1 · When does the average settle?
Run chlorine. Record after 10, 100 and 1,000 atoms. How close is each average to 35.45?
AP Chemistry · Topic 1.2 · Mass spectra of elements
Fire atoms of an element through a spectrometer, watch its spectrum build, and see where the periodic table's mass comes from. Then change the mix, name mystery elements, and draw spectra and check each peak against its abundance.
| # | Mode | Element | Atoms or mix | Average (amu) | Table (amu) |
|---|
Record adds the current run or mix. Copy puts the table on the clipboard for a spreadsheet.
Run chlorine. Record after 10, 100 and 1,000 atoms. How close is each average to 35.45?
Mix chlorine at 100%, 75.76% and 50% chlorine-35. Where is the average each time? Is any atom at that mass?
Run argon, then potassium. Argon has fewer protons. Why is its average mass larger?
Draw neon's spectrum. Neon-21 is only 0.27% of the atoms. Where does it go, and how tall?
Bromine gas shows peaks near 80 and near 160. Which particles make each group? Use the 158, 160 and 162 peaks to find % bromine-79.
About the model. The instrument is a schematic: atoms are ionized to 1+ ions, sped up, and bent by a magnet so that lighter ions curve more; each mass lands at its own detector. The gap between paths is exaggerated, and how the instrument works is not assessed on the AP exam. Each atom fired is drawn at random with the element's natural abundances, so a small sample's average wanders and a large one settles on the periodic-table value.
Isotopic masses and abundances are the NIST representative values used in the Topic 1.2 deck; every element's weighted average matches the AP periodic table to 0.01 amu. Spectra here show one element's singly charged atomic ions only; spectra of compounds or mixtures of elements are not assessed. "Draw it" scores a drawing 1 point when every peak is drawn at its mass (a tiny peak still needs a mark; any small height counts) with a height within 1.5 percentage points or 10% of its abundance. That tolerance is this sim's practice criterion, not an AP rule; released scoring guides ask for peaks at the right masses with heights in the right ratio. "Whole spectrum" goes beyond the AP exam: in chlorine gas and bromine gas the instrument also sees X₂⁺ molecules. Their peak heights follow from pairing atoms at random (light + light, light + heavy two ways, heavy + heavy) and are scaled so the tallest peak is 100; the atom peaks and bromine's small HBr⁺ peaks at 80 and 82 are schematic sizes, after the shape of the NIST electron-ionization spectra.
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