Revision notes for AQA GCSE Chemistry Atoms, elements and compounds. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Atoms, elements and compounds
What you'll learn
- What atoms, elements and compounds are.
- How chemical symbols and formulae represent substances.
- How compounds are formed and separated by chemical reactions.
- How to begin writing and balancing chemical equations.
The basic idea: all substances are made of atoms
A substance is a material with a definite composition, such as oxygen, water or sodium chloride. At GCSE, the key starting point is simple:
All substances are made of atoms.
Definition
Atom and element
An atom is the smallest part of an element that can exist. An element is a substance made from only one type of atom.
For example, oxygen is an element because it contains only oxygen atoms. Copper is an element because it contains only copper atoms. Water is not an element because it contains hydrogen atoms and oxygen atoms chemically joined together.
The diagram below shows the difference between a single atom, an element, a compound and a mixture.
Elements and the periodic table
There are about 100 different elements that you need to recognise at GCSE. They are shown in the periodic table, which is a chart that organises elements in a useful order.
A group is a vertical column in the periodic table. For this course, you should be comfortable using the names and symbols of:
- the first 20 elements
- the elements in Group 1
- the elements in Group 7
- other common elements from the specification, such as iron, copper, zinc and silver
Definition
Chemical symbol
A chemical symbol is a one-letter or two-letter abbreviation for an element. The first letter is always a capital letter. If there is a second letter, it is lower-case.
Examples:
- O represents an atom of oxygen.
- Na represents an atom of sodium.
- Cl represents an atom of chlorine.
- Fe represents an atom of iron.
Common Mistake
Capital letters matter
CO means carbon monoxide, a compound containing carbon and oxygen. Co means cobalt, an element. In chemistry, changing a capital letter can completely change the meaning.
The first 20 elements
You will be given a periodic table in the exam, but you should still become fluent with these common symbols:
- 1–10: hydrogen H, helium He, lithium Li, beryllium Be, boron B, carbon C, nitrogen N, oxygen O, fluorine F, neon Ne
- 11–20: sodium Na, magnesium Mg, aluminium Al, silicon Si, phosphorus P, sulfur S, chlorine Cl, argon Ar, potassium K, calcium Ca
Group 1 elements you often meet include lithium Li, sodium Na and potassium K. Group 7 elements you often meet include fluorine F, chlorine Cl, bromine Br and iodine I.
Formulae: showing what a substance contains
A formula uses chemical symbols and numbers to show which atoms are present in a substance.
For example:
- H₂O is water: hydrogen and oxygen.
- CO₂ is carbon dioxide: carbon and oxygen.
- NaCl is sodium chloride: sodium and chlorine.
A small number written after a symbol is called a subscript. It tells you how many of that atom are present in one unit of the formula. If there is no subscript, it means 1.
Example
Counting atoms in a formula
How many of each atom are shown in Mg(OH)₂?
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Identify the bracketed group. In Mg(OH)₂, the 2 outside the brackets multiplies everything inside the brackets.
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Count magnesium first. Mg has no subscript, so there is 1 magnesium atom.
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Multiply the atoms inside the brackets. The OH group contains 1 oxygen and 1 hydrogen, so two OH groups contain 2 oxygen atoms and 2 hydrogen atoms.
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Mg(OH)₂ therefore shows 1 magnesium atom, 2 oxygen atoms and 2 hydrogen atoms.
Key Idea
Formulae show fixed proportions
A compound always contains its elements in fixed proportions. Water is always H₂O, so it always has hydrogen and oxygen in the ratio 2:1.
Compounds: elements chemically combined
A compound is a substance made from two or more different elements chemically combined.
A chemical bond is a strong attraction that holds atoms or ions together. You will study bonding in more detail later, but for now remember that “chemically combined” means the atoms are not just mixed — they are joined in a new substance.
For example, sodium reacts with chlorine to make sodium chloride:
2Na(s) + Cl₂(g) → 2NaCl(s)
Sodium is a reactive metal. Chlorine is a poisonous gas. Sodium chloride is table salt. The compound has very different properties from the elements that made it.
