Atoms, elements and compounds — AQA GCSE Combined Science
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Atoms, elements and compounds explained
This statement establishes the particle model that underpins all of chemistry.
Read the full explanation
Every substance, whether an element, a compound or a mixture, is built from atoms, so the properties of a material come from the atoms it contains and how they are arranged. An element contains only one type of atom, and that single atom is the smallest piece of the element that can exist. For example, a piece of copper is made of countless copper atoms; if you kept dividing it, you would eventually reach one copper atom, and splitting that atom would give particles that are no longer copper. Atoms are extremely small, roughly 1 × 10⁻¹⁰ m across, so they cannot be seen with a school light microscope. They are not indivisible, however: each atom has a nucleus containing protons and neutrons, surrounded by electrons.
Atoms of each element are represented by a chemical symbol, eg O represents an atom of oxygen, Na represents an atom of sodium.
Chemists use chemical symbols as a universal shorthand for elements. Each element has its own symbol, so O means one atom of oxygen and Na means one atom of sodium. Symbols are one or two letters: the first letter is always a capital, and a second letter, if present, is always lower case, as in Na, Mg and Cl. Many symbols come from the English name, but some come from the Latin name, which is why sodium is Na (from natrium), potassium is K (from kalium) and iron is Fe (from ferrum). A symbol can represent one atom of the element, and in a formula it shows how many atoms of that element are present, for example H₂O contains two hydrogen atoms and one oxygen atom. Learning symbols allows scientists worldwide to read and write formulae consistently.
There are about 100 different elements. Elements are shown in the periodic table.
An element is a pure substance made of only one type of atom. There are about 100 different elements. Each element has a unique symbol, such as H, O or Na, and is placed in the periodic table in order of increasing atomic number. The table groups elements with similar properties in vertical columns called groups, and shows metals on the left and non-metals on the right. For example, all Group 1 elements react vigorously with water, while Group 7 elements form coloured vapours. Knowing an element's position helps predict its reactivity and physical properties. Compounds are formed from elements by chemical reactions.
Compounds are formed from elements by chemical reactions. Chemical reactions always involve the formation of one or more new substances, and often involve a detectable energy change. Compounds contain two or more elements chemically combined in fixed proportions and can be represented by formulae using the symbols of the atoms from which they were formed. Compounds can only be separated into elements by chemical reactions.
A compound forms when two or more elements react chemically, producing one or more new substances with different properties from the starting elements. For example, iron and sulfur react to form iron sulfide, which is not magnetic like iron. Chemical reactions often release or absorb energy, seen as temperature, light or sound changes. Compounds have fixed proportions of elements, shown by formulae such as H₂O or CO₂, where the subscripts give the atom ratio. Unlike mixtures, compounds cannot be separated by physical methods like filtration or distillation; they require chemical reactions such as electrolysis or heating with a reducing agent.
Chemical reactions can be represented by word equations or equations using symbols and formulae.
A chemical reaction rearranges atoms; the products contain the same atoms as the reactants, only joined differently. A word equation names reactants and products, for example methane + oxygen → carbon dioxide + water. A symbol equation replaces names with formulae, for example CH₄ + 2O₂ → CO₂ + 2H₂O, and must be balanced so every element has equal atom counts on both sides. State symbols (s), (l), (g) and (aq) may be added. To write one, identify reactants and products, write correct formulae using valencies, then adjust coefficients only, never subscripts, until each element balances. Check by counting atoms of each element on both sides.
Students will be supplied with a periodic table for the exam and should be able to:
In the exam, you receive a periodic table. You must use it to find element names and symbols, particularly for the first 20 elements, Groups 1 and 7, and others in the specification. You should be able to name compounds from given formulae or symbol equations, such as naming NaCl as sodium chloride. Additionally, you must write word equations and balanced chemical equations for reactions. For example, writing the word equation 'hydrogen + oxygen → water' and the balanced symbol equation '2H₂ + O₂ → 2H₂O'. The periodic table helps identify elements to construct these equations accurately. Note that for main-group elements, the group number equals the outer-shell electrons, except for Group 0.
use the names and symbols of the first 20 elements in the periodic table, the elements in Groups 1 and 7, and other elements in this specification
This skill means you can move confidently between an element's name and its chemical symbol. You need the first 20 elements, from hydrogen (H) to calcium (Ca), plus Group 1 elements such as sodium (Na) and potassium (K), and Group 7 elements such as chlorine (Cl) and bromine (Br). You also need other elements named in the specification, for example iron (Fe), copper (Cu) and zinc (Zn). Learn symbols with their exact capitalisation: the first letter is always a capital and any second letter is always lower case, so Co is cobalt but CO would mean carbon monoxide. A useful method is to cover the name, write the symbol from memory, then check the periodic table. Practise both directions, because an exam may give a name and ask for the symbol, or give a symbol and ask for the name.
