Atoms — Protons, Neutrons, Electrons
In 1911, Ernest Rutherford fired 8,000 alpha particles at a gold foil just 0.00004 cm thick and found 1 in 8,000 bounced straight back — revealing that atoms are mostly empty space around a tiny, dense nucleus.
Printable Worksheets
Print or save as PDF — or build a custom worksheet from any module's questions.
Q1 · Without looking anything up, sketch what you think an atom looks like. Label any parts you've heard of.
Q2 · A piece of gold and a balloon both contain atoms. Why do they behave so differently if they are made of the same kind of building blocks?
● Know
- The three sub-atomic particles: proton, neutron, electron
- The charge and relative mass of each particle
- Where each particle sits inside an atom
● Understand
- Why almost all of an atom's mass is in the nucleus
- Why atoms are mostly empty space
- How the 2–8–8 shell rule predicts where electrons sit
● Can do
- Label a Bohr-style shell diagram of a small atom
- Use the 2–8–8 rule to fill the first three shells
- State the charge of a whole neutral atom
- Proton
- Neutron
- Electron
- Nucleus
- Atom
- Negative particle that orbits in shells
- Positive particle in the nucleus
- Smallest particle of an element
- Dense centre of the atom
- Particle with no charge in the nucleus
Imagine a marble (the nucleus) placed at the centre of the Melbourne Cricket Ground (the atom). The MCG's outer fence is where the electrons orbit — and in between there is nothing but empty space. Every atom is built from just 3 kinds of particle, all packed or orbiting in this vast emptiness. Each has a different charge and a different mass:
| Particle | Charge | Relative mass | Where it sits |
|---|---|---|---|
| Proton | +1 | 1 | In the nucleus |
| Neutron | 0 | 1 | In the nucleus |
| Electron | −1 | ≈ 0 (about 1/2000) | Orbiting the nucleus in shells |
The protons and neutrons stay packed together in a tiny region in the middle called the nucleus. Because each one has a mass of 1 and electrons have almost zero mass, nearly all the mass of an atom is in the nucleus.
The electrons whizz around the nucleus in fixed paths called shells.
A has charge +1 and sits in the . A has no charge. An has charge 1 and orbits in shells.
Wrong: "An atom is a solid little ball." Atoms are mostly empty space. If the nucleus were a pea in the middle of a football stadium, the electrons would be tiny dots whizzing around the seats.
Right: An atom is mostly empty space. The nucleus is tiny compared to the whole atom.
Wrong: "Electrons have the same mass as protons." Electrons are about 2000 times lighter than protons. That's why we say their mass is almost zero.
Right: Electrons are extremely light — almost all the mass of an atom is in the nucleus.
Wrong: "The neutron has a small negative charge." Neutrons have no charge at all (0). The name comes from "neutral".
Right: Neutron = neutral. Charge of 0.
Electrons don't fly around the nucleus randomly. They sit in fixed paths called shells. Each shell can only hold a certain number of electrons.
| Shell | Max electrons |
|---|---|
| 1st (innermost) | 2 |
| 2nd | 8 |
| 3rd | 8 (for the first 20 elements) |
So the first three shells follow the 2–8–8 rule. Electrons fill the inner shell first, then move outward. For example:
- Carbon has 6 electrons → 2 in shell 1, 4 in shell 2 → 2, 4.
- Sodium has 11 electrons → 2, 8, 1.
- Calcium has 20 electrons → 2, 8, 8, 2.
In a normal, neutral atom, the number of protons (+) always equals the number of electrons (−). The positives and negatives cancel out and the whole atom has a charge of 0.
- Carbon: 6 protons, 6 electrons → net charge 0.
- Oxygen: 8 protons, 8 electrons → net charge 0.
- Sodium: 11 protons, 11 electrons → net charge 0.
Neutrons don't change the charge at all because they are neutral. They only add to the mass.
A Bohr-style shell diagram shows the nucleus in the middle and circles around it for each shell. Dots on the circles represent electrons.
To draw one for any element with ≤ 20 electrons:
- Write the number of protons and neutrons inside the nucleus circle.
- Draw a circle around the nucleus and put up to 2 electron dots on it.
- Draw a second circle outside and put up to 8 electron dots on it.
- Draw a third circle if needed and put up to 8 electron dots on it.
Worked example — Carbon (6 protons, 6 neutrons, 6 electrons): Inner shell holds 2 electrons; second shell holds the remaining 4. Pattern = 2, 4.
Try it for oxygen (8 electrons) and sodium (11 electrons) in your book.
A magnesium atom has 12 protons, 12 neutrons and 12 electrons. Predict: (a) the overall charge of the atom, and (b) how the 12 electrons fill the shells using the 2–8–8 rule.
How close was your prediction?
Nice — you correctly applied the 2–8–8 rule.
Good — practice filling shells from the inside out one more time.
At the start of this lesson you were told: If an atom were the size of a football stadium, the nucleus would be smaller than a pea — and the rest is empty space. Did that surprise you?
Now draw an updated atom diagram in your book (label the nucleus, protons, neutrons and electron shells), then write below: how does the football-stadium comparison help explain the mostly-empty structure of an atom?
Q1. Complete a small table showing the charge, relative mass and location of the proton, neutron and electron. (3 marks)
Q2. A neutral atom has 11 protons. State its electron arrangement using the 2–8–8 rule and explain how you worked it out. (4 marks)
Q3. Why is it more accurate to say "an atom is mostly empty space" than "an atom is a tiny solid ball"? Refer to the size of the nucleus, the position of the electrons and the relative masses. (4 marks)
Answers
▾MCQ 1
B — The neutron is in the nucleus and has no charge. Protons are also in the nucleus but have +1 charge.
MCQ 2
D — Protons and neutrons each have a mass of 1; electrons are ~1/2000. Almost all the mass is in the nucleus.
MCQ 3
A — Fill from the inside out: 2 in shell 1, 8 in shell 2, leaves 4 for shell 3 → 2, 8, 4 (this is silicon).
MCQ 4
C — A neutral atom has equal numbers of protons (+) and electrons (−), so the charges cancel out and the overall charge is zero.
MCQ 5
B — Electrons have charge −1 and very little mass (about 1/2000 of a proton). They orbit in shells, not the nucleus. The first shell holds 2, not 8.
Short Answer 1
Model answer: Proton: charge +1, relative mass 1, in the nucleus. Neutron: charge 0, relative mass 1, in the nucleus. Electron: charge −1, mass ≈ 0 (about 1/2000), in shells around the nucleus.
Short Answer 2
Model answer: A neutral atom has equal protons and electrons, so there are 11 electrons. Filling shells: 2 in shell 1 (9 left), 8 in shell 2 (1 left), 1 in shell 3. Arrangement = 2, 8, 1. This is sodium.
Short Answer 3
Model answer: Nearly all the mass of the atom is packed into the nucleus, which is extremely small — if the atom were the size of a football stadium, the nucleus would be smaller than a pea. The electrons are tiny and they orbit in shells far from the nucleus, with empty space between them and the nucleus. Since the volume of the nucleus is a tiny fraction of the atom's volume and the rest is just electrons (almost zero mass) moving in mostly empty space, "mostly empty space" describes an atom much more accurately than "a tiny solid ball".