The Structure of the Atom

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1. Key words

2. Atoms are made of smaller particles

In Stage 7 you learned that every substance is made of particles, and that an element is made of only one type of atom. Atoms are far too small to see, even with the best light microscope — a line of about 10 million atoms would be only 1 mm long. But scientists found that atoms themselves are made of even smaller particles: protons, neutrons and electrons.

An atom has the same number of protons and electrons. Each proton has one positive charge and each electron has one negative charge, so the charges cancel. This is why an atom is neutral — it has no overall charge. Neutrons have no charge, so the number of neutrons makes no difference to the overall charge.

3. The Rutherford model of the atom

The picture of the atom used today at this stage is the Rutherford model (also called the nuclear model). It has four key ideas you must be able to describe:

1. At the centre is a tiny nucleus. It is positive because it contains the protons. It also contains the neutrons.
2. Almost all the mass of the atom is in the nucleus.
3. The negative electrons move around the nucleus, a long way from it.
4. Most of the atom is empty space.

How small is the nucleus? If an atom were blown up to the size of a football stadium, the nucleus would be about the size of a pea on the centre spot, and the electrons would be like specks of dust flying around the outer seats. Everything in between is empty space. This comparison is an analogy — it helps you picture something you can never see.

4. How the model was discovered

Models in science change when new evidence is found. The story of the atom is a famous example.

5. The gold foil experiment — evidence for the nucleus

Alpha particles are tiny, fast-moving particles with a positive charge. Geiger and Marsden fired them at a sheet of gold foil only a few hundred atoms thick, and recorded where they went. If the plum pudding model were right, the positive charge would be spread thinly everywhere, so all the alpha particles should go straight through or bend only very slightly. That is not what happened.

6. What holds an atom together?

You already know from magnets and static electricity that opposite charges attract and like charges repel. The same rule works inside atoms.

• The nucleus is positive (it contains protons). The electrons are negative.
• Opposite charges attract, so every electron is pulled towards the nucleus. This pull is called electrostatic attraction.
• Electrostatic attraction is what holds the atom together: it keeps the electrons in the atom, moving around the nucleus, instead of flying away.
• Neutrons have no charge, so they are not attracted to the electrons. (A different, very strong force holds the protons and neutrons together inside the nucleus — you will meet it in later years.)

7. Models and analogies

Nobody has ever seen a proton or an electron directly, so scientists use models. An analogy is a kind of model that compares the atom with something familiar. Every analogy has strengths and weaknesses — Checkpoint can ask for both.

8. Worked examples

Worked example 1 — Complete the table about the particles in an atom. [3]

1. Row 1: proton → positive (+).
2. Row 2: the particle with no charge is the neutron; it is found in the nucleus.
3. Row 3: electron → negative (−), moving around the nucleus (outside the nucleus).
4. Marking: usually 1 mark per correct row. Common mistake: writing "neutral" for the electron, or putting electrons inside the nucleus.

Worked example 2 — An atom of boron has 5 protons and 6 neutrons. (a) How many electrons does it have? [1] (b) How many particles are in the nucleus? [1] (c) Explain why the atom has no overall charge. [1]

1. (a) Atoms are neutral, so electrons = protons = 5.
2. (b) Nucleus = protons + neutrons = 5 + 6 = 11 particles.
3. (c) The 5 positive charges of the protons are cancelled by the 5 negative charges of the electrons; neutrons have no charge.
4. Common mistake: in (a), giving 6 (the number of neutrons) — the electron number always matches the proton number in an atom.

Worked example 3 — In the gold foil experiment, a few alpha particles were deflected. Alpha particles are positive. What does this tell you about the atom? [2]

1. There is a positive charge inside the atom (1)…
2. …which repels the positive alpha particles, because like charges repel (1).
3. Common mistake: "they hit electrons". Electrons are far too light to push an alpha particle off course — it was the positive nucleus.

Worked example 4 — Explain what holds the electrons in an atom. Use ideas about charge in your answer. [2]

1. The nucleus is positive (it contains protons) and the electrons are negative (1).
2. Opposite charges attract, so there is an electrostatic attraction that pulls the electrons towards the nucleus and keeps them in the atom (1).
3. Exam tip: name both charges and use the word attract/attraction. "Gravity" or "the neutrons hold them" scores 0.

Worked example 5 — A teacher says: "If an atom were the size of a football stadium, the nucleus would be the size of a pea in the middle." (a) What does this analogy show about the nucleus? [1] (b) Give one way in which the analogy is not like a real atom. [1]

1. (a) The nucleus is very small compared with the whole atom (the atom is mostly empty space).
2. (b) Any one: a stadium is full of solid seats and air, but an atom is mostly empty space / a pea has no charge but the nucleus is positive / the pea does not hold most of the mass of the stadium.
3. Exam tip: a question about an analogy wants a comparison with the real thing — always mention the real atom in your answer.

Worked example 6 — An atom has 8 protons, 8 neutrons and 8 electrons. It gains 2 extra electrons. Is it still neutral? Explain. [2]

1. No (1).
2. It now has 10 negative electrons but only 8 positive protons — there are 2 more negative charges, so its overall charge is negative (−2) (1).
3. (A charged particle like this is called an ion. You will study ions in later years; at Stage 8 you just need to count the charges.)

Practice

All pages in this course

Start here

• Stage 8 Science: The Tests and How to Prepare
• Thinking and Working Scientifically: Models, Investigations and Analysing Results

Biology

• Joints and Antagonistic Muscles
• Blood and Its Components
• Breathing, Gas Exchange and Diffusion in the Lungs
• A Balanced Diet, Energy Stores and Lifestyle
• Aerobic Respiration
• Ecosystems, Bioaccumulation and Invasive Species

Chemistry

• The Structure of the Atom
• Purity, Concentration and Chromatography
• Word Equations, Exothermic and Endothermic Reactions
• The Reactivity of Metals
• Impure Products and Solubility of Salts

Physics

• Speed and Distance–Time Graphs
• Balanced and Unbalanced Forces, and Moments
• Pressure, Particles and Diffusion
• Reflection and Refraction
• Dispersion and Colour
• Magnetic Fields and Electromagnets

Earth and space

• The Earth's Magnetic Field and Earth's Resources
• Climate, Weather and Climate Change
• Galaxies and Asteroids

Practice tests

• Stage 8 Science — Practice Test 1
• Stage 8 Science — Practice Test 2
• Stage 8 Science — Practice Test 3