name compounds of these elements from given formulae or symbol equations
This skill requires reading a chemical formula or symbol equation to give the correct compound name. You use element names and symbols, including the first 20 elements, Groups 1 and 7, and transition metals. For ionic compounds like NaCl, name it sodium chloride. Transition metals may need Roman numerals to show valency, such as iron(II) chloride for FeCl₂. For covalent compounds, Greek prefixes indicate the number of atoms: 'mono-' for one, 'di-' for two, and 'tri-' for three. For example, CO₂ is carbon dioxide. In equations like 2Mg + O₂ → 2MgO, identify MgO as magnesium oxide. Split the formula into symbols, name the first element, then the second, applying prefixes or numerals where appropriate.
write word equations for the reactions in this specification
A word equation names the reactants and products of a chemical reaction, with an arrow showing the direction of change. You write the names of the starting substances on the left, separated by a plus sign where there is more than one, then an arrow, then the names of the substances formed on the right, again separated by plus signs. For example, magnesium reacting with oxygen is written as magnesium + oxygen → magnesium oxide. The names must be the correct chemical names, not everyday labels, and the equation must account for every reactant and product in the reaction described. Word equations apply to all the reactions named in the specification, including combustion, neutralisation, displacement, precipitation and reactions of acids with metals and carbonates.
write formulae and balanced chemical equations for the reactions in this specification.
A chemical formula shows the elements in a substance and the number of atoms of each, using symbols and subscript numbers, for example H₂O or CO₂. A balanced chemical equation uses these formulae to show reactants and products with the same number of each type of atom on both sides. You balance by adjusting the large numbers in front of formulae, called coefficients, never by changing the subscripts inside a formula. For example, hydrogen burning in oxygen is 2H₂ + O₂ → 2H₂O. State symbols (s), (l), (g) and (aq) may be added to show physical states. This skill applies to all reactions named in the specification, including neutralisation, displacement, combustion and reactions of acids with metals and carbonates.
(HT only) write balanced half equations and ionic equations where appropriate.
This skill requires representing electron transfer and solution reactions using balanced equations. A half equation shows one species losing or gaining electrons, such as Na → Na⁺ + e⁻. Balance atoms first, then balance the charge by adding electrons. An ionic equation shows only the reacting ions that undergo a change, omitting spectator ions. For example, in the reaction between silver nitrate and sodium chloride solutions, the ionic equation is Ag⁺(aq) + Cl⁻(aq) → AgCl(s). The sodium and nitrate ions are spectator ions. You must balance both atoms and charge in every equation. State symbols are often included to show the physical state. This is assessed by asking you to complete, construct or interpret these equations.
Your focus
- Describe that all substances are made of atoms.
- Define an atom as the smallest part of an element that can exist.
- Give examples showing that different elements are made of different atoms.
Show all 33 objectives
- Recall that each element is represented by its own chemical symbol.
- Write and interpret symbols such as O, Na, Mg and Cl correctly.
- Use a formula to state the number of atoms of each element present.
- Define an element as a substance made of only one type of atom.
- Locate elements in the periodic table using their symbols and atomic numbers.
- Explain how the position of an element in the periodic table relates to its properties.
- Describe how compounds are formed from elements by chemical reactions.
- Interpret chemical formulae to identify the elements and their fixed proportions in a compound.
- Explain why compounds can only be separated into elements by chemical reactions.
- Convert a named reaction into a correct word equation.
- Construct a balanced symbol equation from given formulae.
- Check an equation by counting atoms of each element on both sides.
- Use the periodic table to find names and symbols of key elements.
- Name compounds from given formulae and write word equations for reactions.
- Write formulae and balanced chemical equations for reactions in the specification.
- Recall the names and symbols of the first 20 elements in the periodic table.
- Recall the names and symbols of the elements in Groups 1 and 7.
- Recall the names and symbols of other elements named in the specification, such as iron, copper and zinc.
- Construct the correct name of a compound from its formula, applying prefixes for covalent compounds and Roman numerals for transition metals.
- Identify the elements present in a given chemical formula or symbol equation.
- Name compounds formed in symbol equations, distinguishing compounds from elements.
