Edexcel IGCSE Chemistry: Principles of Chemistry (4CH1)

This guide covers Topic 1 Principles of Chemistry, the first of four topics in Pearson Edexcel International GCSE Chemistry (4CH1), first teaching September 2017. The qualification is untiered: both Chemistry Paper 1 and Paper 2 draw on all four topic areas, with Paper 2 additionally assessing the bold, “C”-referenced content shown in the specification for greater depth.

Where this fits in 4CH1

Topic 1 introduces the foundational ideas – states of matter, atomic structure, the periodic table, and the three main types of chemical bonding – that Topics 2-4 (Inorganic chemistry, Physical chemistry, Organic chemistry) all build on. Every later topic assumes secure knowledge of ionic, covalent and metallic bonding from this topic.

Syllabus coverage

PEARSON EDEXCEL INTERNATIONAL GCSE CHEMISTRY (4CH1) — TOPIC 1 PRINCIPLES OF CHEMISTRY

• (a) States of matter — the arrangement, movement and energy of particles in solids, liquids and gases, and interconversion between states
• (b) Elements, compounds and mixtures — classifying substances and understanding melting/boiling point behaviour
• (c) Atomic structure — the structure of the atom and its subatomic particles
• (d) The Periodic Table — how elements are arranged and organised
• (e) Chemical formulae, equations and calculations — writing and balancing chemical equations and formulae
• (f) Ionic bonding — the formation and properties of ionic compounds
• (g) Covalent bonding — the formation and properties of covalently bonded substances
• (h) Metallic bonding — the structure and properties of metals
• (i) Electrolysis — the principles of electrolysis and its uses

How to approach it

Because this topic covers three distinct types of chemical bonding (ionic, covalent, metallic), build a comparison table linking each bonding type to the physical properties it produces (melting point, conductivity, solubility), since exam questions frequently ask candidates to explain a substance’s properties in terms of its bonding and structure. Practise writing and balancing chemical equations and formulae (e) as a routine skill separate from the conceptual content, since accuracy here is assumed throughout the rest of the specification. Since Paper 2 tests this topic in greater depth than Paper 1 (via the bold, “C”-referenced content), check which paper you are preparing for so you know how deep your revision of each sub-topic needs to go.

Official syllabus

Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3, September 2024 — qualifications.pearson.com.

States of matter and particle theory

Solids have particles in a regular arrangement, vibrating about fixed positions. Liquids have particles close together but able to move past one another. Gases have particles far apart, moving randomly at speed. Changes of state are physical: energy is supplied to overcome forces of attraction, and no new substance forms.

Diffusion is the net movement of particles from high to low concentration. Lighter particles diffuse faster at a given temperature, which is why ammonia and hydrogen chloride meet nearer the hydrochloric acid end of a glass tube.

Atomic structure and the Periodic Table

Protons and neutrons sit in the nucleus; electrons occupy shells filling 2, 8, 8. Atomic number gives the number of protons and defines the element; mass number is protons plus neutrons. Isotopes differ in neutron number only, so their chemistry is identical.

Group number equals the number of outer-shell electrons; period number equals the number of occupied shells. Elements react to achieve a full outer shell, which is the single idea underlying all the bonding that follows.

Bonding

Ionic bonding occurs between metals and non-metals: electrons transfer, ions form, and strong electrostatic attraction holds a giant lattice together. This gives high melting points, and conduction only when molten or dissolved, because ions must be free to move.

Covalent bonding occurs between non-metals: electron pairs are shared. Simple molecular substances have weak intermolecular forces between molecules — not within them — so melting points are low and they do not conduct. Giant covalent structures such as diamond and graphite have very high melting points because covalent bonds must be broken. Graphite conducts because each carbon bonds to only three others, leaving one delocalised electron per atom.

Metallic bonding is a lattice of positive ions in a sea of delocalised electrons, giving conduction and malleability.

Calculations

moles = mass / Mr
moles = concentration x volume (dm3)
percentage yield = (actual / theoretical) x 100

Empirical formula is found by dividing the mass or percentage of each element by its Ar, then dividing all results by the smallest.

Worked example

24.0 g of magnesium burns completely in oxygen. Calculate the mass of magnesium oxide formed.

2Mg + O2 -> 2MgO

moles Mg  = 24.0 / 24 = 1.0 mol
ratio Mg : MgO is 2 : 2, so moles MgO = 1.0 mol
Mr of MgO = 24 + 16 = 40
mass MgO  = 1.0 x 40 = 40.0 g

The mass increases because oxygen has been added — conservation of mass applies to the whole system, including the gas.

Common mistakes

Saying covalent bonds break when a simple molecular substance melts — only the weak forces between molecules are overcome. Stating ionic compounds conduct as solids. Forgetting to balance the equation before using mole ratios, which invalidates every later step. Writing that graphite conducts “because it is a metal”. Giving empirical formula answers without dividing through by the smallest ratio.

Quick revision checklist

• Describe the three states in terms of arrangement, movement and energy.
• Determine protons, neutrons and electrons, and explain isotopes.
• Link group and period to electronic structure.
• Explain ionic, covalent, giant covalent and metallic bonding, and use each to account for melting point and conductivity.
• Balance equations and carry out mole, mass, concentration and percentage yield calculations.
• Calculate an empirical formula from masses or percentages.