The Structure of the Atom

This lesson covers the fundamental structure of the atom as required by the AQA GCSE Physics specification (4.4.1). You need to understand the basic model of the atom, the sub-atomic particles it contains, and how they are arranged. This knowledge is the foundation for understanding radioactivity, nuclear reactions, and the behaviour of matter at the smallest scales.

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The Nuclear Model of the Atom

The atom consists of a tiny, dense nucleus at the centre, surrounded by electrons that orbit the nucleus at different energy levels (sometimes called shells). The nucleus contains two types of sub-atomic particle: protons and neutrons. Together, protons and neutrons are called nucleons.

Key features of the nuclear model:

• The nucleus is extremely small compared to the overall size of the atom. The radius of the nucleus is about 1 x 10^-14 m, while the radius of the atom is about 1 x 10^-10 m. This means the nucleus is roughly 10,000 times smaller than the atom itself.
• Most of the mass of the atom is concentrated in the nucleus, because protons and neutrons are much heavier than electrons.
• The electrons orbit the nucleus at relatively large distances. Most of the atom is empty space.
• The electrons are held in orbit by the electrostatic force of attraction between the negatively charged electrons and the positively charged protons in the nucleus.

Exam Tip: If asked to describe the nuclear model, always mention three things: (1) a small, dense, positively charged nucleus, (2) electrons orbiting at a distance, and (3) most of the atom is empty space. These three points will secure full marks.

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Sub-Atomic Particles

There are three sub-atomic particles you need to know. Each has a different charge and relative mass.

Particle	Location	Relative Mass	Relative Charge
Proton	Nucleus	1	+1
Neutron	Nucleus	1	0
Electron	Orbiting nucleus	Very small (approx. 1/1836)	-1

Key Points

• Protons and neutrons have approximately the same mass. For GCSE purposes, we say they each have a relative mass of 1.
• Electrons have a mass that is negligible compared to protons and neutrons. This is why the mass of an atom is almost entirely due to its nucleus.
• A proton has a charge of +1 and an electron has a charge of -1. These charges are equal in magnitude but opposite in sign.
• A neutron has no charge (it is electrically neutral).

Exam Tip: The relative mass of an electron is sometimes given as 0.0005 or 1/1836. For most GCSE questions, you can simply say the mass of an electron is "negligible" or "very small" compared to a proton or neutron. Do not say it is zero — it does have mass, just a very small amount.

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Atoms Are Electrically Neutral

In a neutral atom, the number of protons equals the number of electrons. Because each proton has a +1 charge and each electron has a -1 charge, the charges cancel out, making the atom electrically neutral overall.

• A hydrogen atom has 1 proton and 1 electron: total charge = +1 + (-1) = 0.
• A carbon atom has 6 protons and 6 electrons: total charge = +6 + (-6) = 0.
• A uranium atom has 92 protons and 92 electrons: total charge = +92 + (-92) = 0.

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The Size of Atoms

Atoms are incredibly small. The radius of a typical atom is about 1 x 10^-10 m (0.1 nanometres). To put this in perspective:

• A human hair is about 0.1 mm (1 x 10^-4 m) thick, so roughly one million atoms could fit across the width of a human hair.
• The nucleus is even smaller — about 1 x 10^-14 m in radius, which is about 1/10,000th the radius of the atom.

If the atom were scaled up to the size of a football stadium, the nucleus would be about the size of a marble on the centre spot. Everything else would be empty space with tiny electrons orbiting far from the centre.

graph TD
    A["Atom (radius ~ 1 x 10^-10 m)"] --> B["Nucleus (radius ~ 1 x 10^-14 m)"]
    A --> C["Electron Cloud"]
    B --> D["Protons (+1 charge, mass 1)"]
    B --> E["Neutrons (0 charge, mass 1)"]
    C --> F["Electrons (-1 charge, negligible mass)"]

    style A fill:#2c3e50,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#3498db,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#95a5a6,color:#fff
    style F fill:#3498db,color:#fff

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Electron Configuration

Electrons orbit the nucleus in energy levels (shells). Each energy level can hold a certain maximum number of electrons:

Energy Level (Shell)	Maximum Electrons
1st (closest to nucleus)	2
2nd	8
3rd	8

Electrons always fill the lowest available energy level first before moving to the next shell outward. This is because electrons in lower energy levels are closer to the nucleus and experience a stronger electrostatic attraction.

Examples

Element	Atomic Number	Electron Configuration
Hydrogen	1	1
Helium	2	2
Lithium	3	2, 1
Carbon	6	2, 4
Oxygen	8	2, 6
Sodium	11	2, 8, 1
Chlorine	17	2, 8, 7
Calcium	20	2, 8, 8, 2

Exam Tip: You can work out the electron configuration from the atomic number (which equals the number of protons, which equals the number of electrons in a neutral atom). Fill the first shell with up to 2, the second with up to 8, the third with up to 8, and so on.

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Ions

When an atom gains or loses electrons, it becomes an ion. Ions are charged particles:

• If an atom loses one or more electrons, it becomes a positive ion (cation) because it now has more protons than electrons.
• If an atom gains one or more electrons, it becomes a negative ion (anion) because it now has more electrons than protons.

Atom	Protons	Electrons in Atom	Change	Electrons in Ion	Ion Charge
Sodium (Na)	11	11	Loses 1	10	+1 (Na+)
Chlorine (Cl)	17	17	Gains 1	18	-1 (Cl-)
Magnesium (Mg)	12	12	Loses 2	10	+2 (Mg2+)
Oxygen (O)	8	8	Gains 2	10	-2 (O2-)

Important: When an atom becomes an ion, only the number of electrons changes. The number of protons and neutrons stays the same. This means the element does not change — a sodium ion is still sodium.

