Protons, neutrons, electrons and isotopes
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 4 practice questions, 3 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Same element, different weights
- Subatomic particles
- Counting particles
Read the transcript of the teaching part
Last time
Before we start: last time, in Ionisation energies, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Same element, different weights
Chlorine gas from a cylinder is a mixture of two kinds of atom. They behave identically in every reaction, yet one is heavier than the other. The reason lies in the nucleus, and it explains why the relative atomic mass of chlorine is thirty-five point five, and not a whole number.
Subatomic particles
A proton has relative mass one and relative charge plus one. A neutron has relative mass one and no charge. An electron has a relative mass of about one eighteen-hundredth, and charge minus one. Its mass is negligible. Nearly all the mass of an atom is in the tiny, dense nucleus. Electrons occupy the space around it. The proton number Z is the number of protons. The nucleon number A is protons plus neutrons.
Counting particles
Manganese-55 has proton number twenty-five. So it has 25 protons. Neutrons: 55 minus 25, which is 30. In a neutral atom, the number of electrons equals the number of protons: 25. For an ion, adjust the electrons. A positive ion has lost electrons; a negative ion has gained them.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Protons, neutrons and electrons.
The whole chapter: Atomic Structure, Atomic Structure.
Chapter 1: Atomic structure
- Protons, neutrons, electrons and isotopes 6 min
- Electronic configuration, orbitals and atomic radius 6 min
- Ionisation energies 7 min
Also for A Level Chemistry 9701
Which topics come up most in A Level Chemistry 9701 past papers, counted from every paper since 2004: Paper 2 topic frequency, Paper 4 topic frequency.