Ch 1 · The Nuclear Model of the Atom · IGCSE Physics 0625
Topic 1 of 2 in Nuclear Physics — IGCSE Physics 0625 — 18 lessons.
Introduction
Almost all the mass of an atom is packed into a nucleus about 100 000 times smaller than the atom itself. This picture — the nuclear model — came from a surprising experiment in which a few alpha particles bounced straight back off a thin gold foil. Understanding what is inside the nucleus explains isotopes, radioactivity (next lesson), and the enormous energy released in nuclear power stations and in the Sun.
Key ideas
1. Structure of the atom. An atom has a tiny, positively charged nucleus at its centre, surrounded by negatively charged electrons in orbit around it. A neutral atom has equal numbers of protons and electrons, so the charges cancel.
🧑🏫 Teacher's explanation
Imagine an atom like a massive football stadium where the entire mass of the stadium is concentrated into a single marble sitting on the center pitch. Most of the stadium is just empty space where fans (electrons) fly around. If you fired tiny, fast-moving marbles (alpha particles) at the stadium, most would fly straight through the empty stands without hitting anything. However, if a marble hits that tiny, heavy marble in the center, it will bounce off violently. If that center marble has a positive charge, it might even push the incoming marbles away without them even touching it.
📖 Definitions
🔍 Worked example — Easy
An atom of Carbon has a nucleon number of 14 and a proton number of 6. Calculate the number of neutrons in this atom.
🔍 Worked example — Standard
Complete the following nuclide equation for the fission of Uranium-235: ^235_92U + ^1_0n arrow ^141_56Ba + ^A_ZX + 3\^1_0n\
🔍 Worked example — Exam-style
In a star, two hydrogen nuclei fuse to form a helium nucleus and a neutron. Write the balanced nuclide equation for this fusion reaction.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the number of protons and neutrons in this nucleus. (b) Explain why this atom has the same chemical properties as ^12_6C. (c) Describe how the alpha-particle scattering experiment provided evidence that the nucleus is positively charged. [4 marks]
Worked example
Example 1 — reading nuclide notation. For ^56_26Fe, state the numbers of protons, neutrons and electrons in (a) a neutral atom, (b) an Fe^3+ ion.
🔍 Worked example — Easy
An atom of Sodium is represented by the nuclide notation ^23_11Na. Calculate the number of protons, neutrons, and electrons in a neutral Sodium atom.
🔍 Worked example — Standard
A Magnesium ion has the symbol Mg^2+. Using the notation ^24_12Mg, determine the number of protons, neutrons, and electrons in this ion.
⚠️ Common errors
Common mistakes and examiner tips
Practice
Answers: (1) Proton +1, neutron 0, electron −1. (2) 13 protons, 27 - 13 = 14 neutrons, 13 electrons. (3) Same: 92 protons. Different: U-235 has 143 neutrons, U-238 has 146 neutrons. (4) +2 (a Mg^2+ ion). (5) Fission splits a heavy nucleus into two smaller nuclei (nuclear power stations); fusion joins light nuclei into a heavier nucleus (the Sun and stars). Both release energy. (6) A very small number of alpha particles were deflected back through large angles (more than 90°). (7) A neutron, ^1_0n: top 2 + 2 = 3 + 1, bottom 1 + 1 = 2 + 0. (8) 236 = 139 + 95 + x, so x = 2; check 92 = 54 + 38. (9) The total mass of the products is slightly less than the mass of the uranium nucleus and neutron; the lost mass is released as energy, mostly as kinetic energy of the fission products.