Evidence boundary
Use supplied isotope and spectrum data. This page is public formative synthesis.
Explicit science instruction
Mass number is an integer count for one nucleus. Isotopic mass is a measured quantity for one isotope. Periodic-table atomic mass is an abundance-weighted population average.
A spectrum can reveal more than one isotope population. Once relative abundance is estimated, a weighted average explains why the table value can fall between isotope masses.
Vocabulary: mass number; isotopic mass; abundance; weighted average; evidence chain.
Worked example with reasoning
A synthetic spectrum shows masses 63 and 65 at about 69% and 31%. The weighted average lies between 63 and 65 and closer to 63 because that isotope is more abundant.
Guided question
Write four sentences distinguishing mass number, isotopic mass, abundance, and weighted atomic mass.
Independent work
- Submit one annotated synthetic spectrum.
- Show one weighted-average calculation.
- Write a CER explaining why the average need not be an integer.
- Add one interpretation limitation.
Spiral retrieval bridge
- From Week 02: what determines elemental identity?
- For ²⁷Al³⁺, calculate neutrons and electrons.
- Why can rare data points matter when comparing models?
- From Week 01: why do extra display digits not guarantee accuracy?
Misconception check
A non-integer periodic-table mass does not describe one fractional nucleus; it summarizes a population of isotopes.
Formative understanding check
Why can two samples of the same element have slightly different weighted average masses?
Reveal worked response
The samples can contain different isotope abundances while retaining the same proton number and elemental identity.
Mentor evidence conference
Ask Archaemenes to find any place where mass number, isotope mass, or weighted atomic mass are being used as if they were the same quantity.
Teacher guidance
Foundation: Define the three mass terms with one example.
Core: Integrate spectrum evidence and a weighted calculation.
Extended: Explain how abundance shifts an average without changing elemental identity.
Return to Friday Evidence Workspace