Key Concepts in Chemistry

What you'll learn

This guide covers the fundamental principles that underpin all GCSE Chemistry topics. You'll master atomic structure, chemical formulae, balanced equations, state symbols and mole calculations. These concepts form the foundation for every other topic in your Edexcel GCSE Chemistry specification, so developing confidence here is essential for exam success.

Key terms and definitions

Atom — the smallest particle of an element that retains its chemical properties, consisting of a nucleus containing protons and neutrons, surrounded by electrons in shells

Element — a substance made of only one type of atom, which cannot be broken down into simpler substances by chemical means

Compound — a substance formed when two or more elements are chemically bonded together in fixed proportions

Molecule — two or more atoms chemically bonded together; can be atoms of the same element or different elements

Relative atomic mass (Ar) — the average mass of one atom of an element compared to 1/12th the mass of one carbon-12 atom

Relative formula mass (Mr) — the sum of the relative atomic masses of all atoms in a chemical formula

Mole — the unit for amount of substance; one mole contains 6.02 × 10²³ particles (Avogadro constant)

Limiting reactant — the reactant that is completely used up in a chemical reaction, determining the maximum amount of product formed

Core concepts

Atomic structure and the periodic table

All substances are made of atoms. Each atom consists of a central nucleus containing protons (positive charge, relative mass 1) and neutrons (no charge, relative mass 1). Electrons (negative charge, negligible mass) orbit the nucleus in shells or energy levels.

Key numbers:

• Atomic number = number of protons (defines the element)
• Mass number = protons + neutrons
• In a neutral atom: number of protons = number of electrons

Electron configuration determines chemical properties. The first shell holds up to 2 electrons, the second and third shells hold up to 8 electrons each. For example, sodium (atomic number 11) has the configuration 2,8,1.

Elements in the periodic table are arranged by atomic number. Elements in the same group have the same number of electrons in their outer shell, giving them similar chemical properties. For instance, Group 1 elements (lithium, sodium, potassium) all have 1 outer electron and react vigorously with water.

Isotopes and relative atomic mass

Isotopes are atoms of the same element with different numbers of neutrons. They have identical chemical properties but different physical properties.

For example, chlorine has two main isotopes:

• Chlorine-35 (75% abundance)
• Chlorine-37 (25% abundance)

The relative atomic mass accounts for all isotopes and their abundance:

Ar of chlorine = (35 × 75) + (37 × 25) / 100 = 35.5

This explains why relative atomic masses on the periodic table are rarely whole numbers.

Chemical formulae and equations

Chemical formulae show the number and type of atoms in a substance:

• Elements: O₂, Cl₂, P₄
• Simple compounds: H₂O, CO₂, NH₃
• Ionic compounds: NaCl, MgO, CaCO₃

Balancing equations ensures the law of conservation of mass is obeyed — atoms cannot be created or destroyed in chemical reactions.

Method:

1. Write the word equation
2. Write the symbol equation with correct formulae
3. Count atoms of each element on both sides
4. Add numbers (coefficients) in front of formulae to balance
5. Never change subscript numbers within formulae

Example:

• Unbalanced: Mg + O₂ → MgO
• Balanced: 2Mg + O₂ → 2MgO

State symbols indicate physical states:

• (s) = solid
• (l) = liquid
• (g) = gas
• (aq) = aqueous solution (dissolved in water)

Relative formula mass calculations

Calculate Mr by adding the Ar values of all atoms in the formula.

Example 1: Water (H₂O)

• Ar of H = 1, Ar of O = 16
• Mr = (2 × 1) + 16 = 18

Example 2: Calcium carbonate (CaCO₃)

• Ar of Ca = 40, Ar of C = 12, Ar of O = 16
• Mr = 40 + 12 + (3 × 16) = 100

This skill is essential for all mole calculations and percentage composition problems.

The mole concept and calculations

One mole of any substance contains 6.02 × 10²³ particles (Avogadro constant). The mass of one mole equals the relative formula mass in grams.

Key formula:

Number of moles = mass (g) / Mr

Example: How many moles in 20g of calcium carbonate (CaCO₃)?

• Mr of CaCO₃ = 100
• Moles = 20 / 100 = 0.2 mol

Rearranging the formula:

• Mass = moles × Mr
• Mr = mass / moles

Conservation of mass in reactions means total mass of reactants = total mass of products. If a reaction appears to gain mass (e.g., magnesium burning), a gas from the air has reacted. If mass appears to decrease (e.g., thermal decomposition), a gas has been released.

Reacting masses and limiting reactants

Chemical equations show the molar ratios in which substances react.

