Atoms, Elements, Compounds and Mixtures

This lesson covers the fundamental building blocks of chemistry — atoms, elements, compounds and mixtures — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to understand what each of these terms means, how they relate to one another, and how to use chemical symbols and formulae correctly.

---

What Is an Atom?

An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are incredibly small — typically around 0.1 to 0.3 nanometres (nm) in diameter, where 1 nm = 1 × 10⁻⁹ m.

Key facts about atoms:

• Atoms are the building blocks of all matter — every substance you encounter is made of atoms.
• Each element is made of its own unique type of atom, defined by the number of protons in its nucleus.
• Atoms cannot be broken down into simpler substances by ordinary chemical reactions.
• There are approximately 118 known elements, each with a different type of atom.

Exam Tip: When asked to define an atom in the exam, state that it is "the smallest part of an element that can take part in a chemical reaction." Do not say atoms are indivisible — they contain sub-atomic particles (protons, neutrons and electrons).

---

Elements

An element is a substance that contains only one type of atom. Elements cannot be broken down into simpler substances by chemical means. Each element is represented by a unique chemical symbol on the periodic table.

Common Elements and Their Symbols

Element	Symbol	Type
Hydrogen	H	Non-metal
Helium	He	Non-metal (noble gas)
Carbon	C	Non-metal
Nitrogen	N	Non-metal
Oxygen	O	Non-metal
Sodium	Na	Metal
Magnesium	Mg	Metal
Chlorine	Cl	Non-metal
Iron	Fe	Metal
Copper	Cu	Metal
Gold	Au	Metal

Rules for Chemical Symbols

• The first letter is always a capital letter.
• If there is a second letter, it is always lower case.
• Some symbols come from Latin names — for example, Fe from ferrum (iron), Na from natrium (sodium), Au from aurum (gold), Cu from cuprum (copper).

Exam Tip: Be very careful with capital and lower case letters. "CO" means carbon monoxide (a compound of carbon and oxygen), but "Co" means cobalt (a single element). Getting this wrong in an exam will lose you marks.

---

Compounds

A compound is a substance that contains two or more different elements that are chemically bonded together. The atoms in a compound are held together by chemical bonds — either ionic bonds (between metals and non-metals) or covalent bonds (between non-metals).

Common Compounds

Compound	Formula	Elements Present
Water	H₂O	Hydrogen, Oxygen
Carbon dioxide	CO₂	Carbon, Oxygen
Sodium chloride	NaCl	Sodium, Chlorine
Magnesium oxide	MgO	Magnesium, Oxygen
Calcium carbonate	CaCO₃	Calcium, Carbon, Oxygen
Sulfuric acid	H₂SO₄	Hydrogen, Sulfur, Oxygen

Key Points About Compounds

• Compounds have different properties from the elements they contain.
• For example, sodium is a reactive metal and chlorine is a toxic gas, but sodium chloride (table salt) is a harmless white solid.
• Compounds can only be separated into their elements by chemical reactions, not by physical methods like filtration or distillation.
• The ratio of atoms in a compound is fixed — water is always H₂O, never H₃O or HO₂.

Exam Tip: A very common exam question asks you to explain the difference between a compound and a mixture. Remember: in a compound the elements are chemically bonded in fixed proportions, whereas in a mixture the substances are not chemically bonded and can be in any proportion.

---

Mixtures

A mixture contains two or more substances that are not chemically bonded together. The components of a mixture retain their own individual properties and can be separated by physical methods.

Comparing Compounds and Mixtures

Feature	Compound	Mixture
Chemical bonds	Elements are chemically bonded	Substances are NOT chemically bonded
Fixed ratio	Always a fixed ratio of atoms	Can be in any proportion
Properties	Different from the constituent elements	Each substance retains its own properties
Separation	Only by chemical reactions	By physical methods (filtration, distillation, chromatography, evaporation)

Methods of Separating Mixtures

1. Filtration — separates an insoluble solid from a liquid (e.g. sand from water).
2. Evaporation — removes the solvent to leave behind a dissolved solid (e.g. obtaining salt from salt water).
3. Distillation — separates liquids with different boiling points, or a solvent from a solution.
4. Chromatography — separates dissolved substances that move at different rates through a stationary phase.

---

Chemical Formulae

A chemical formula shows the number and type of atoms in a substance. The small number written after an element's symbol (the subscript) tells you how many atoms of that element are present. If there is no subscript, there is one atom.

Formula	Atoms Present
H₂	2 hydrogen atoms
O₂	2 oxygen atoms
H₂O	2 hydrogen atoms and 1 oxygen atom
CO₂	1 carbon atom and 2 oxygen atoms
H₂SO₄	2 hydrogen, 1 sulfur and 4 oxygen atoms
Ca(OH)₂	1 calcium, 2 oxygen and 2 hydrogen atoms
Mg(NO₃)₂	1 magnesium, 2 nitrogen and 6 oxygen atoms

Working with Brackets

When a formula contains brackets, the subscript outside multiplies everything inside the bracket:

• Ca(OH)₂ → 1 Ca, 2 × O = 2 O, 2 × H = 2 H
• Mg(NO₃)₂ → 1 Mg, 2 × N = 2 N, 2 × 3 O = 6 O

Exam Tip: When counting atoms in a formula with brackets, always multiply the subscript outside by every element inside. A common mistake is forgetting to multiply — in Mg(NO₃)₂ there are 6 oxygen atoms (3 × 2), not 3.

