Ch 1 · 5.1.1 The Atom · O Level Physics 5054
Topic 1 of 8 in Nuclear Physics — O Level Physics 5054 — 29 lessons.
Introduction
Everything is made of atoms, but atoms are far too small to see. So how do we know what is inside one? Over a hundred years ago, scientists fired tiny particles at a sheet of gold foil and watched where they went. A few bounced almost straight back, and that changed our picture of the atom for ever. (Syllabus section 5.1.1.)
Key ideas — The structure of the atom
An atom has:
🧑🏫 Teacher's explanation
Imagine a massive football stadium. If the entire stadium represents the whole atom, the nucleus is not the size of a football; it is actually a tiny pea sitting right in the middle of the pitch. Everything else in the stadium—the stands, the seats, the air—is just empty space where electrons fly around. This tells us that atoms are mostly nothingness!
📖 Definitions
🔍 Worked example — Easy
An atom contains 6 protons in its nucleus and 6 electrons orbiting it. Calculate the total charge of the atom in terms of elementary charge e.
🔍 Worked example — Standard
In an alpha-particle scattering experiment, 99.9\% of particles pass straight through the target. If 10,000 particles are fired, how many are expected to pass through?
🔍 Worked example — Exam-style
A student observes that alpha particles are deflected at large angles during a scattering experiment. State what this observation provides evidence for regarding the nucleus.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State what the observation that most particles pass straight through the gold sheet provides evidence for. [1] (b) State what the observation that some particles are deflected at large angles provides evidence for. [2] [3 marks]
Key ideas — The alpha-particle scattering experiment
What was done. A thin beam of alpha particles (small, fast, positively charged particles) was fired at a very thin sheet of gold foil in a vacuum. Detectors around the foil recorded where the alpha particles went.
🧑🏫 Teacher's explanation
Imagine you are throwing a handful of tiny, fast-moving marbles at a large, thin sheet of hanging laundry. If the laundry is just a light fabric, most marbles will pass straight through the gaps between the threads. However, if there is a heavy, solid lead weight sewn into the middle of the sheet, most marbles will still pass through, but a few might hit the weight and bounce off at weird angles. If a marble hits the weight dead-on, it might even fly straight back at you.
📖 Definitions
🔍 Worked example — Easy
An alpha particle is fired at a gold atom. If the atom is modeled as a solid sphere of radius 1 × 10^-10 m, and the alpha particle has a radius of 1 × 10^-15 m, why does the scattering experiment suggest the nucleus is much smaller than the atom?
🔍 Worked example — Standard
In the scattering experiment, an alpha particle is deflected by 120^°. Based on the observations, what does this specific deflection tell us about the nucleus?
🔍 Worked example — Exam-style
A student claims that the deflection of alpha particles proves the nucleus is negatively charged. Explain why this is incorrect using the concept of electrostatic forces.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the observation that provides evidence that the nucleus is positively charged. [1] (b) Explain how this observation supports the conclusion that the nucleus is positively charged. [2] [3 marks]
Worked example
Explain why only a very small fraction of alpha particles were deflected through large angles.
🔍 Worked example — Easy
An alpha particle is fired towards an atom. Most pass through undeflected. What does this suggest about the volume of the nucleus compared to the atom?
🔍 Worked example — Standard
In the Rutherford scattering experiment, some alpha particles are deflected at very large angles (greater than 90^°). Explain why this occurs.
🔍 Worked example — Exam-style
An alpha particle with mass m is deflected by a nucleus. If the deflection angle is very large, what can be concluded about the mass distribution of the atom?
⚠️ Common errors
Common mistakes
Practice
Answers: (1) A small, positively charged nucleus containing most of the mass, with negatively charged electrons orbiting it; mostly empty space. (2) They passed straight through; the atom is mostly empty space and the nucleus is very small. (3) Some alpha particles (positive) were deflected through large angles: they were repelled by a concentrated positive charge. (4) They came very close to a nucleus, which is very dense and contains most of the atom's mass, so it repelled them back.