Three Mass Spectrometer Lines from One Meteorite: Element, Mixture or Two Elements?
A school laboratory receives two powdered samples taken from different parts of the same meteorite fragment. The mass spectrum of Sample A shows three peaks at mass-to-charge ratios 24, 25 and 26 with relative abundances of 79.0%, 10.0% and 11.0% respectively. Sample B, from another part of the fragment, is reported to have a relative atomic mass of 24.18. Both samples are silvery, conduct electricity, and fizz in dilute hydrochloric acid to release a gas that burns with a 'pop'. A partner in the group makes two claims: first, that a substance giving three lines in a mass spectrometer cannot be an element; second, that Samples A and B must be different elements because their relative atomic masses are different. The teacher refuses to accept either claim without calculation and asks the group to prepare a written justification before the samples are sent away for confirmatory analysis.
1. Calculate the relative atomic mass of Sample A and state, with reasoning, which sample is closer to the value printed in the Periodic Table (24.3).
Read the worked answer
Calculation
RAM = Σ(isotope mass × relative abundance) ÷ 100
= (24 × 79.0 + 25 × 10.0 + 26 × 11.0) ÷ 100
= (1896 + 250 + 286) ÷ 100 = 2432 ÷ 100 = 24.32
Comparison
Sample A (24.32) is essentially the printed value of 24.3, so its isotopic composition matches the natural mixture. Sample B (24.18) is lower, which means it contains a slightly larger fraction of the lightest isotope and less of the two heavier ones.
Reasoning to state explicitly
The Periodic Table value is a weighted average over the naturally occurring mixture, not a fixed constant for every specimen of the element. A specimen that formed under different conditions, or that has been irradiated, can carry a slightly different isotope ratio and therefore a slightly different relative atomic mass without being a different element. The value alone cannot decide identity - the proton number does.
2. Sample A gives three lines in the mass spectrometer, yet the teacher insists it is one pure element rather than a mixture. Explain how three lines are possible, and give the numbers of protons, neutrons and electrons in the ion formed when an atom of the mass-26 isotope loses two electrons.
Read the worked answer
Why three lines
A mass spectrometer separates particles by mass-to-charge ratio, not by chemical identity. Atoms of the same element with 12 protons but 12, 13 or 14 neutrons have masses of 24, 25 and 26 and therefore land at three separate positions. Three lines simply mean three isotopes.
Distinguishing isotopes from a mixture of elements
The decisive evidence is not the number of lines but the pattern: the peaks are exactly one mass unit apart, which fits successive neutrons in the same nucleus, and their abundances (79 : 10 : 11) are the fixed natural ratio for this element. A mixture of three different elements would not give peaks evenly spaced by one unit nor a reproducible abundance ratio, and the components could be separated by physical means.
Particle count for the mass-26 ion
Protons = 12 (unchanged - this is what makes it the same element)
Neutrons = 26 − 12 = 14
Electrons = 12 − 2 = 10
3. Evaluate the partner's two claims and describe one practical test that would settle the question of whether Samples A and B are the same element.
Read the worked answer
Claim 1: three lines prove it is a mixture - rejected. The three lines arise from isotopes, which are atoms of one element with different neutron numbers. Chemical identity is set by the proton number, which is the same (12) for all three peaks. The evenly spaced peaks and the reproducible 79 : 10 : 11 ratio are the signature of isotopes, not of a three-component mixture.
Claim 2: different relative atomic masses prove different elements - rejected. Relative atomic mass depends on the isotope ratio, which can vary slightly between specimens. Two samples with the same proton number but different neutron proportions will differ in relative atomic mass while remaining the same element.
What would actually settle it
Compare the atomic emission spectra of the two samples in a flame or discharge tube. Emission lines come from electron transitions, and the electron arrangement depends only on the proton number. Identical line positions confirm one element; different line positions confirm two. A simpler supporting check is that both samples must give identical chemical behaviour - same flame colour, same reaction with dilute acid, same formula of oxide and chloride. A density measurement would be unreliable, because density depends on packing and structure, not just on identity.