---

Classification of Matter — Overview

graph TD
    A["All Matter"] --> B["Pure Substances"]
    A --> C["Mixtures"]
    B --> D["Elements"]
    B --> E["Compounds"]
    D --> F["One type of atom only<br/>e.g. O₂, Fe, C"]
    E --> G["Two or more different atoms<br/>chemically bonded<br/>e.g. H₂O, NaCl"]
    C --> H["Two or more substances<br/>NOT chemically bonded<br/>e.g. air, salt water"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#27ae60,color:#fff
    style F fill:#1a1a2e,color:#fff
    style G fill:#1a1a2e,color:#fff
    style H fill:#1a1a2e,color:#fff

---

Summary

• An atom is the smallest particle of an element that can take part in a chemical reaction.
• An element contains only one type of atom.
• A compound contains two or more different elements chemically bonded together in fixed proportions.
• A mixture contains two or more substances that are not chemically bonded.
• Chemical symbols use one or two letters (first always capital, second always lower case).
• Chemical formulae show the number and type of atoms in a substance using subscripts.
• Compounds have different properties from their constituent elements.
• Mixtures can be separated by physical methods; compounds require chemical reactions to separate.

---

Worked Examples — Counting Atoms and Identifying Substance Type

Worked Example 1: Counting atoms in aluminium sulfate

Aluminium sulfate has the formula Al₂(SO₄)₃. How many atoms of each element are present in one formula unit?

Step 1 — Work out each element:

• Al: the subscript 2 is directly on Al → 2 aluminium atoms.
• S: inside the bracket, subscript is 1 (implied). Multiply by the 3 outside → 3 sulfur atoms.
• O: inside the bracket, subscript is 4. Multiply by the 3 outside → 12 oxygen atoms.

Step 2 — Total atoms: 2 + 3 + 12 = 17 atoms per formula unit.

Worked Example 2: Counting atoms in ammonium nitrate

Ammonium nitrate has the formula NH₄NO₃. Here NH₄ is not in brackets because it appears only once.

• N: 1 (from NH₄) + 1 (from NO₃) = 2 nitrogen atoms
• H: 4 (from NH₄) = 4 hydrogen atoms
• O: 3 (from NO₃) = 3 oxygen atoms

Total: 9 atoms.

Worked Example 3: Element, compound or mixture?

Substance	Type	Reason
Copper wire	Element	Only Cu atoms
Brass	Mixture	Copper + zinc; variable ratio; components not chemically bonded
Copper sulfate crystals	Compound	Cu, S and O chemically bonded in fixed ratio (CuSO₄)
Air	Mixture	N₂, O₂, Ar, CO₂ — not chemically bonded
Distilled water	Compound	Pure H₂O — H and O chemically bonded
Sea water	Mixture	Water + dissolved salts (NaCl, MgCl₂ etc.)
Diamond	Element	Only C atoms
Stainless steel	Mixture	Iron + chromium + carbon in variable ratio

Common-mistake callout: Students often write "an alloy is a compound". It is not. An alloy is a mixture of a metal with other elements — the atoms are not chemically bonded in a fixed ratio.

---

Comparison Table — Pure Substances vs Impure Substances

Feature	Pure substance (element or compound)	Impure substance (mixture)
Melts/boils at	A single sharp temperature	A range of temperatures
Composition	Fixed	Variable
Separation	Chemical reaction (compound) or not separable (element)	Physical method
Example	Ice melts sharply at 0 °C	Salty water melts over a range

---

Answering at different grade levels

A Grade 3–4 answer might say: "A compound has elements joined together but a mixture does not."

A Grade 6–7 answer sharpens the language: "A compound contains two or more different elements that are chemically bonded in a fixed ratio, so its properties differ from those of its constituent elements. A mixture contains substances that are not chemically bonded and can be separated by physical methods."

A Grade 8–9 answer adds precision about composition and separation: "In a compound such as sodium chloride (NaCl), the atomic number of each element determines which atoms are present, and those atoms are held together by ionic bonds in a 1:1 ratio. Its relative formula mass (58.5) is fixed. In a mixture such as salt water, no chemical bonds join the solute to the solvent, so the relative atomic mass of each component is unaffected by the mixing, and physical techniques such as distillation can recover pure water."

Further Practice — Short-Answer Checks

1. Classify each as element, compound or mixture: (a) oxygen gas O₂, (b) carbon monoxide CO, (c) tap water, (d) gold jewellery (22-carat), (e) sodium hydroxide solution.

• (a) Element — one type of atom only. (b) Compound — C and O chemically bonded in a fixed ratio. (c) Mixture — dissolved minerals. (d) Mixture — gold alloyed with copper. (e) Mixture — NaOH dissolved in water (the solid NaOH itself is a compound, but the solution is a mixture).

2. Work out the total number of atoms in 1 formula unit of magnesium sulfate heptahydrate, MgSO₄·7H₂O.

• Mg: 1; S: 1; O from sulfate: 4; H₂O ×7 gives 14 H and 7 O.
• Total: 1 + 1 + 4 + 14 + 7 = 27 atoms.

3. Explain why iron in wet air rusts but iron sealed in a vacuum does not.

• Rust formation requires iron to react chemically with both oxygen and water — two reactants from the air. A vacuum contains neither, so the elements that would combine to form the compound iron(III) oxide hydrate are absent, preventing any chemical reaction.

Exam Tip: When a question asks whether a substance is an element, compound or mixture, decide first whether it can be split by physical means (mixture) or only by chemical means (compound). If it cannot be split at all, it is an element.

Edexcel alignment: This content is aligned with Edexcel GCSE Chemistry (1CH0) specification Topic 1 Key concepts in chemistry — specifically 1.2 Mixtures and separations, 1.3 Atomic structure and 1.4 The periodic table. Assessed on Paper 1.