Understanding Atomic structure and the periodic table: OxfordAQA IGCSE Chemistry Explained

Why Atomic Structure and the Periodic Table Matters

This is a dedicated look at OxfordAQA IGCSE Chemistry atomic structure and the periodic table, the foundation section that every later topic in the specification quietly depends on. Whether you searched for atomic structure and the periodic table OxfordAQA IGCSE revision help or landed here looking for IGCSE 9202 atomic structure and the periodic table notes specifically, the ground covered below is the same three subtopics the specification groups together: solids, liquids and gases, a simple model of the atom, and the periodic table itself.

Every other topic in OxfordAQA IGCSE Chemistry leans on this one. Bonding, reactivity trends, electrolysis and even the properties of acids and bases all trace back to how electrons are arranged around a nucleus. Get comfortable here and the rest of the course reads like a set of logical consequences rather than a pile of separate facts to memorise. This deep dive works through the three components of the section, solids, liquids and gases, a simple model of the atom, and the periodic table, with worked examples, common mistakes and self-check questions along the way.

Solids, Liquids and Gases

The particle model explains the three states of matter in terms of how particles are arranged, how they move, and how strongly they attract each other.

Every change of state, melting, freezing, evaporating, condensing, subliming, is a change in energy, not a change in the identity of the substance. Students should be able to name each inter-conversion process and describe the energy transfer that accompanies it: energy is absorbed when a solid melts or a liquid boils, and energy is released when a gas condenses or a liquid freezes.

Evidence for the existence of particles themselves comes from simple diffusion experiments: bromine vapour spreading through air, ammonia and hydrogen chloride gases meeting to form a visible ring of ammonium chloride, or potassium manganate(VII) crystals slowly colouring a beaker of still water without stirring. These experiments show that particles are constantly moving and spreading out, even when nothing appears to be pushing them.

A Simple Model of the Atom

All substances are made of atoms, and a substance made of only one type of atom is an element, represented by a chemical symbol such as O for oxygen. There are about 100 known elements, and they are organised in the periodic table.

Subatomic Particles

An atom has no overall charge because the number of electrons always equals the number of protons. The atomic number tells you the number of protons, and the mass number tells you the total of protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons, which is why relative atomic mass is an average across an element's naturally occurring isotopes rather than a single whole number.

Worked Example: Finding Subatomic Particles

Question: An atom of chlorine can be represented as chlorine-35, with atomic number 17. State the number of protons, neutrons and electrons.

Answer: Protons = atomic number = 17. Electrons = protons (atom is neutral) = 17. Neutrons = mass number minus atomic number = 35 - 17 = 18.

This three-step method, protons from atomic number, electrons matching protons, neutrons by subtraction, answers almost every subatomic particle question you will meet.

Electron Shells

Electrons occupy energy levels, often called shells, filling the lowest available level first. For the first twenty elements, electronic structures can be written as a sequence of numbers, such as 2,8,1 for sodium, meaning two electrons in the first shell, eight in the second, and one in the third. Being able to write this out correctly, and to draw it as a simple diagram with the nucleus at the centre and electrons arranged in concentric shells, is one of the most reliably tested skills in this section.

The Periodic Table

The periodic table arranges elements in order of atomic (proton) number, and elements with similar properties sit in the same vertical column, called a group. The table is periodic because similar chemical behaviour recurs at regular intervals as atomic number increases. Its modern form is credited to Mendeleev, whose early version left gaps for undiscovered elements based on the pattern of properties he observed.

Elements in the same group share the same number of electrons in their outer shell, which is exactly why they behave similarly: chemical reactivity is largely a story about outer-shell electrons. Group 0, the noble gases, are unreactive because their atoms already have a stable outer-shell arrangement, eight electrons for every noble gas except helium, which has a stable arrangement of just two.

Self-Check Questions

• Write the electronic structure of an atom with atomic number 12.
• Explain why isotopes of the same element have the same chemical properties but different physical masses.
• Explain why elements in Group 0 are described as unreactive, in terms of electron arrangement.
• Describe one piece of experimental evidence for the existence of moving particles.
• State the number of protons, neutrons and electrons in an atom with mass number 27 and atomic number 13.