- Write a word equation with reactants on the left and products on the right for a named reaction.
- Use correct chemical names for all reactants and products in a word equation.
- Apply word equations to the reactions listed in the specification, including acid reactions and combustion.
- Write correct chemical formulae from names or ions, including formulae with brackets.
- Balance chemical equations by adjusting coefficients so that each element is conserved.
- Apply formulae and balanced equations to the reactions named in the specification, including acid and combustion reactions.
- Construct balanced half equations for oxidation and reduction processes, including the correct number of electrons.
- Derive ionic equations from full equations by removing spectator ions.
- Apply state symbols correctly to species in half equations and ionic equations.
Atoms, elements and compounds exam tips
Marking Points
- States that all substances are made of atoms, including elements, compounds and mixtures.
- Defines an atom as the smallest part of an element that can exist.
- Recognises that an element contains only one type of atom.
- Explains that atoms are the building blocks of all materials and are too small to see with a light microscope.
- Uses the idea that atoms themselves contain smaller particles, such as protons, neutrons and electrons, so they are not indivisible.
- States that each element has its own chemical symbol.
- Interprets a symbol such as O or Na as representing one atom of that element.
- Applies the capital-then-lower-case rule when writing symbols, for example Na and Cl.
- Recognises that some symbols come from Latin names, such as Na for sodium and Fe for iron.
- Uses symbols within a formula to show the number of atoms of each element present.
- State that an element is a pure substance containing only one type of atom.
- Recall that there are about 100 different elements, each with a unique atomic number.
- Identify the periodic table as the arrangement of elements in order of increasing atomic number.
- Describe how groups organise elements with similar properties in vertical columns, while periods are horizontal rows.
- Use a given element symbol or position to predict its metal/non-metal character or reactivity trend.
- State that compounds are formed when elements react chemically.
- Explain that chemical reactions produce one or more new substances and often involve a detectable energy change.
- Describe compounds as containing two or more elements chemically combined in fixed proportions.
- Interpret or write formulae such as H₂O and CO₂ to show the ratio of atoms in a compound.
- Explain that compounds can only be separated into elements by chemical reactions, not by physical separation techniques.
- Word equations name reactants and products and use an arrow, for example sodium + chlorine → sodium chloride.
- Symbol equations use correct chemical formulae, for example H₂O not HO or H₂O₂, and correct state symbols where required.
- Balancing is achieved by changing coefficients in front of formulae, never by altering subscripts within a formula.
- Atom counts for each element must be equal on both sides, demonstrating conservation of mass.
- A balanced equation can be checked element by element, including polyatomic ions treated as units where unchanged.
- Use the periodic table to identify names and symbols of the first 20 elements, Group 1, Group 7, and other specified elements.
- Name compounds correctly when given their chemical formulae or symbol equations.
- Write accurate word equations for chemical reactions described in the specification.
- Write correct chemical formulae and construct balanced chemical equations for specified reactions.
- Correctly pairs each of the first 20 elements with its symbol, for example potassium with K and calcium with Ca.
- Correctly pairs Group 1 elements with symbols, for example lithium with Li, sodium with Na and potassium with K.
- Correctly pairs Group 7 elements with symbols, for example fluorine with F, chlorine with Cl and bromine with Br.
- Correctly names and gives symbols for other specification elements such as iron (Fe), copper (Cu) and zinc (Zn).
- Uses correct capitalisation, including two-letter symbols such as Mg, Cl and Ca, and distinguishes Co (cobalt) from CO (carbon monoxide).
- Responds accurately in either direction, converting a name to a symbol or a symbol to a name.
- Identifies each element symbol in a given formula, for example Na and Cl in NaCl or Mg and O in MgO.
- Names simple two-element ionic compounds correctly without prefixes, for example sodium chloride from NaCl.
- Uses prefixes such as di- and tri- correctly for covalent compounds, and mono- for one atom, for example carbon monoxide from CO.
- Applies Roman numerals to indicate the valency of transition metals, for example iron(II) chloride for FeCl₂ or copper(II) sulfate for CuSO₄.
- Names compounds from symbol equations by identifying the formula of each product, distinguishing compounds from elements.
- Reactant names are written on the left of the arrow and product names on the right, in the correct order.
- A plus sign separates each pair of substances on the same side of the arrow.
- The arrow, not an equals sign, shows the direction of the reaction.
- Chemical names are used accurately, for example magnesium oxide rather than magnesium ash.
- Every substance named in the reaction is included, so no reactant or product is omitted.
