Elements, Compounds and Mixtures

Introduction

There are millions of different materials, yet chemists sort all matter into just three families: elements , compounds and mixtures . Knowing which family a material belongs to tells you a great deal about it: whether it has fixed properties, whether it can be split by physical methods, and whether a chemical reaction would be needed to break it down.

Core explanation

Elements. An element is a substance made of only one type of atom. It cannot be broken down into anything simpler by a chemical reaction. There are 118 known elements, listed in the periodic table, including hydrogen, carbon, oxygen, iron and gold. Some elements exist as single atoms (helium, He); others exist as molecules of identical atoms joined together (oxygen, O₂; nitrogen, N₂). Metals such as copper are giant arrangements of one kind of atom.

Compounds. A compound is a substance made of two or more different elements chemically joined in a fixed ratio . Water, H₂O, always has two hydrogen atoms bonded to one oxygen atom. Carbon dioxide, CO₂, always has one carbon to two oxygens. Sodium chloride, NaCl, has sodium and chlorine in a 1 : 1 ratio. A compound has new properties quite different from its elements: sodium is a reactive metal and chlorine is a toxic green gas, yet sodium chloride is table salt. Compounds can only be split into their elements by chemical reactions.

Mixtures. A mixture contains two or more elements or compounds that are not chemically joined. The parts keep their own properties, the proportions can vary, and physical methods can separate them. Air is a mixture of elements (nitrogen, oxygen, argon) and compounds (carbon dioxide, water vapour).

Pure or not? Elements and compounds are both pure substances , because each contains only one substance. Mixtures are not pure.

Family Pure? Fixed composition? Separated by --- --- --- --- Element Yes Yes (one type of atom) Cannot be split Compound Yes Yes (fixed ratio) Chemical reactions only Mixture No No (can vary) Physical methods

Using formulas. A single chemical formula for the whole sample, such as Fe, O₂ or H₂O, describes a pure substance. If a material needs several formulas to describe it, such as N₂ + O₂ + Ar for air, it is a mixture. A formula with one element symbol only (Fe, O₂, S₈) is an element; a formula with two or more different symbols (NaCl, CH₄) is a compound.

Step-by-step reasoning

To classify a material:

1. Does the sample contain only one substance? If not, it is a mixture . 2. If it is one substance, does it contain only one type of atom? If yes, it is an element . 3. If it contains two or more types of atom bonded together, it is a compound .

Visual explanation

Imagine three boxes. Box one holds identical single circles, or identical pairs of the same colour: an element. Box two holds identical groups, each made of differently coloured circles stuck together: a compound. Box three holds a jumble of different particles, some single, some paired, none bonded to the others: a mixture.

Real-world analogy

Think of letters, words and sentences. Letters are like elements: basic units that cannot be split. Words are like compounds: letters joined in a fixed order that gives a new meaning. A bag of Scrabble tiles and word cards thrown together is like a mixture: the pieces are just jumbled, not joined.

Real-world example

Pencil "lead" is mainly graphite, a form of the element carbon, mixed with clay, so the pencil core is a mixture. Tap water is mainly the compound water mixed with small amounts of dissolved substances. A steel paperclip is a mixture (an alloy) of iron, carbon and other elements.

Why?

Why does a compound have different properties from its elements? When atoms bond, their electrons are rearranged and a new particle forms. The new particle interacts with its neighbours in a new way, so the substance has a new melting point, colour and reactivity. In a mixture no bonds form between components, so nothing new appears.

Common misconception

"Anything containing more than one element is a mixture." Water contains two elements but is a compound, because the hydrogen and oxygen atoms are chemically joined in a fixed ratio.

Worked example

Question: Classify: (a) oxygen gas, O₂; (b) methane, CH₄; (c) brass (copper and zinc); (d) glucose, C₆H₁₂O₆.

Reasoning: O₂ contains only oxygen atoms. CH₄ and C₆H₁₂O₆ each contain different atoms bonded in fixed ratios. Brass is two metals mingled in variable proportions.

Answer: (a) element, (b) compound, (c) mixture, (d) compound.

Quick check

1. Is carbon dioxide, CO₂, an element, a compound or a mixture? Answer: A compound, because carbon and oxygen are chemically joined in a fixed 1 : 2 ratio.

Exam focus

Particle diagrams are a favourite exam question. Look for how many kinds of particle are present (one means pure, more than one means a mixture) and whether each particle contains one or more types of atom. Remember that O₂ is an element even though it is a molecule.

Advanced insight

The boundary between compound and mixture can blur. Some solids, such as certain iron oxides, have compositions that vary slightly from the ideal formula because of gaps in their crystal structure. These non-stoichiometric compounds are still treated as compounds because their atoms are bonded in a single continuous structure.

Summary

All matter can be sorted into elements (one type of atom), compounds (two or more elements chemically joined in a fixed ratio) and mixtures (two or more substances not chemically joined). Elements and compounds are pure substances; mixtures are not. Compounds need chemical reactions to split them, whereas mixtures separate by physical methods.

Practice questions

1. Define the term compound. Answer: A substance made of two or more different elements chemically joined in a fixed ratio. 2. Classify each as element, compound or mixture: sulfur, S₈; salt water; ammonia, NH₃. Answer: Sulfur is an element, salt water is a mixture and ammonia is a compound. 3. A particle diagram shows identical molecules, each made of two different atoms. What does it represent? Explain. Answer: A pure compound, because there is only one kind of particle and each contains two different types of atom bonded together. 4. Explain why nitrogen gas, N₂, is an element even though its particles contain two atoms. Answer: Both atoms in each molecule are nitrogen atoms, so only one type of atom is present. 5. Why can a compound not be separated into its elements by filtration or distillation? Answer: Its elements are chemically bonded, so a chemical reaction is needed to break the bonds; physical methods only separate substances that are not joined.