as single atoms? A single atom of oxygen or chlorine is unstable and bonds readily, so pairs form. Noble gas atoms already have a stable electron arrangement, so they have no tendency to bond and exist as separate atoms.

Common misconception

"If atoms are joined together, the substance must be a compound." Oxygen, O₂, has two atoms bonded together but both are oxygen, so it is an element. A compound needs atoms of at least two different elements bonded together.

Worked example

Question: A diagram shows six identical particles, each made of one green circle bonded to one white circle. Classify the substance.

Reasoning: All six particles are identical, so it is pure. Each particle contains two different kinds of atom bonded together, so it is a compound. It could represent hydrogen chloride, HCl.

Answer: A pure compound made of molecules.

Quick check

1. Is a box containing only Cl₂ molecules an element, a compound or a mixture? Answer: An element, because every particle contains only chlorine atoms.

Exam focus

Particle-diagram questions are very common. Read each box carefully, count the kinds of particle first, and then look inside each particle. Remember that a mixture can contain elements, compounds or both, and that diatomic elements such as H₂, N₂, O₂ and Cl₂ are still elements.

Advanced insight

Not all compounds are made of separate molecules. Sodium chloride and silicon dioxide form giant structures in which atoms or ions are bonded in a continuous network. Their diagrams show a repeating pattern of two colours with no distinct molecules, and their formulae describe a ratio rather than a molecule.

Summary

Molecules of elements contain atoms of one kind, such as O₂; molecules of compounds contain different atoms bonded together, such as H₂O. In particle diagrams, a pure substance has only one kind of particle, while a mixture has several kinds that are not bonded to each other. Colour and spacing reveal both the substance type and its state.

Practice questions

1. Classify H₂, H₂O and a mixture of H₂ and O₂. Answer: H₂ is an element, H₂O is a compound, and H₂ with O₂ is a mixture of two elements. 2. Describe a particle diagram of carbon dioxide gas. Answer: Widely spaced, identical particles, each one black circle bonded to two red circles. 3. Why is argon drawn as single circles while nitrogen is drawn as pairs? Answer: Argon atoms are stable on their own and do not bond, whereas nitrogen atoms bond in pairs to form N₂ molecules. 4. A box contains identical particles, each made of two red circles. Is it an element or a compound? Explain. Answer: An element, because each particle contains only one kind of atom, even though the atoms are bonded together.