New
New
Year 11
OCR
Higher

Mass spectrometry

I can explain how mass spectrometry is used to identify isotopes and molecular structures, and calculate relative atomic masses.

New
New
Year 11
OCR
Higher

Mass spectrometry

I can explain how mass spectrometry is used to identify isotopes and molecular structures, and calculate relative atomic masses.

Lesson details

Key learning points

  1. Isotopes are atoms of the same element with different numbers of neutrons.
  2. Relative abundances of isotopes can be used to calculate the relative atomic mass (RAM) of an element.
  3. Mass spectrometry ionises samples to create positive ions for analysis.
  4. The M⁺ peak represents the molecular ion, indicating the molecular mass.
  5. Fragmentation in mass spectrometry helps identify molecular structure by breaking molecules into smaller ions.

Common misconception

All peaks in a mass spectrum represent intact (and neutral) molecules or atoms.

Clarify that in mass spectrometry, all detected peaks represent ionised species, not neutral molecules or atoms. The peaks can correspond to either the molecular ion (M⁺) or smaller ionised fragments, as well as different isotopes of an element.

Keywords

  • Isotope - An atom of an element with the same number of protons, but different number of neutrons.

  • Relative atomic mass - An average value that takes account of the abundance of the isotopes of the element.

  • Molecular mass - The sum of the atomic masses of all the atoms in a molecule.

  • Molecular ion - The ion produced when the molecule is ionised by loss of an electron from the molecule.

  • Spectrometer - An apparatus used for recording and measuring spectra, especially as a method of analysis.

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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6 Questions

Q1.
What do isotopes of an element have in common?
Same atomic mass
Same number of neutrons
Correct answer: Same number of protons
Q2.
Atoms form when they gain or lose electrons.
Correct Answer: ions, ion
Q3.
How do you calculate the number of neutrons in an atom?
Atomic number - Mass number
Mass number + Atomic number
Correct answer: Mass number - Atomic number
Q4.
Which subatomic particle determines the identity of an element?
Electron
Neutron
Correct answer: Proton
Q5.
What does the relative atomic mass of an element represent?
The total mass of an element's isotopes combined.
Correct answer: An average value that takes account of the abundance of the isotopes.
The mass of the most abundant isotope of an element.
The mass of one mole of the element in grams.
Q6.
What is the relative atomic mass of chlorine, given isotopes Cl-35 (75%) and Cl-37 (25%)?
Correct Answer: 35.5

6 Questions

Q1.
What does the M⁺ peak represent in a mass spectrum?
The base peak
The heaviest fragment
Correct answer: The molecular ion
Q2.
What does fragmentation in mass spectrometry help identify?
Atomic number
Electron configuration
Correct answer: Molecular structure
Q3.
How is the relative atomic mass calculated using mass spectrometry data?
Counting the total number of isotopes
Using the atomic number
Correct answer: (mass of isotope 1 × abundance) + (mass of isotope 2 × abundance)/100
Q4.
What is a molecular ion?
A molecule with an unequal number of protons and neutrons.
A molecule that has formed a covalent bond.
Correct answer: An ion produced when the molecule is ionised by loss of an electron.
A neutral molecule with no charge.
Q5.
Calculate the relative atomic mass of bromine, given isotopes Br-79 (50.7%) and Br-81 (49.3%). Give your answer to one decimal place.
Correct Answer: 79.9
Q6.
Calculate the relative atomic mass of neon, given isotopes Ne-20 (90.48%), Ne-21 (0.27%), Ne-22 (9.25%). Give your answer to two decimal places.
Correct Answer: 20.22