Revision notes for AQA GCSE Physics The structure of an atom. Open the guide for explanations and worked examples. Written against the AQA GCSE Physics (8463) specification, so the content matches what's examinable rather than general Physics background.
The structure of an atom
What you'll learn
- How small atoms are, and how to read their size in standard form.
- The basic structure of an atom: nucleus, protons, neutrons and electrons.
- Why most of an atom is empty space, but most of its mass is in the nucleus.
- How electrons can move between energy levels by absorbing or emitting electromagnetic radiation.
Starting point: what is an atom?
An atom is the smallest part of an element that still behaves like that element. For example, a single carbon atom is still carbon.
Atoms are not solid little balls. They have a tiny central region called the nucleus, with electrons arranged around it at different distances.
Definition
Atom
An atom is a very small particle made from a central nucleus surrounded by electrons.
Atoms are extremely small
The radius of an atom is about:
ratom≈1×10−10 mr_\text{atom} \approx 1 \times 10^{-10}\ \text{m}ratom≈1×10−10 mThe radius is the distance from the centre of something to its edge.
That number, 1×10−101 \times 10^{-10}1×10−10 metres, is written in standard form. Standard form is a way of writing very large or very small numbers using powers of ten.
Definition
Standard form
A number in standard form is written as a number between 1 and 10, multiplied by a power of ten, such as 1×10−101 \times 10^{-10}1×10−10.
For atoms, the power of ten is negative because the number is much smaller than 1 metre.
Example
Understanding the size of an atom
An atom has a radius of about 1×10−10 m1 \times 10^{-10}\ \text{m}1×10−10 m. What does this mean as an ordinary decimal?
- The exponent −10-10−10 means the decimal point moves 10 places to the left from 1.
- So 1×10−10 m1 \times 10^{-10}\ \text{m}1×10−10 m becomes 0.0000000001 m0.0000000001\ \text{m}0.0000000001 m.
- This shows that an atom is far too small to see with a normal microscope.
Tip
Reading powers of ten
A negative power, like 10−1010^{-10}10−10, means a very small number. A positive power, like 10610^6106, means a very large number.
The basic structure of an atom
At the centre of an atom is the nucleus. It is made of two types of particle:
- Protons, which have a positive charge.
- Neutrons, which have no charge.
Around the nucleus are electrons, which have a negative charge.
Definition
Subatomic particles
Subatomic particles are the smaller particles that make up an atom: protons, neutrons and electrons.
Here is the basic structure you need for GCSE Physics:
| Part of atom | Charge | Where it is found |
|---|---|---|
| Proton | Positive | In the nucleus |
| Neutron | No charge | In the nucleus |
| Electron | Negative | Around the nucleus |
The nucleus is positive overall because it contains positive protons and neutral neutrons. The electrons are negative, so they are attracted to the positive nucleus.
Key Idea
Basic atom structure
An atom has a tiny positive nucleus made of protons and neutrons, surrounded by negative electrons.
The nucleus is tiny compared with the atom
The nucleus is much smaller than the whole atom.
The radius of the nucleus is less than one ten-thousandth of the radius of the atom. In other words:
rnucleus<110 000ratomr_\text{nucleus} < \frac{1}{10\,000} r_\text{atom}rnucleus<100001ratomSo although diagrams show the nucleus quite large so you can see it, real atoms are mostly empty space.
Common Mistake
Taking atom diagrams too literally
Diagrams of atoms are almost always not to scale. If the nucleus were drawn to its real scale, it would be far too small to see clearly on the page.
Example
Estimating the maximum nucleus radius
An atom has radius 1×10−10 m1 \times 10^{-10}\ \text{m}1×10−10 m. The nucleus radius is less than one ten-thousandth of this. Estimate the largest possible nucleus radius.
