What Is a Molecule?

Introduction

Atoms are the building blocks of matter, but most atoms do not exist on their own. In the air you breathe, oxygen atoms are paired up; in a glass of water, hydrogen and oxygen atoms are locked together in groups of three. These tightly bound groups are called molecules . Understanding what a molecule is — and what it is not — is one of the most important steps in learning chemistry.

Core explanation

Definition. A molecule is a group of two or more atoms held together by chemical bonds and acting as a single unit. In most molecules the atoms are joined by covalent bonds , in which neighbouring atoms share pairs of electrons.

Joined, not just nearby. Atoms that happen to be close together are not a molecule. In a mixture of hydrogen and oxygen gases, hydrogen molecules and oxygen molecules move around separately. Only when they react do the atoms become bonded into new water molecules. The key feature of a molecule is that its atoms are chemically joined and move together as one particle.

A molecule has a fixed make-up. Every water molecule contains exactly two hydrogen atoms and one oxygen atom, written H₂O. Every carbon dioxide molecule contains one carbon atom and two oxygen atoms, CO₂. This fixed make-up is shown by the molecular formula .

A molecule has a shape. The atoms in a molecule are held in definite positions. A water molecule is bent, with an angle of about 104.5° between the two O–H bonds. A carbon dioxide molecule is straight. Shape affects how molecules behave, as later topics will show.

The smallest unit with the substance's properties. For a molecular substance, a single molecule is the smallest particle that still has the chemical identity of that substance. One water molecule is still water; split it into hydrogen and oxygen atoms and it is no longer water.

Two kinds of molecule.

- Molecules of elements contain atoms of only one element, such as O₂ and N₂. - Molecules of compounds contain atoms of two or more different elements, such as H₂O and NH₃.

Strong inside, weaker between. The bonds holding atoms together inside a molecule are strong. The forces between separate molecules are much weaker. That is why water boils at 100 °C without the molecules breaking apart: boiling separates whole molecules from one another.

Step-by-step reasoning

To decide whether a particle is a molecule:

1. Check that it contains at least two atoms. 2. Check that the atoms are joined by chemical bonds, not just mixed. 3. Check that it moves and reacts as one unit. 4. If all three are true, it is a molecule.

Visual explanation

Use a 3D viewer or a model kit. A single sphere is one atom. Now snap one red sphere to two white spheres with short connectors: that is a water molecule. Spin it and the three spheres turn together, because they are bonded. A pile of loose spheres, by contrast, falls apart when you move it.

Real-world analogy

A molecule is like a team of three people roped together on a mountain. They move as one unit and cannot drift apart. A crowd of walkers on the same path is more like a mixture: they are close, but each walks separately.

Real-world example

When you smell coffee, perfume or freshly cut grass, molecules have drifted through the air to your nose. Each type of scent molecule has a particular make-up and shape, and receptors in your nose recognise that shape. The smell exists only because the atoms travel together as a complete molecule.

Why?

Why do atoms form molecules at all? Many atoms are more stable when they share electrons with other atoms than when they are alone. Forming bonds releases energy, and the bonded group is lower in energy, so it holds together.

Common misconception

"When water boils, the molecules break into hydrogen and oxygen." No. Boiling separates whole water molecules from each other. The bonds inside each molecule stay intact, and the steam is still made of H₂O molecules.

Worked example

Question: A sealed container holds a mixture of hydrogen gas (H₂) and oxygen gas (O₂). Is the whole mixture one big molecule? How many different kinds of molecule are present?

Reasoning: The H₂ molecules and O₂ molecules are not bonded to each other; they move independently. Each H₂ is a molecule and each O₂ is a molecule.

Answer: No; the mixture contains two kinds of separate molecules, H₂ and O₂.

Quick check

1. What must be true of atoms for them to form a molecule? Answer: They must be joined by chemical bonds so that they act as one unit.

Exam focus

Learn a precise definition: a molecule is two or more atoms chemically bonded together. Be ready to say whether a molecule belongs to an element or a compound, and remember that physical changes such as melting and boiling do not break molecules apart.

Advanced insight

Molecules range from two atoms, as in H₂, to millions, as in DNA or the polymers in plastic bags. Size does not change the definition: if the atoms are covalently bonded into a single discrete unit, it is a molecule. Some substances, such as diamond and sodium chloride, have no separate molecules at all, which is a topic for later.

Summary

A molecule is two or more atoms held together by chemical bonds, usually covalent, acting as one unit. It has a fixed make-up shown by its molecular formula and a definite shape. Molecules may be of elements (O₂) or compounds (H₂O). Bonds inside molecules are strong, while forces between molecules are weaker.

Practice questions

1. Define a molecule. Answer: A group of two or more atoms held together by chemical bonds that acts as a single unit. 2. Is N₂ a molecule of an element or a compound? Explain. Answer: An element, because both atoms are nitrogen. 3. What is the difference between a water molecule and a mixture of hydrogen and oxygen gases? Answer: In a water molecule the hydrogen and oxygen atoms are chemically bonded as one unit; in the mixture the H₂ and O₂ molecules are separate and not bonded to each other. 4. Explain why ice melting does not break water molecules. Answer: Melting overcomes the weak forces between molecules, not the strong covalent bonds within them, so each molecule remains H₂O.