H3 Chemistry 9813 · Study focus: H3 Chemistry: Interpret M+1, M+2 and M+4 isotope patterns
H3 Chemistry: Interpret M+1, M+2 and M+4 isotope patterns
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Interpret M+1, M+2 and M+4 isotope patterns
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Read one-halogen signatures
One chlorine gives M:M+2 about3:1; one bromine gives about1:1.
The two-unit spacing follows isotope mass difference.
Build two-halogen patterns
Two chlorine atoms give about9:6:1 at M,M+2,M+4; two bromines give1:2:1.
Binomial combinations count zero, one or two heavy isotopes.
Natural 13C produces M+1; its relative size grows approximately with carbon count.
M+1 is distinct from halogen M+2 evidence.
| Case | Positions | Ratio | Isotope combinations |
|---|---|---|---|
| 1 Cl | M, M+2 | 3:1 | 35Cl; 37Cl |
| 1 Br | M, M+2 | 1:1 | 79Br; 81Br |
| 2 Cl | M, M+2, M+4 | 9:6:1 | 35/35; 35/37 twice; 37/37 |
| 2 Br | M, M+2, M+4 | 1:2:1 | 79/79; 79/81 twice; 81/81 |
| M+1 | M+1 | ≈1.1% × C count | one 13C |
Text alternative: Text lists positions, ratios, isotope combinations and counts.
Interpret M+1
Natural 13C produces M+1; its relative size grows approximately with carbon count.
M+1 is distinct from halogen M+2 evidence.
Count one chlorine
A cluster112:114=3:1 supports one Cl atom.
It does not support one Br, whose pair is nearly equal.
- Assign the halogen.
Open the feedback checkpoint after attempting
- Credit spacing and ratio.
Count two chlorines
A 9:6:1 triplet at M/M+2/M+4 supports two Cl atoms.
The middle member contains one35Cl and one37Cl, with two arrangements.
- Explain 6 coefficient.
Open the feedback checkpoint after attempting
- Credit binomial abundance/combinatorics.
Count two bromines
A 1:2:1 triplet supports two Br atoms.
Equal natural Br abundances yield coefficients1,2,1.
- Assign exact count.
Open the feedback checkpoint after attempting
- Credit two Br, not one.
Repair M+2 absolutism
M+2 evidence must use ratios and formula context; Cl, Br and other isotopes can contribute.
One M+2 stick alone cannot establish exact identity.
- Repair: ‘any M+2 proves chlorine.’
Open the feedback checkpoint after attempting
- Require ratio/cluster context.
Combine carbon and halogen evidence
A 3:1 M/M+2 pair plus M+1 about6.6% supports one Cl and roughly six carbons.
Next: fragments at /learning/h3-mass-spectrometry-fragment-ions-lesson.html.
- Interpret both features.
Open the feedback checkpoint after attempting
- Credit independent halogen and carbon inferences.