What does Edexcel GCSE Chemistry Topic 1 cover?

Topic 1 covers atomic structure (protons, neutrons and electrons, atomic number, mass number and isotopes), the development of the model of the atom, the periodic table and electronic configurations, ionic, covalent and metallic bonding and the structures they form (giant ionic, simple molecular, giant covalent, fullerenes, graphene, polymers and metals), chemical formulae and balanced equations, and the core calculations of the course.

How do you calculate relative atomic mass?

Relative atomic mass is a weighted mean. Multiply each isotope's mass number by its percentage abundance, add the results, then divide by 100. For chlorine with 75 percent of mass 35 and 25 percent of mass 37, the relative atomic mass is (35 times 75 plus 37 times 25) divided by 100, which is 35.5.

What are the three types of bonding in Topic 1?

Ionic bonding is electron transfer between a metal and a non-metal, forming a giant ionic lattice. Covalent bonding is shared pairs of electrons between non-metals, forming simple molecules or giant covalent structures. Metallic bonding is the attraction between positive metal ions and a sea of delocalised electrons. Each structure explains the substance's melting point and conductivity.

How do you find an empirical formula?

Divide the mass (or percentage) of each element by its relative atomic mass to get the number of moles, then divide all the answers by the smallest to find the simplest whole-number ratio. Scale up if needed. For an oxide with 6.4 g copper and 1.6 g oxygen, both give 0.1 mol, a 1 to 1 ratio, so the empirical formula is CuO.

What is the most common mistake in Topic 1?

Common mistakes are taking a simple average of isotope masses instead of weighting by abundance, changing the small numbers inside a formula when balancing, using masses directly instead of converting to moles in empirical-formula and reacting-mass questions, and forgetting to convert cm3 to dm3 in concentration. Show full working so method marks are secure.

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Edexcel GCSE Chemistry Topic 1 Key concepts in chemistry: a complete overview

A deep-dive Edexcel GCSE Chemistry guide to Topic 1 Key concepts in chemistry. Covers atomic structure and isotopes, the periodic table and electronic configuration, ionic, covalent and metallic bonding and the structures they form, chemical formulae and equations, and the core calculations including empirical formulae, reacting masses, concentration and the mole.

Generated by Claude Opus 4.816 min readTopic 1Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What Topic 1 actually demands
Atomic structure and isotopes
The periodic table and electronic configuration
Bonding and structure
Formulae, equations and calculations
How Topic 1 is examined
Check your knowledge

What Topic 1 actually demands

Key concepts in chemistry is the foundation of the whole course and the only topic examined on both papers. It links the structure of the atom to the periodic table, the three types of bonding to the properties of materials, and the formulae of compounds to the calculations that run through every other topic. Edexcel rewards precise particle-level explanation and confident, well-shown calculation here.

This guide walks through the topic in specification order, then sets out the exam patterns Edexcel repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Atomic structure and isotopes

An atom has a tiny nucleus of protons (relative mass 1, charge $+1$ ) and neutrons (mass 1, charge 0), surrounded by electrons (negligible mass, charge $-1$ ) in shells. The atomic number is the number of protons and the mass number is protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons, and relative atomic mass is the weighted mean of the isotope masses. The model of the atom developed from Dalton's spheres, through Thomson's plum pudding, to Rutherford's nuclear atom (from alpha-particle scattering) and Bohr's shells.

The periodic table and electronic configuration

The modern table arranges elements by increasing atomic number, so that groups (columns) share the same number of outer electrons and periods (rows) share the same number of shells. Mendeleev's earlier table ordered by atomic mass but grouped by properties and left gaps for undiscovered elements. The first 20 elements fill shells holding 2, then 8, then 8 electrons, and the configuration gives the position directly.

Bonding and structure

The three bonding types and their structures explain the properties of materials:

Ionic bonding (metal plus non-metal) forms a giant ionic lattice with high melting point, conducting only when molten or dissolved.
Covalent bonding (non-metals) forms simple molecules (low melting point, non-conducting) or giant covalent structures such as diamond, graphite, silicon dioxide, fullerenes and graphene.
Metallic bonding forms a lattice of positive ions in a sea of delocalised electrons, giving conductivity and malleability.

Formulae, equations and calculations

Formulae are written so charges balance; equations are balanced with big numbers and carry state symbols. The law of conservation of mass means the mass of products equals the mass of reactants. The core calculations are relative formula mass, percentage by mass, empirical formulae (mass divided by $A_r$ , then simplest ratio), reacting masses (mass to moles, ratio, moles to mass), concentration in g/dm $^3$ , and the mole ( $6.02 \times 10^{23}$ particles, mass in grams equal to the $M_r$ ).