- State symbols are not required in a word equation, but the names must match the substances actually reacting.
- Formulae are written correctly from the names or ions given, using the correct symbols and subscripts.
- The equation has the same number of atoms of each element on both sides.
- Coefficients are placed in front of whole formulae and are used to balance, not subscripts.
- State symbols, where required, are correct for the reaction conditions described.
- Ionic formulae use the correct charges, for example Na⁺ and O²⁻, so that the compound is electrically neutral.
- The final equation is checked atom by atom to confirm it is balanced.
- Half equations must balance atoms and charge; electrons are shown with the correct number and side.
- Ionic equations include only the reacting ions and the product formed; spectator ions are omitted.
- State symbols such as (s), (l), (g) and (aq) are used correctly where required.
- Coefficients are whole numbers in the simplest ratio; equations are balanced for both mass and charge.
- In oxidation half equations, electrons appear on the right; in reduction half equations, electrons appear on the left.
Examiner Tips
- 💡Use the exact phrase 'smallest part of an element' when defining an atom, because vague wording such as 'smallest thing' loses credit.
- 💡Link the definition to a named element, for example copper or oxygen, to show the idea applies to a real substance.
- 💡If asked why atoms cannot be seen, refer to their very small size, roughly 1 × 10⁻¹⁰ m, rather than saying they are invisible.
- 💡Learn the symbols for common elements, including those with Latin origins such as Na, K and Fe, because they are frequently used in formulae.
- 💡When naming an element from its symbol, check the capital and lower-case letters carefully before answering.
- 💡Practise counting atoms in formulae such as CO₂ and CaCO₃ so you can link symbols to numbers of atoms.
- 💡When asked to define an element, include both 'pure substance' and 'only one type of atom' to secure the mark.
- 💡Use the periodic table provided in the exam to locate an element by symbol or atomic number rather than relying on memory.
- 💡When describing a reaction, name the new substance formed and state one observable energy change to cover both parts of the statement.
- 💡Use the formula to count atoms: in CO₂ there is one carbon atom and two oxygen atoms, giving a fixed 1:2 ratio.
- 💡If asked how to separate a compound, name a chemical method such as electrolysis or thermal decomposition, not a physical method.
- 💡Underline each element in turn and tally atoms on both sides before finalising your answer.
- 💡If a question gives a word equation, convert names to formulae carefully, using the periodic table and common ion charges.
- 💡Show balancing by writing coefficients clearly; do not erase working that shows your method.
- 💡For 6-mark equation questions, balance first, then add state symbols, then recheck every element.
- 💡Always check the periodic table for the correct spelling of element names and exact symbols.
- 💡When writing balanced equations, only change the large numbers (coefficients) in front of formulae, never the small subscript numbers.
- 💡Make a two-column list of names and symbols and test yourself in both directions until you can recall them without the periodic table.
- 💡When a question gives a formula, identify each symbol first and then write the element name, checking capitalisation carefully.
- 💡If you cannot recall a symbol, use the periodic table provided in the exam rather than leaving the answer blank.
- 💡Write symbols clearly so that a lower-case l is not mistaken for a capital I or the digit 1.
- 💡Check if the compound is ionic or covalent before naming; only use prefixes like di- or tri- for covalent compounds.
- 💡When naming compounds containing transition metals like iron or copper, always determine the charge of the metal ion and include it as a Roman numeral in brackets.
- 💡Underline the chemical names in the question before writing so you do not miss a reactant or product.
- 💡Check that each name is a real chemical substance and that the elements in it match those in the reactants.
- 💡Practise the common reactions from the specification, such as acid + metal → salt + hydrogen, until the naming pattern is automatic.
- 💡Count atoms element by element and write the tally beside the equation until it balances.
- 💡Balance formulae containing several elements last, and leave hydrogen and oxygen until near the end where possible.
- 💡Check that every formula is a valid substance before balancing, because a wrong formula cannot be fixed by coefficients.
- 💡Check charge balance after balancing atoms: total charge on the left must equal total charge on the right.
- 💡For ionic equations, identify the precipitate, gas or molecule formed, then include only the ions that form it.
Common Mistakes
- Saying an atom is the smallest particle that exists: correct this by specifying the smallest part of an element, because atoms contain protons, neutrons and electrons.
- Confusing atoms with molecules: correct this by noting that a molecule is two or more atoms bonded together, whereas an atom is a single particle of an element.
- Thinking compounds contain only one type of atom: correct this by stating that a compound contains atoms of two or more elements chemically combined.
