Molecules of Compounds

Introduction

Hydrogen is a flammable gas and oxygen is a gas that helps things burn, yet together they make water, which puts fires out. Carbon is a black solid and oxygen a colourless gas, yet together they make carbon dioxide, the gas that gives fizzy drinks their bubbles. When atoms of different elements bond together, they form molecules of compounds — completely new substances with their own properties.

Core explanation

Definition. A molecule of a compound contains atoms of at least two different elements joined by chemical bonds. The atoms are present in a fixed ratio, shown by the molecular formula.

Four important examples.

Compound Formula Atoms per molecule Shape Bond angle --- --- --- --- --- Water H₂O 2 H, 1 O bent (V-shaped) about 104.5° Carbon dioxide CO₂ 1 C, 2 O linear 180° Ammonia NH₃ 1 N, 3 H pyramidal about 107° Methane CH₄ 1 C, 4 H tetrahedral about 109.5°

Water, H₂O. The oxygen atom sits in the middle, bonded to two hydrogen atoms. The molecule is bent rather than straight. This bent shape makes water molecules slightly charged at each end, which is why water dissolves so many substances.

Carbon dioxide, CO₂. The carbon atom is in the centre, with a double bond to each oxygen atom. The three atoms lie in a straight line.

Ammonia, NH₃. One nitrogen atom is bonded to three hydrogen atoms, forming a low pyramid with nitrogen at the top. Ammonia has a sharp, choking smell and is used to make fertilisers.

Methane, CH₄. One carbon atom is bonded to four hydrogen atoms that point to the corners of a tetrahedron. Methane is the main component of natural gas.

New substance, new properties. A compound's properties are not an average of its elements' properties. The atoms are bonded in a new arrangement, so the substance behaves differently. Sodium is a reactive metal and chlorine a toxic gas, but their compound, sodium chloride, is table salt. Water, made from two gases, is a liquid at room temperature.

Same elements, different compounds. The same elements can form different molecules. Carbon and oxygen form both CO and CO₂; hydrogen and oxygen form both H₂O and H₂O₂. Each has its own formula, shape and properties.

Step-by-step reasoning

To describe a molecule of a compound from its formula:

1. List the different elements from their symbols. 2. Read the subscripts to count each type of atom (no subscript means 1). 3. Add the counts for the total number of atoms. 4. Confirm there are at least two different elements, so it is a compound.

Visual explanation

Build four models: a red ball with two white balls in a V shape (H₂O); a black ball with a red ball either side in a straight line (CO₂); a blue ball with three white balls hanging below like a tripod (NH₃); a black ball with four white balls pointing out like the corners of a pyramid (CH₄). Rotating them in 3D shows how each has its own characteristic shape.

Real-world analogy

Letters combine into words. The letters E, A and T can make "eat", "tea" or "ate" — the same letters, different words, different meanings. Atoms combine into molecules in the same way, and the arrangement matters as much as the ingredients.

Real-world example

Plants take in carbon dioxide from the air and water from the soil and, using light energy, build glucose (C₆H₁₂O₆) and release oxygen. This is photosynthesis. The atoms of carbon, hydrogen and oxygen are rearranged from small molecules into a larger one that stores energy for the plant.

Why?

Why does water have a bent shape rather than a straight one? The oxygen atom has two pairs of outer electrons that are not used in bonding. These lone pairs push the two O–H bonds closer together, giving the bent shape and a bond angle of about 104.5°.

Common misconception

"A water molecule contains a hydrogen molecule and an oxygen atom." No. H₂O means two hydrogen atoms and one oxygen atom, each hydrogen bonded to the oxygen. There is no H₂ molecule inside water; the hydrogen atoms are not bonded to each other.

Worked example

Question: How many atoms of each element, and how many atoms in total, are in one molecule of ammonia, NH₃, and one of methane, CH₄?

Reasoning: NH₃: N has no subscript, so 1 nitrogen; H₃ means 3 hydrogen. CH₄: 1 carbon and 4 hydrogen.

Answer: Ammonia: 1 N and 3 H, 4 atoms in total. Methane: 1 C and 4 H, 5 atoms in total.

Quick check

1. Why is CO₂ a molecule of a compound, but O₃ is not? Answer: CO₂ contains two different elements (carbon and oxygen); O₃ contains only oxygen.

Exam focus

Be able to read a molecular formula and count atoms, name the elements present and state whether the substance is an element or a compound. Learn the shapes of H₂O (bent), CO₂ (linear), NH₃ (pyramidal) and CH₄ (tetrahedral), as they appear in questions about models.

Advanced insight

Shape controls properties. Carbon dioxide and water both contain polar bonds, but CO₂'s straight shape makes the bond effects cancel, so the molecule is non-polar. Water's bent shape means the effects do not cancel, so water is polar. That single difference helps explain why water is a liquid at room temperature while CO₂ is a gas.

Summary

A molecule of a compound contains atoms of two or more different elements bonded in a fixed ratio. Key examples are H₂O (bent), CO₂ (linear), NH₃ (pyramidal) and CH₄ (tetrahedral). Compounds have properties quite different from their elements, and the same elements can form several different compounds.

Practice questions

1. Define a molecule of a compound. Answer: A molecule containing atoms of two or more different elements chemically bonded together. 2. How many atoms are in one molecule of glucose, C₆H₁₂O₆? Answer: 6 + 12 + 6 = 24 atoms. 3. Describe the shape of a carbon dioxide molecule. Answer: Linear: the carbon atom is in the middle with an oxygen atom on each side, all in a straight line (180°). 4. Explain why water's properties are different from those of hydrogen and oxygen. Answer: Water is a new substance in which the atoms are bonded in a new arrangement, so it has its own properties rather than those of its elements.