Chemistry · GCSE · Atoms, Elements & Compounds

Mass number and isotopes

Mass number and isotopesFree4:22Heddy Score 3 of 7, below the standard

What you need to know

  • For a neutral atom, the atomic number gives the number of protons and electrons; an ion has gained or lost electrons, so its charge must be used to find its electron count.
  • The mass number of a particular isotope is the total number of protons and neutrons; subtract the proton number from that isotope’s mass number to find neutrons.
  • Relative atomic mass is a weighted average of isotope masses, so rounding it only estimates a common mass number and does not identify the mass number of every element.

In this clip

0:00–4:07The periodic table
Also coversIons — atoms with a charge
Also coversMass number and isotopes
Heddy Score3/7, Below the standard
See the seven stars
  • Trusted source · passed. The creator's own upload, live and playable here.
  • On the map · not judged yet. A teacher watched it: it mainly teaches this map topic.
  • Spec-true · not judged yet. Accurate, and right for the current UK specification.
  • Focused · passed. One idea, short enough to watch in one go.
  • Accessible · passed. Good captions, clear sound and pictures.
  • Safe for children · not judged yet. Right for 11 to 16 year olds, with no sales pitch.
  • Proven · not judged yet. Earned from our students: they learn from it, find it helpful and watch it to the end.
★ passed · ☆ pending · ✕ failed
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Our own students' answers. These are what move ★7 Proven.
On YouTube

999K views · 9.45K likes · MooMooMath and Science 642K subscribers

As reported by YouTube on 2026-11-03. Not part of the Heddy Score.
How the Heddy Score works

Why we picked it

The worked periodic-table examples practise finding proton and electron numbers and calculating neutrons when an isotope mass number is known.

MooMooMath and Science on YouTubechecked 2026-10-20

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