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An atom has a tiny, dense, positively charged nucleus containing protons and neutrons at its centre, with negatively charged electrons around it. Free notes.
The Structure of the Atom is part of Atoms and isotopes in GCSE AQA Higher Physics.
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An atom has a tiny, dense, positively charged nucleus containing protons and neutrons at its centre, with negatively charged electrons around it. Free notes.
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Use this lesson to stay inside Atoms and isotopes while connecting the detail back to the wider Atomic structure topic.
Read the core explanations from The Structure of the Atom before testing yourself from memory.
Everything around you - your desk, the air you breathe, even your own body - is made of atoms. These particles are so incredibly small that you could fit about 10 million atoms across the width of your fingernail.
Atoms have a radius of about 1 × 10⁻¹⁰ metres. To put this in perspective, if an atom were the size of a football stadium, the nucleus would be like a marble sitting in the centre circle. Most of what we think of as 'solid' matter is actually empty space.
Understanding atomic structure explains why elements behave differently and how chemical reactions work. It's the foundation for everything else in chemistry and physics.
Picture an atom like a miniature solar system, but with some crucial differences. At the centre sits the nucleus - a tiny, dense core containing protons and neutrons. Whizzing around this nucleus are electrons, but not in neat circular orbits like planets.
The nucleus is incredibly small compared to the whole atom. Its radius is less than 1/10,000 of the atom's radius. Yet this tiny space contains nearly all the atom's mass. It's like having 99.9% of a football stadium's weight concentrated in that central marble.
The electrons occupy the vast empty space around the nucleus. They carry a negative charge, while the nucleus is positively charged due to the protons inside it. This opposite attraction keeps the electrons from flying away.
The three types of subatomic particles have very different properties. Understanding these differences is crucial for explaining how atoms behave and why elements have their unique characteristics.
| Particle | Location | Relative Mass | Relative Charge |
|---|---|---|---|
| Proton | Nucleus | 1 | +1 |
| Neutron | Nucleus | 1 | 0 |
| Electron | Around nucleus | 1/1840 (negligible) | -1 |
Notice how electrons are nearly 2000 times lighter than protons or neutrons. This is why we can ignore electron mass when calculating atomic mass. The number of protons determines what element you have - change the proton number and you get a completely different element.
Neutrons act like nuclear 'glue', helping to hold the positively charged protons together in the nucleus. Without neutrons, the protons would repel each other and the nucleus would fly apart.
Every element has its own identity card in the form of atomic number and mass number. The atomic number tells you how many protons are in the nucleus - this never changes for a given element. Carbon always has 6 protons, oxygen always has 8.
The mass number is the total count of protons plus neutrons in the nucleus. Since electrons have negligible mass, we ignore them in this calculation. For example, a carbon atom with 6 protons and 6 neutrons has a mass number of 12.
You can work out the number of neutrons by subtracting: neutrons = mass number - atomic number. This simple calculation is essential for understanding isotopes and nuclear processes.
Worked example: Finding Particle Numbers from Atomic Data
An atom has atomic number 11 and mass number 23. Find the number of protons, neutrons and electrons.
Answer: 11 protons, 12 neutrons, 11 electrons
The scale of atoms challenges our everyday experience. If you could line up atoms like beads on a string, you'd need 10 billion of them to stretch across just one centimetre. That's more atoms than there are people on Earth, all fitting in the width of your little fingernail.
The nucleus is even more mind-boggling. Remember that football stadium analogy? If the atom were Wembley Stadium (90 metres across), the nucleus would be a marble about 1 centimetre wide sitting in the centre circle. Everything else is empty space where electrons zip around.
This emptiness explains some surprising facts. When you touch a table, you're not actually touching it - the electrons in your finger repel the electrons in the table surface. What feels solid is really electromagnetic forces between tiny particles separated by vast empty spaces.
Scientists use scientific notation to handle these extreme numbers. Writing 0.0000000001 metres is clumsy and error-prone. Writing 1 × 10⁻¹⁰ metres is cleaner and shows the scale clearly.
Exam questions on atomic structure follow predictable patterns. You'll often be given atomic and mass numbers and asked to find particle numbers. The key is remembering the simple relationships: atomic number = protons, mass number = protons + neutrons, and in neutral atoms, electrons = protons.
Watch out for the common trap of confusing atomic number with mass number. Atomic number is always the smaller number and appears at the bottom left of element symbols. Mass number is larger and goes at the top left. Many students mix these up under exam pressure.
When describing atomic structure, use precise scientific language. Say 'the nucleus contains protons and neutrons' rather than 'the middle bit has the heavy particles'. Examiners reward accurate terminology, and it shows you understand the concepts properly.
For calculations involving scientific notation, be extra careful with powers of 10. A common mistake is writing 1 × 10⁻¹⁰ as 1 × 10¹⁰, which makes atoms impossibly large instead of impossibly small. Always check your answer makes physical sense.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The relative mass of an electron is 1/1840, which is negligible, compared with 1 for a proton or neutron. Electrons are nearly 2000 times lighter, so their mass is ignored when calculating atomic mass.
An atom has a tiny, dense, positively charged nucleus containing protons and neutrons at its centre, with negatively charged electrons around it. The electrons occupy the vast empty space around the nucleus and are attracted to the positive nucleus.
Atoms have a radius of about 1 × 10⁻¹⁰ metres. They are so small that about 10 million atoms would fit across the width of a fingernail, and scientific notation is used to handle such extreme numbers.
The radius of the nucleus is less than 1/10,000 of the atom's radius, yet it contains nearly all the atom's mass. If an atom were the size of a football stadium, the nucleus would be like a marble sitting in the centre circle.
An atom is mostly empty space because nearly all its mass is in a nucleus whose radius is less than 1/10,000 of the atom's radius, and electrons zip around in the space between. Much of what we think of as solid matter is actually empty space.
The number of protons determines which element an atom is: changing the proton number gives a completely different element. Carbon always has 6 protons and oxygen always has 8.
An atom with atomic number 11 and mass number 23 has 11 protons, 12 neutrons and 11 electrons. Neutrons = mass number − atomic number = 23 − 11 = 12, and a neutral atom has equal numbers of protons and electrons.
A proton has a relative mass of 1 and a relative charge of +1, a neutron has a relative mass of 1 and charge 0, and an electron has a relative mass of 1/1840 (negligible) and a charge of -1. Protons and neutrons are in the nucleus; electrons are around it.
About 10 million atoms would fit across the width of a fingernail, because atoms have a radius of about 1 × 10⁻¹⁰ metres. Lining atoms up like beads, 10 billion would stretch across one centimetre.
Electrons are kept around the nucleus by the attraction between opposite charges. The electrons carry a negative charge, while the nucleus is positively charged because of the protons inside it.
Neutrons act like nuclear glue, helping to hold the positively charged protons together in the nucleus. Without neutrons, the protons would repel each other and the nucleus would fly apart.
A table feels solid even though atoms are mostly empty space because the electrons in a finger repel the electrons in the table surface. What feels solid is really electromagnetic forces between tiny particles separated by vast empty spaces.
Scientific notation is used because writing 0.0000000001 metres is clumsy and error-prone, while 1 × 10⁻¹⁰ metres is cleaner and shows the scale clearly. A common mistake is writing 1 × 10¹⁰, which would make atoms impossibly large.
The mass number is bigger than the atomic number and goes at the top left of the element symbol, while the atomic number is the smaller number at the bottom left. Confusing the two is a common trap.
Protons and neutrons are found in the nucleus, a tiny, dense core at the centre of the atom. Electrons are found around the nucleus, occupying the vast empty space around it.