Atoms and Molecules in Reactions

Introduction

When hydrogen burns in oxygen, two colourless gases combine to produce water. What actually happens to the particles? Particle diagrams let us look inside a reaction. They show the bonds between atoms breaking and new bonds forming, with the atoms themselves surviving unchanged. This picture is the heart of Dalton's explanation of chemical change.

Core explanation

Drawing particles. In a particle diagram, each atom is a circle, with different colours or sizes for different elements. Atoms joined together touch or overlap to show a molecule. A single element is shown by identical circles; a compound by molecules containing more than one kind of circle.

What a reaction does. In a chemical reaction, the bonds holding atoms together in the reactants break, and the atoms regroup into new combinations, forming products. The atoms themselves are not created, destroyed or changed into atoms of another element.

Example 1 — making water. 2H₂ + O₂ → 2H₂O

- Reactants: two H₂ molecules (four white circles in pairs) and one O₂ molecule (two red circles joined). - Products: two H₂O molecules, each a red circle with two white circles attached. - Count: 4 H and 2 O on each side.

Atoms are conserved; molecules are not. Three molecules on the left become two molecules on the right. The number of molecules can change, but the number of each type of atom cannot.

Mass is conserved. Because the same atoms are present before and after, total mass stays the same. Using Mr: reactants 2 × 2 + 32 = 36; products 2 × 18 = 36.

Example 2 — making ammonia. N₂ + 3H₂ → 2NH₃. One nitrogen molecule and three hydrogen molecules (four molecules in all) become two ammonia molecules. Atoms: 2 N and 6 H on each side. Masses: 28 + 6 = 34 and 2 × 17 = 34.

Example 3 — methane burning. CH₄ + 2O₂ → CO₂ + 2H₂O. Here the number of molecules stays at three on each side, but every molecule is different. Masses: 16 + 64 = 80 and 44 + 36 = 80.

Leftover particles. Real mixtures do not always contain reactants in the exact ratio. If a diagram shows three O₂ molecules but only two H₂ molecules, the two H₂ molecules use up only one O₂ molecule, so two O₂ molecules remain unreacted in the product box. A correct diagram shows these leftover molecules unchanged alongside the products.

Step-by-step reasoning

To draw the particle diagram for C + O₂ → CO₂:

1. Draw the reactant box: one black carbon circle and one O₂ molecule (two joined red circles). 2. Count atoms: 1 C and 2 O. 3. Draw the product box with the same atoms regrouped: one black circle between two red circles. 4. Check atoms balance: 1 C and 2 O in the products. 5. Check mass: 12 + 32 = 44 = Mr of CO₂.

Visual explanation

Picture two boxes side by side with an arrow between them. The left box holds pairs of white circles and a pair of red circles. The right box holds "Mickey Mouse" shapes — a red circle with two white ears. Every circle on the left appears somewhere on the right; only the partners have changed.

Real-world analogy

Take apart two models built from interlocking bricks and build a new model from the same bricks. The number of models may change, and they look completely different, but every brick is still there and the total mass of bricks is unchanged.

Real-world example

In the Haber process, nitrogen from the air and hydrogen react to make ammonia for fertilisers. Engineers track atoms exactly as in a particle diagram, because the nitrogen atoms in every tonne of fertiliser came from nitrogen molecules in the air.

Why?

Why does the number of molecules change while mass does not? Mass belongs to atoms, not to the way they are grouped. Regrouping four hydrogen atoms and two oxygen atoms from three molecules into two molecules keeps every atom, so the total mass stays the same.

Common misconception

"When a substance burns, its atoms are destroyed." Burning rearranges atoms into new substances, such as carbon dioxide and water, which often escape as gases. The atoms still exist; the apparent loss of mass happens only because products move away into the air.

Worked example

Question: A diagram shows 3 N₂ molecules and 6 H₂ molecules before reaction. Using N₂ + 3H₂ → 2NH₃, what does the diagram look like afterwards?

Reasoning: 6 H₂ molecules react with 6 ÷ 3 = 2 N₂ molecules, forming 2 × 2 = 4 NH₃ molecules. One N₂ molecule is left over. Check atoms: before, 6 N and 12 H; after, 4 NH₃ (4 N, 12 H) plus one N₂ (2 N), giving 6 N and 12 H.

Answer: Four NH₃ molecules and one unreacted N₂ molecule.

Quick check

1. In 2H₂ + O₂ → 2H₂O, how many molecules are there before and after the reaction? Answer: Three before and two after.

Exam focus

In particle diagram questions, count every atom of each element on both sides, include any leftover reactant molecules, and use a key to show which circle is which element. Marks are often lost for drawing products with atoms that do not balance.

Advanced insight

Particle diagrams show the overall change, not the actual pathway. Most reactions happen through several steps involving short-lived fragments. For example, hydrogen and oxygen react through a chain involving reactive species such as H, O and OH, even though the diagram simply shows H₂ and O₂ becoming H₂O.

Summary

Particle diagrams show atoms as circles grouped into molecules. In a reaction, bonds break and atoms regroup into new substances. The number of each kind of atom, and therefore the total mass, is conserved, while the number of molecules may change. Diagrams should include any reactant molecules left over.

Practice questions

1. Describe the particle diagram for 2H₂ + O₂ → 2H₂O. Answer: Two pairs of hydrogen atoms and one pair of oxygen atoms become two molecules, each an oxygen atom joined to two hydrogen atoms. 2. Show that mass is conserved in CH₄ + 2O₂ → CO₂ + 2H₂O (C = 12, H = 1, O = 16). Answer: Reactants 16 + 64 = 80; products 44 + 36 = 80. 3. Explain why the number of molecules may change in a reaction but the number of atoms cannot. Answer: Atoms are only rearranged into new groupings, so the same atoms can form more or fewer molecules, but none is created or destroyed. 4. Four H₂ molecules are mixed with three O₂ molecules. How many water molecules form, and what is left over? Answer: Four H₂ react with two O₂ to form four H₂O molecules; one O₂ molecule is left over.