How OxfordAQA Examines This Topic

Across OxfordAQA IGCSE Chemistry practice questions on this section, the recurring skill being tested is translating between representations: a written description, a numerical atomic/mass number pair, an electronic structure written as digits, and a diagram of shells. Questions frequently ask you to move from one representation to another, for example given a diagram, write the electronic structure, or given atomic and mass numbers, state the number of neutrons. Practising these conversions in both directions, rather than only reading definitions, is the single most effective way to prepare.

Diffusion and state-change questions tend to ask for an explanation in terms of particle movement and energy, so keep a stock of precise vocabulary ready: "particles gain kinetic energy," "particles overcome forces of attraction," "particles move further apart," rather than looser descriptions like "the substance gets hotter and changes."

Building Revision Notes for This Section

These OxfordAQA IGCSE Chemistry revision notes work well condensed onto a single page per subtopic:

1. A labelled diagram of an atom with proton, neutron and electron positions marked.
2. A short table of the three subatomic particles and their charges and masses.
3. A blank periodic table outline with Groups 1, 0 and 7 highlighted, since these three groups carry almost all of the group-property questions in this specification.
4. A written definition, in your own words, of isotope, atomic number and mass number.

Testing yourself by covering the diagram and redrawing it from memory is far more useful preparation than simply re-reading a completed page, and it is a habit worth carrying into every other topic in this course, not just this one. Good OxfordAQA IGCSE Chemistry notes are tested, not just written.

Worked Example: Predicting a Group Property

Question: Fluorine and chlorine are both in Group 7. Predict, giving a reason, which element has the higher melting point.

Answer: Chlorine has the higher melting point. Within Group 7, melting point increases going down the group, so chlorine, being below fluorine, is expected to melt at a higher temperature than fluorine.

Questions like this one test whether you can apply a group trend rather than simply recite it, so practise stating the trend and then applying it to a specific pair of elements, in that order.

Answering the Self-Check Questions

Working through the self-check questions above yourself before reading on is the better approach, but here is what a full-mark answer looks like for each.

• Electronic structure of atomic number 12 (magnesium): 2,8,2 - two electrons fill the first shell, eight fill the second, and the remaining two sit in the third shell.
• Isotopes and chemical properties: isotopes have the same number of protons and therefore the same number of electrons, and it is the number and arrangement of electrons that determines chemical behaviour; differing neutron numbers change mass but not electron arrangement.
• Group 0 and unreactivity: noble gas atoms already have a full, stable outer shell of electrons, so they have no tendency to lose, gain or share electrons to form bonds.
• Evidence for particle movement: potassium manganate(VII) crystals placed at the bottom of a beaker of water gradually colour the whole beaker purple without any stirring, showing that particles move and spread out on their own.
• Aluminium-27, atomic number 13: 13 protons, 13 electrons, and 27 minus 13 equals 14 neutrons.

Where This Topic Leads Next

Everything covered in atomic structure and the periodic table feeds directly into the next major section of the course, structure, bonding and the properties of matter, where atoms start losing, gaining and sharing the very electrons discussed here to form compounds. If electron arrangement and shell structure still feel shaky, it is worth consolidating them before moving forward, because bonding diagrams assume this knowledge is already automatic. A student who can confidently draw the electronic structure of sodium, chlorine and calcium without hesitation is already most of the way to understanding ionic bonding in the next topic.

Chemistry OxfordAQA IGCSE questions on this section reward precision far more than volume of writing. A short, correctly labelled diagram often earns full marks where a paragraph of prose earns partial credit at best, so practise being concise and accurate rather than exhaustive.

OxfordAQA IGCSE Chemistry Explained, One Diagram at a Time

If there is a single habit that keeps OxfordAQA IGCSE Chemistry explained clearly in your own head rather than as a fog of half-remembered facts, it is drawing rather than describing. Sketch the atom, sketch the shells, sketch the outline of the periodic table with Groups 1, 0 and 7 marked, every single time you revise this topic, even after you think you already know it. Diagrams surface gaps that reading past them quietly hides, and by the time you sit real OxfordAQA IGCSE Chemistry practice questions on atomic structure, that habit will have paid for itself many times over.