How Topic 1 is examined

A typical Edexcel profile for this topic:

Atomic notation and isotopes. Counting particles and weighted-mean relative atomic mass.
Position and configuration. Deducing group and period from electronic configuration.
Bonding and properties. Explaining melting point and conductivity from structure, including diamond versus graphite.
Calculations. Empirical formulae, reacting masses, percentage by mass and concentration.

Check your knowledge

A mix of recall and calculation questions covering Topic 1. Attempt them under timed conditions, then check against the solutions.

State the relative mass and charge of a proton, a neutron and an electron. (3 marks)
An atom is $^{39}_{19}K$ . State its protons, neutrons and electrons. (3 marks)
Bromine has isotopes $^{79}Br$ ( $50\%$ ) and $^{81}Br$ ( $50\%$ ). Calculate its relative atomic mass. (2 marks)
Write the electronic configuration of sulfur (atomic number 16) and give its group and period. (3 marks)
Explain why magnesium oxide has a higher melting point than sodium chloride. (2 marks)
Calculate the percentage by mass of oxygen in water $H_2O$ ( $A_r$ : H = 1, O = 16). (2 marks)
A compound has $4.6$ g sodium and $1.6$ g oxygen. Find its empirical formula ( $A_r$ : Na = 23, O = 16). (3 marks)
Explain why graphite conducts electricity but diamond does not. (2 marks)

Solutions

Step 1: Q1 - subatomic particles

Each of the three subatomic particles has a defined relative mass and charge. Protons and neutrons sit in the nucleus; electrons occupy shells around it.

Proton: mass $1$ , charge $+1$ . Neutron: mass $1$ , charge $0$ . Electron: negligible mass, charge $-1$ .

Step 2: Q2 - reading nuclear notation

The bottom number (subscript) is the atomic number, giving the proton count. The top number (superscript) is the mass number, so neutrons are mass minus protons. A neutral atom has equal protons and electrons.

For $^{39}_{19}\text{K}$ : protons $= 19$ ; neutrons $= 39 - 19 = 20$ ; electrons $= 19$ .

Step 3: Q3 - relative atomic mass

Relative atomic mass is a weighted mean, not a simple average. Multiply each isotope mass by its percentage abundance, sum the results, then divide by 100:

\frac{(79 \times 50) + (81 \times 50)}{100} = \frac{8000}{100} = 80

Step 4: Q4 - electronic configuration and position

Sulfur has atomic number 16, so 16 electrons fill the shells in order: 2, then 8, then the remaining 6. The number of outer electrons gives the group; the number of shells gives the period.

Configuration $2, 8, 6$ : Group 6, Period 3.

Step 5: Q5 - melting points of ionic compounds

A higher melting point means stronger electrostatic attraction between ions is needed to overcome. Magnesium oxide ( $\text{Mg}^{2+}$ and $\text{O}^{2-}$ ) has doubly charged ions, so its lattice energy is much greater than sodium chloride ( $\text{Na}^+$ and $\text{Cl}^-$ ) with singly charged ions.

Magnesium oxide has $2+$ and $2-$ ions, so the electrostatic attraction is stronger than the $1+$ and $1-$ ions in sodium chloride, needing more energy to separate.

Step 6: Q6 - percentage by mass

The percentage by mass of an element is the mass of that element in one formula unit divided by the total relative formula mass, multiplied by 100. For water, $M_r = (2 \times 1) + 16 = 18$ :

\frac{16}{18} \times 100 = 88.9\%

Step 7: Q7 - empirical formula

Divide the mass of each element by its relative atomic mass to find moles, then divide all values by the smallest to get the simplest whole-number ratio:

Moles Na $= \dfrac{4.6}{23} = 0.2$ ; moles O $= \dfrac{1.6}{16} = 0.1$ ; ratio $0.2 : 0.1 = 2 : 1$ , so the empirical formula is $\text{Na}_2\text{O}$ .

Step 8: Q8 - graphite versus diamond conductivity

The difference comes down to delocalised electrons. In graphite each carbon forms three covalent bonds in a layer, leaving one electron per atom that is free to move and carry charge. In diamond every carbon forms four bonds in a tetrahedral lattice, leaving no free electrons at all.

Sources & how we know this

Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)