The nuclear atom Cambridge International AS & A Level Physics revision
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In plain words
A century ago, physicists fired alpha particles at a sheet of gold foil. Almost all of them went straight through, but a very few bounced right back. The atom, it turned out, is nearly all empty space, with a tiny, dense, charged core.
5 things to know
- Alpha-particle scattering: most alpha particles went straight through, so the atom is mostly empty space. A very few were turned through large angles, so the positive charge and nearly all the mass are packed into a tiny nucleus.
- The nuclear model: a small nucleus made of protons and neutrons (together called nucleons), with electrons moving around it.
- Nucleon number A = number of protons + number of neutrons. Proton number Z = number of protons. A nuclide is written with A above Z in front of its symbol, for example ²³⁸₉₂U.
- Isotopes are atoms of the same element, with the same number of protons but different numbers of neutrons.
- Small masses are measured in unified atomic mass units: 1 u = 1.66 × 10⁻²⁷ kg, which is about the mass of one nucleon.
Tips and tricks
- Pair each observation with what it shows. "Most went straight through" goes with "mostly empty space"; "a few came back" goes with "a tiny, dense, charged nucleus".
- Number of neutrons = A − Z.
It lands in your notebook with its questions as flashcards.
The nuclear atom: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What did the large-angle scattering of a few alpha particles show?
Only a concentrated charge could turn an alpha particle back.
A nucleus has a nucleon number of 14 and a proton number of 6. How many neutrons does it have?
14 − 6.
What are isotopes?
The same element, with a different mass.
Roughly what is the mass of one nucleon?
The unified atomic mass unit is 1.66 × 10⁻²⁷ kg.
In the nuclear model, where is most of the mass of an atom?
Protons and neutrons are each about 1800 times heavier than an electron.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
The nuclear atom
Cambridge International AS & A Level Physics 9702 · 7 marks · papermunch.org
Name ______________________________ Date ______________
State what the alpha-scattering experiment showed about the atom, giving the evidence for each point.[3]
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Most alpha particles passed straight through, so the atom is mostly empty space. A very few were deflected through large angles, so the positive charge and most of the mass are concentrated in a very small nucleus.
Find the number of protons and the number of neutrons in a nucleus of ²³⁵₉₂U.[2]
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92 protons and 143 neutrons.
State what isotopes are.[2]
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Atoms (nuclei) with the same number of protons but different numbers of neutrons.
Answers: The nuclear atom
- 1. Most alpha particles passed straight through, so the atom is mostly empty space. A very few were deflected through large angles, so the positive charge and most of the mass are concentrated in a very small nucleus.
- 2. 92 protons and 143 neutrons.
- 3. Atoms (nuclei) with the same number of protons but different numbers of neutrons.