Definition
Chemical reaction
A chemical reaction is a process where one or more new substances are formed. Chemical reactions often involve a detectable energy change, such as a temperature change, light being given out or a flame.
Compounds can only be separated into their elements by chemical reactions. They cannot be separated into elements by simple physical methods such as filtering, evaporating or using a magnet.
Common Mistake
Compound or mixture?
A compound has different elements chemically bonded in a fixed ratio. A mixture contains substances together but not chemically bonded, so the substances can often be separated physically.
Naming compounds from formulae
When naming simple compounds, first identify the elements present.
For many compounds made from a metal and a non-metal, the name follows this pattern:
metal name + non-metal ending in “-ide”
Examples:
- NaCl is sodium chloride.
- MgO is magnesium oxide.
- CaCl₂ is calcium chloride.
If a compound contains oxygen and another element, the ending often becomes “oxide”, as in magnesium oxide or carbon dioxide.
Example
Naming a simple compound
Name the compound CaCl₂.
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Split the formula into element symbols: Ca and Cl. Ca is calcium, and Cl is chlorine.
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Decide the naming pattern. Calcium is a metal and chlorine is a non-metal, so the non-metal ending changes to “-ide”.
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The subscript 2 tells you there are two chlorine particles for every calcium particle, but the name is still calcium chloride.
Tip
Use the periodic table actively
If you are unsure what a symbol means, find it in the periodic table. Then check the capital letters carefully: Mg is magnesium, not “MG”.
Word equations and symbol equations
A word equation represents a reaction using the names of substances.
For example:
magnesium(s) + oxygen(g) → magnesium oxide(s)
A symbol equation represents a reaction using chemical symbols and formulae.
Mg(s) + O₂(g) → MgO(s)
However, this symbol equation is not balanced yet.
A balanced equation has the same number of each type of atom on both sides. This matters because atoms are rearranged in chemical reactions — they are not created or destroyed.
A large number placed in front of a formula is called a coefficient. Coefficients are used to balance equations.
Example
Balancing a symbol equation
Balance the reaction between magnesium and oxygen to make magnesium oxide.
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Start with the correct formulae and state symbols: Mg(s) + O₂(g) → MgO(s). Oxygen is O₂ because oxygen gas exists as pairs of oxygen atoms.
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Count the atoms on each side. The left side has 1 Mg atom and 2 O atoms. The right side has 1 Mg atom and 1 O atom.
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Balance oxygen by putting 2 in front of MgO: Mg(s) + O₂(g) → 2MgO(s). The right side now has 2 O atoms, but it also has 2 Mg atoms.
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Balance magnesium by putting 2 in front of Mg: 2Mg(s) + O₂(g) → 2MgO(s). Both sides now have 2 Mg atoms and 2 O atoms.
Common Mistake
Do not change the formula to balance
Never change MgO into MgO₂ just to balance oxygen. Changing a subscript changes the substance. Balance by changing coefficients only.
A quick Higher Tier note
If you are sitting Higher Tier, later topics may ask for ionic equations and half equations.
An ion is an atom or group of atoms with an electrical charge. An ionic equation shows only the ions that actually change in a reaction. A half equation shows electrons being lost or gained.
You do not need to master these yet for the basic idea of atoms, elements and compounds. The important link is that equations must still balance for atoms, and Higher Tier equations must also balance for charge.
Exam technique
In the exam
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Use the periodic table to check symbols, but write them with exact capital letters: Na, not NA.
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For compounds, look for the elements first, then apply naming rules such as “-ide” or “oxide”.
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When balancing symbol equations, change coefficients only; never alter the small subscript numbers inside formulae.
Self review
Check yourself
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What is the difference between an element and a compound?
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Why does H₂O represent a compound in a fixed proportion?
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Balance this equation: H₂(g) + O₂(g) → H₂O(l)
A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes
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