-
Write the comparison as a calculation:
rnucleus<110 000×ratomr_\text{nucleus} < \frac{1}{10\,000} \times r_\text{atom}rnucleus<100001×ratom -
Substitute the atom radius:
rnucleus<110 000×1×10−10 mr_\text{nucleus} < \frac{1}{10\,000} \times 1 \times 10^{-10}\ \text{m}rnucleus<100001×1×10−10 m -
Dividing by 10 000 is the same as multiplying by 10−410^{-4}10−4:
rnucleus<1×10−14 mr_\text{nucleus} < 1 \times 10^{-14}\ \text{m}rnucleus<1×10−14 m -
So the nucleus radius is less than about 1×10−14 m1 \times 10^{-14}\ \text{m}1×10−14 m.
Most of the mass is in the nucleus
Even though the nucleus is tiny, it contains nearly all the mass of the atom.
This is because protons and neutrons are much more massive than electrons. Electrons do have mass, but at GCSE you usually treat their mass as very small compared with the mass of protons and neutrons.
Key Idea
Mass in an atom
Most of the atom’s mass is concentrated in the nucleus, because the nucleus contains the protons and neutrons.
This is a really important idea: atoms are mostly empty space, with a tiny, massive nucleus at the centre.
Electrons are arranged in energy levels
Electrons are not just randomly scattered around the nucleus. They are arranged at different distances from the nucleus. These distances are called energy levels.
An energy level is sometimes called an electron shell in GCSE questions.
Definition
Energy level
An energy level is a fixed allowed distance from the nucleus where electrons can exist. Higher energy levels are further from the nucleus.
Electrons closer to the nucleus are in lower energy levels. Electrons further from the nucleus are in higher energy levels.
Key Idea
Electron arrangement
Electrons occupy energy levels around the nucleus. Energy levels further from the nucleus have higher energy.
Electrons can move between energy levels
An electron can move from one energy level to another, but it must gain or lose energy to do this.
The energy is transferred by electromagnetic radiation. This is a type of wave that can carry energy, such as light, ultraviolet, infrared, X-rays and gamma rays.
Definition
Electromagnetic radiation
Electromagnetic radiation is energy transferred by electromagnetic waves, including visible light and other parts of the electromagnetic spectrum.
There are two key cases:
- If an electron absorbs electromagnetic radiation, it gains energy and moves further from the nucleus to a higher energy level.
- If an electron emits electromagnetic radiation, it loses energy and moves closer to the nucleus to a lower energy level.
Example
Deciding whether radiation is absorbed or emitted
An electron moves from an outer energy level to an inner energy level. State whether electromagnetic radiation is absorbed or emitted.
- Moving from an outer level to an inner level means the electron moves closer to the nucleus.
- Levels closer to the nucleus are lower energy levels, so the electron has lost energy.
- Lost energy is given out as electromagnetic radiation, so the radiation is emitted.
Common Mistake
Mixing up absorption and emission
If the electron moves away from the nucleus, it must absorb energy. If it moves towards the nucleus, it emits energy.
A simple way to remember it
Think of the electron like someone climbing stairs.
Analogy
Electron energy levels as stairs
To go up to a higher step, you must gain energy. To come down to a lower step, energy is released.
This is only an analogy, but it helps with the direction: higher energy levels are further from the nucleus.
What you should be able to say
By the end of this section, you should be able to describe an atom like this:
An atom has a radius of about 1×10−10 m1 \times 10^{-10}\ \text{m}1×10−10 m. It has a tiny positively charged nucleus made from protons and neutrons. The nucleus is less than one ten-thousandth of the radius of the atom, but contains most of the atom’s mass. Negatively charged electrons are arranged around the nucleus in energy levels. Electrons can move to higher energy levels by absorbing electromagnetic radiation, or to lower energy levels by emitting electromagnetic radiation.
Exam technique
In the exam
- Use the key words nucleus, protons, neutrons, electrons, energy levels, absorbed and emitted accurately.
- If a question asks about scale, say that the nucleus is tiny compared with the atom and that the atom is mostly empty space.
- For electron changes, link the direction to the energy transfer: moving out means radiation is absorbed; moving in means radiation is emitted.
Self review
Check yourself
- What particles are found in the nucleus, and what are their charges?
- Why is most of an atom’s mass in the nucleus?
- What happens to an electron when it absorbs electromagnetic radiation?
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