Atomic Structure
A concise revision guide to subatomic particles, atomic number, mass number, nuclide notation, isotopes, relative isotopic mass and relative formula mass for Edexcel A Level Chemistry.
Subatomic Particles
Atoms are made from three main subatomic particles: protons, neutrons and electrons.
Protons and neutrons are found in the nucleus, while electrons occupy regions of space around the nucleus. Protons and neutrons are also called nucleons.
Almost all of the mass of an atom is concentrated in the nucleus. The rest of the atom is mostly empty space.
| Particle | Position | Relative mass | Relative charge |
|---|---|---|---|
| Proton | Nucleus | 1 | +1 |
| Neutron | Nucleus | 1 | 0 |
| Electron | Orbitals or shells around the nucleus | 1/1840 | -1 |
This simplified Bohr model shows the nucleus at the centre with electrons arranged in shells around it.
Atomic Number and Mass Number
The atomic number, symbol Z, is the number of protons in the nucleus of an atom.
The mass number, symbol A, is the total number of protons and neutrons in the atom.
For a neutral atom, the number of electrons is equal to the number of protons.
Example: In 73Li, the atom has 3 protons and 7 – 3 = 4 neutrons. A neutral lithium atom also has 3 electrons.
Isotopes
Isotopes are atoms with the same number of protons but different numbers of neutrons.
Because isotopes of the same element have the same number of protons, they have the same atomic number. Their mass numbers are different because the number of neutrons is different.
Isotopes normally have very similar chemical properties because they have the same electronic structure. They may have slightly different physical properties because their masses are different.
| Isotope | Symbol | Number of protons | Number of neutrons | Number of electrons in a neutral atom |
|---|---|---|---|---|
| chlorine-35 | 3517Cl | 17 | 18 | 17 |
| chlorine-37 | 3717Cl | 17 | 20 | 17 |
Exam focus: Isotopes differ in neutrons, not protons. If the number of protons changes, it is a different element.
Relative Isotopic Mass
Relative isotopic mass is the mass of one atom of an isotope compared with one twelfth of the mass of one atom of carbon-12.
Carbon-12 is used as the standard because one atom of carbon-12 is defined as exactly 12 atomic mass units.
This relative scale avoids using extremely small masses in kilograms when comparing atoms and isotopes.
| Term | Meaning | Exam wording to use |
|---|---|---|
| Atomic mass unit | One twelfth of the mass of one atom of carbon-12. | Use 1/12 and carbon-12. |
| Relative isotopic mass | The mass of one atom of an isotope on the carbon-12 scale. | Refer to one atom of an isotope. |
| Relative atomic mass | The weighted mean mass of atoms of an element compared with 1/12 of carbon-12. | This is covered in more depth in RAM calculations. |
Relative Molecular Mass and Relative Formula Mass
The relative formula mass, Mr, is found by adding together the relative atomic masses of all the atoms shown in a formula.
The term relative molecular mass, sometimes written RMM, should only be used for substances that exist as molecules. Relative formula mass is the safer term because it applies to molecular substances, ionic compounds and giant structures. The specification states that the term relative formula mass should be used for compounds with giant structures.
Relative formula mass, Mr: the sum of the relative atomic masses of all atoms in the formula of a substance.
| Substance | Formula | Calculation | Mr |
|---|---|---|---|
| Carbon dioxide | CO2 | 12.0 + (2 × 16.0) | 44.0 |
| Calcium carbonate | CaCO3 | 40.1 + 12.0 + (3 × 16.0) | 100.1 |
| Magnesium nitrate | Mg(NO3)2 | 24.3 + 2(14.0 + 3 × 16.0) | 148.3 |
Exam focus: brackets multiply everything inside them. In Mg(NO3)2, there are two nitrogen atoms and six oxygen atoms.
Relative formula mass is calculated by counting each atom in the formula and adding the correct relative atomic masses.
Common Exam Points
Atomic structure questions often reward precise definitions and careful counting. The most important terms are atomic number, mass number, isotope, neutron number and relative isotopic mass.
Do not confuse atomic number and mass number
Atomic number is the number of protons. Mass number is the total number of protons and neutrons.
Use A – Z for neutrons
The number of neutrons is found by subtracting the atomic number from the mass number.
Neutral atoms have equal protons and electrons
If the atom is neutral, the positive charge from protons is balanced by the negative charge from electrons.
Isotopes have the same electronic structure
This explains why isotopes of the same element have similar chemical properties.
Check Your Understanding
Use these short activities to check subatomic particles, nuclide notation, isotopes and relative isotopic mass before moving on.
QuickSnap
For this page, the essential idea is that the nucleus contains protons and neutrons, while electrons occupy regions around the nucleus. Atomic number counts protons, mass number counts protons plus neutrons, and isotopes have the same number of protons but different numbers of neutrons.
Subatomic particles
Protons have charge +1, neutrons have charge 0, and electrons have charge -1. Protons and neutrons have relative mass 1, while electrons have a much smaller relative mass of about 1/1840.
Atomic number and mass number
Atomic number, Z, gives the number of protons. Mass number, A, gives the number of protons plus neutrons.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Carbon-12 scale
Relative isotopic mass compares one atom of an isotope with one twelfth of the mass of one atom of carbon-12.
Master Atomic Structure and The Periodic Table for Edexcel A Level Chemistry
Continue from these free revision notes into the full Topic 1 Atomic Structure and The Periodic Table course, with guided video teaching, diagnostic MCQ practice, teacher-marked short-answer questions and a specification assignment with a personalised progress report.
Guided video teaching
Learn atomic structure, isotopes, mass spectrometry, ionisation energy, electron configuration and periodic trends through structured video lessons with worked examples and walkthroughs.
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Auto-marked MCQ quizzes provide immediate diagnostic feedback for every answer choice.
Teacher-marked SAQs
Submit written exam responses and receive chemistry specialist feedback with improvement guidance.
Progress tracking
Identify strengths and weaknesses across atomic structure, isotopes, mass spectrometry, ionisation energies, electron configuration and periodic trends with targeted reporting.
See how the course works
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Atomic Structure FAQs
Use these quick answers to check the definitions most commonly assessed in early atomic structure questions.
What is the atomic number?
The atomic number is the number of protons in the nucleus of an atom. It is also called Z.
What is the mass number?
The mass number is the total number of protons and neutrons in the nucleus of an atom. It is also called A.
How do you calculate the number of neutrons?
Use number of neutrons = mass number – atomic number. In symbols, this is A – Z.
What are isotopes?
Isotopes are atoms with the same number of protons but different numbers of neutrons.
Why do isotopes have similar chemical properties?
Isotopes of the same element have the same number of electrons and the same electronic structure, so they normally react in similar ways.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.