For the equation: 2Mg + O₂ → 2MgO

• 2 moles of Mg react with 1 mole of O₂ to produce 2 moles of MgO
• 48g of Mg reacts with 32g of O₂ to produce 80g of MgO

Calculating reacting masses:

1. Write the balanced equation
2. Calculate moles of the known substance
3. Use the equation ratio to find moles of unknown substance
4. Convert moles back to mass

The limiting reactant is completely used up. Any reactant in excess will have some left over. The limiting reactant determines the maximum amount of product that can form.

Concentration and volumes

Concentration measures the amount of solute dissolved in a solvent.

Key formulae:

Concentration (g/dm³) = mass of solute (g) / volume of solution (dm³)

Concentration (mol/dm³) = moles of solute / volume of solution (dm³)

Converting units:

• 1 dm³ = 1000 cm³
• To convert cm³ to dm³: divide by 1000

Example: 4g of sodium hydroxide dissolved in 500 cm³ of water

• Volume = 500/1000 = 0.5 dm³
• Concentration = 4/0.5 = 8 g/dm³

Worked examples

Example 1: Balancing equations (2 marks)

Question: Balance the equation for the combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O

Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Mark scheme:

• Correct balancing of all atoms (1 mark)
• All coefficients in simplest ratio (1 mark)

Working: Left side has 3 C atoms, 8 H atoms. Right side needs 3 CO₂ for carbon and 4 H₂O for hydrogen. This gives 10 O atoms needed on right, requiring 5 O₂ on left.

Example 2: Mole calculation (3 marks)

Question: Calculate the mass of carbon dioxide produced when 10g of calcium carbonate decomposes completely. CaCO₃ → CaO + CO₂ (Ar: Ca = 40, C = 12, O = 16)

Answer:

• Mr of CaCO₃ = 40 + 12 + (3 × 16) = 100 (1 mark)
• Moles of CaCO₃ = 10/100 = 0.1 mol (1 mark)
• Mr of CO₂ = 12 + (2 × 16) = 44
• From equation: 1 mol CaCO₃ produces 1 mol CO₂
• Mass of CO₂ = 0.1 × 44 = 4.4g (1 mark)

Example 3: Concentration calculation (3 marks)

Question: A student dissolves 5.85g of sodium chloride in water to make 250 cm³ of solution. Calculate the concentration in mol/dm³. (Mr of NaCl = 58.5)

Answer:

• Moles of NaCl = 5.85/58.5 = 0.1 mol (1 mark)
• Volume in dm³ = 250/1000 = 0.25 dm³ (1 mark)
• Concentration = 0.1/0.25 = 0.4 mol/dm³ (1 mark)

Common mistakes and how to avoid them

• Changing subscripts when balancing equations — never alter the numbers within formulae (e.g., H₂O must stay as H₂O). Only add coefficients in front of formulae. The subscripts define what the substance is.

• Forgetting to convert cm³ to dm³ — concentration calculations require volumes in dm³. Always divide cm³ values by 1000 before calculating. Set out your working clearly to show the conversion.

• Confusing Ar and Mr — relative atomic mass (Ar) is for single elements from the periodic table; relative formula mass (Mr) is the sum for compounds. Check which one the question asks for.

• Not using the equation ratio — in reacting mass calculations, you must use the numbers from the balanced equation to convert between moles of different substances. A 1:1 ratio cannot be assumed.

• Rounding too early — keep full calculator values until the final answer, then round to an appropriate number of significant figures (usually 2 or 3). Early rounding causes accumulating errors.

• Missing state symbols — questions specifically asking for state symbols will lose marks if you omit them. Check the command word "including state symbols" carefully.

Exam technique for "Key Concepts in Chemistry"

• "Calculate" questions require working — always show your method step-by-step. Even if your final answer is wrong, you can gain method marks. Write the formula you're using, substitute numbers, then calculate.

• Use the periodic table provided — Ar values are given in the exam. Don't memorise them, but do practise finding them quickly. For compounds, carefully identify every element and its subscript number.

• Check equation balancing carefully — count atoms systematically for each element on both sides. Use a table if needed. The most common errors are with oxygen in combustion reactions.

• Units matter — include correct units in your final answer (g, mol, g/dm³, mol/dm³). One mark questions may award the mark only if units are correct. If units aren't specified in the question, use standard SI units.

Quick revision summary

Atoms consist of protons, neutrons and electrons. Elements contain one type of atom; compounds contain chemically bonded atoms of different elements. Calculate Mr by summing Ar values. Balance equations by adding coefficients, never changing subscripts. One mole of substance has mass equal to Mr in grams. Use moles = mass/Mr for calculations. In chemical reactions, use balanced equations to determine molar ratios between reactants and products. Concentration links mass or moles of solute to volume of solution. Always show full working in calculations.