- Writing both letters as capitals, such as NA: correct this to Na, because only the first letter is a capital.
- Treating a symbol as a word abbreviation only: correct this by noting that Na represents one atom of sodium, not the word 'sodium'.
- Reading H₂O as one hydrogen atom: correct this by using the subscript to show two hydrogen atoms and one oxygen atom.
- Confusing elements with compounds: an element contains one type of atom, while a compound contains two or more elements chemically combined. Correct by checking the chemical formula: O₂ is an element, H₂O is a compound.
- Thinking there are exactly 100 elements: the specification states there are about 100 different elements, though 118 are currently recognised. Correct by using 'about 100 different elements'.
- Assuming elements in the same period have similar properties: properties repeat in groups (vertical columns), not periods. Correct by linking similar reactivity to group number.
- Confusing compounds with mixtures: a mixture contains elements or compounds not chemically combined and can be separated physically. Correct by checking whether a chemical formula with fixed ratios is given.
- Thinking energy change always means a temperature rise: some reactions are endothermic and cool the surroundings. Correct by stating 'detectable energy change' rather than only 'heat released'.
- Believing compounds can be separated by distillation or filtration: these separate mixtures. Correct by stating that chemical reactions such as electrolysis are needed to break compounds into elements.
- Changing a subscript to balance, such as writing H₂O as H₂O₂; correction: adjust only the large coefficient, for example 2H₂O.
- Forgetting that diatomic elements exist as pairs, writing O instead of O₂; correction: use H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂ when these elements appear alone.
- Miscounting atoms in formulae containing brackets or multiple groups, for example Ca(OH)₂; correction: multiply every atom inside the bracket by the subscript outside.
- Omitting state symbols when the question asks for them; correction: add (s), (l), (g) or (aq) after each formula as appropriate.
- Misidentifying element symbols due to incorrect capitalisation. Correction: ensure the first letter is uppercase and the second is lowercase (e.g., Co, not CO).
- Failing to balance chemical equations correctly. Correction: ensure the number of atoms of each element is the same on both sides of the equation.
- Overgeneralising group number rules. Correction: remember that while group number equals outer-shell electrons for Groups 1-7, Group 0 elements have full outer shells.
- Writing symbols with incorrect capitalisation, such as CL for chlorine or na for sodium; the correction is Cl and Na, because the first letter is a capital and the second is lower case.
- Confusing elements with similar names or symbols, such as magnesium (Mg) and manganese (Mn); the correction is to link each name to its exact symbol and position in the periodic table.
- Treating a two-letter symbol as two separate elements, for example reading NaCl as sodium and carbon and lithium; the correction is to recognise Na as sodium and Cl as chlorine.
- Assuming every element named in a question is in the first 20; the correction is to learn the additional specification elements, including iron, copper and zinc.
- Applying Greek prefixes to ionic compounds, such as calling MgCl₂ magnesium dichloride; the correction is to name it magnesium chloride as prefixes are typically for covalent compounds.
- Misunderstanding the prefix 'mono-', thinking it means more than one atom; the correction is to recognise 'mono-' means exactly one atom, as in carbon monoxide.
- Omitting Roman numerals for transition metals with variable valency, such as calling FeCl₂ iron chloride; the correction is to write iron(II) chloride to specify the +2 charge on the iron ion.
- Writing products on the left and reactants on the right: correct this by checking that the substances you start with are always on the left.
- Using an equals sign instead of an arrow: replace it with → to show the direction of change.
- Naming a product incorrectly, such as writing sodium chloride as sodium chlorine: correct it by using the proper compound name formed from the elements involved.
- Changing a subscript to balance, such as turning H₂O into H₂O₂: correct this by changing only the coefficient in front of the formula.
- Forgetting to multiply through when a coefficient is added, for example writing 2H₂O but counting only one oxygen: recount all atoms after each change.
- Using incorrect ion charges in a formula, such as writing sodium oxide as NaO: correct it by balancing charges to give Na₂O.
- Forgetting to balance charge: writing Na → Na⁺ + e⁻ is correct, but writing Na → Na⁺ without an electron is wrong. Correction: add the electron to balance the 1⁺ charge on the right.
- Including spectator ions in an ionic equation: for example, writing Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq). Correction: cancel ions that appear unchanged on both sides.
- Unbalanced atoms in a half equation: writing Cl₂ + 2e⁻ → Cl⁻. Correction: balance chlorine atoms first to give Cl₂ + 2e⁻ → 2Cl⁻.