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Electrons and Electromagnetic Radiation

Electrons can move between energy levels if they absorb or emit energy. This is a fundamental idea that is revisited throughout GCSE Physics, particularly in the context of the atomic model and the electromagnetic spectrum.

• An electron can absorb electromagnetic radiation (for example, a photon of light). If the absorbed energy exactly matches the difference between two energy levels, the electron is promoted to a higher energy level further from the nucleus. This is sometimes described as the atom being in an excited state.
• An electron can later fall back down to a lower energy level. When it does, it emits electromagnetic radiation with an energy equal to the difference between the two levels.
• If an electron absorbs enough energy, it can be removed from the atom entirely. This process is called ionisation, and the atom becomes a positive ion.

You do not need to calculate the energy of photons at GCSE, but you do need to know that electrons gain and lose energy in fixed amounts (not a continuous range), which is why heated elements produce characteristic emission spectra with sharp lines rather than a continuous rainbow.

Exam Tip: If an exam question mentions an atom absorbing or emitting light, always refer to electrons moving between energy levels. Do not say the atom itself "gets bigger" — the nucleus is unchanged, and it is the electrons that change energy level.

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Worked Example: Describing the Structure of a Neutral Aluminium Atom

Aluminium has atomic number 13 and mass number 27. Describe the complete structure.

Step 1: Number of each sub-atomic particle.

• Protons = 13 (equal to the atomic number).
• Electrons = 13 (equal to the protons in a neutral atom).
• Neutrons = 27 − 13 = 14.

Step 2: Arrangement of particles.

• The 13 protons and 14 neutrons are packed into a small, dense nucleus at the centre of the atom with a radius of roughly 1 x 10^-14 m. The nucleus has an overall charge of +13.
• The 13 electrons occupy energy levels around the nucleus. Filling the shells in order 2, 8, 8: first shell 2, second shell 8, third shell 3. So the electron configuration is 2, 8, 3.
• The whole atom has a radius of roughly 1 x 10^-10 m, so the nucleus is about 10,000 times smaller than the atom.

Step 3: Overall charge.

• Total nuclear charge = +13. Total electron charge = −13. Net charge = 0 (neutral).

This worked example shows the full set of skills an exam answer is likely to require: extracting data from nuclide notation, calculating the number of neutrons, writing an electron configuration, and commenting on scale and charge.

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Common Exam Mistakes

• Saying the nucleus contains electrons — electrons orbit outside the nucleus.
• Saying electrons have no mass — they have a very small mass, not zero.
• Confusing atoms and ions — an atom is neutral; an ion has gained or lost electrons.
• Saying the atom is "solid" or "full" — most of the atom is empty space.
• Giving the wrong relative charges — protons are +1, electrons are -1, neutrons are 0.

Common mistake: Students sometimes say that "atoms have shells" but then describe these shells as solid layers. Shells are not physical structures — they are energy levels. An electron in the second shell is not sitting on a surface; it occupies a region of space with a specific quantised energy. Using the correct language ("energy level" rather than "layer") is rewarded by examiners.

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Summary

• The atom has a small, dense nucleus containing protons (+1 charge, mass 1) and neutrons (0 charge, mass 1).
• Electrons (-1 charge, negligible mass) orbit the nucleus at different energy levels.
• The radius of an atom is about 1 x 10^-10 m; the radius of the nucleus is about 1 x 10^-14 m.
• In a neutral atom, the number of protons equals the number of electrons.
• Electrons fill the lowest energy levels first (2, 8, 8 pattern).
• Ions are formed when atoms gain or lose electrons, becoming electrically charged.
• Most of the atom is empty space.
• Electrons can absorb electromagnetic radiation to move to a higher energy level, or emit radiation when they fall back down.

Exam Tip: A common 4-mark question asks you to describe the structure of the atom. Structure your answer logically: start with the nucleus (protons and neutrons), then describe the electrons orbiting in shells, then state that the atom is mostly empty space and that the nucleus is very small compared to the overall atom. Include the relative charges and masses for full marks.

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Answering at different grade levels

Exam-style question (4 marks): Describe the structure of a neutral atom. Refer to the positions, relative charges, and relative masses of the sub-atomic particles.

Grade 4–5 answer: "In an atom there is a nucleus in the middle with protons and neutrons. Electrons go around it. Protons are positive, electrons are negative, neutrons have no charge. The nucleus has most of the mass."

Examiner comment: Identifies the key terms but does not give relative values for mass or charge, does not describe the relative size of the nucleus, and does not explain why the atom is neutral. This would secure about 2 of the 4 marks.

Grade 8–9 answer: "An atom has a tiny, dense nucleus at its centre with a radius of around 1 x 10^-14 m, about 10,000 times smaller than the whole atom (radius roughly 1 x 10^-10 m). The nucleus contains protons (relative charge +1, relative mass 1) and neutrons (relative charge 0, relative mass 1), so almost all the atomic mass is concentrated there. Electrons (relative charge −1, negligible mass) orbit the nucleus in fixed energy levels, filling the lowest available level first (2, 8, 8 pattern). In a neutral atom, the number of protons equals the number of electrons, so the total charge is zero. Most of the atom's volume is empty space."

Examiner comment: Uses correct terminology (nucleus, energy levels, relative mass/charge), quantifies the sizes of the atom and nucleus, explains neutrality through balancing charge, and addresses the common misconception that the atom is "full." Full marks.

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AQA alignment: This content is aligned with AQA GCSE Physics (8463) specification section 4.4 Atomic structure — specifically 4.4.1.1 The structure of an atom. Assessed on Paper 1.