KS4 Science · Chemistry · Year 10
Atomic structure
Protons, neutrons and electrons — and how to read the periodic table.
About 25 minutes
Step 1 · Wonder why it matters
Hook
Mostly empty space
If an atom were the size of a football stadium, its nucleus would be about the size of a pea on the centre spot. Almost everything else is empty space.
Yet that tiny nucleus holds nearly all of the atom's mass. Let's see what it is made of.
Step 2 · See it
Picture
See the particles
Atoms are made of three subatomic particles.
| Particle | Where | Relative charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Shells around the nucleus | −1 | Very small (about 1/1840) |
Step 3 · Understand how it works
Logic
Reading the periodic table
Each element has two numbers. The atomic number (smaller) is the number of protons. The mass number (larger) is protons + neutrons.
Atoms are neutral, so the number of electrons equals the number of protons.
Number of neutrons = mass number − atomic number.
Electrons fill shells from the inside out: up to 2 in the first shell, then up to 8 in the second and 8 in the third (for the first 20 elements).
- Atomic number
- The number of protons in an atom.
- Mass number
- The total number of protons and neutrons.
- Isotopes
- Atoms of the same element with different numbers of neutrons.
Worked example
Sodium has atomic number 11 and mass number 23. Find its protons, neutrons, electrons and electron configuration.
- 1Protons = atomic number = 11.
- 2Electrons = protons = 11.
- 3Neutrons = 23 − 11 = 12.
- 4Fill shells: 2 in the first, 8 in the second, 1 in the third.
Answer: 11 protons, 12 neutrons, 11 electrons; configuration 2, 8, 1
Step 4 · Have a go
Do
Have a go
Use a periodic table (search “periodic table” if you do not have one).
- 1For carbon (6, 12), find protons, neutrons and electrons.
- 2For chlorine (17, 35), do the same and write its electron configuration.
- 3Draw a dot-and-cross shell diagram for oxygen (8, 16).
- 4Chlorine also exists as chlorine-37. How is it different from chlorine-35? What is the word for this?
Step 5 · Remember it
Recall
Quick quiz
Have a periodic table handy.
0 of 5 answered
1.Which particles are found in the nucleus?
2.An atom has atomic number 13 and mass number 27. How many neutrons?
3.What is the electron configuration of magnesium (atomic number 12)?
4.Isotopes of an element have different numbers of…
5.What is the relative charge of an electron?
Step 6 · Link it to your world
Reflect
Think about it
Our model of the atom changed many times as scientists collected new evidence.
Why do you think scientific models change over time? Is a model “wrong” if it gets replaced?
Saved to your notebook on this device only.
Finished all six steps?
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