Mass Spectrum of an Element
- The mass and relative abundance of each isotope of an element in a sample can be determined using a mass spectrometer
- A mass spectrometer provides this information in the form of a mass spectrum
Sample mass spectrum
The mass spectrum of chlorine with peaks for the isotopes chlorine-35 and chlorine-37 have abundances of 25% and 75% respectively
- The average atomic mass can be estimated and the identity of an element determined using the approximate isotopic mass and abundance from a mass spectrum
Worked example
Based on the mass spectrum of a pure element shown above, what is the most likely identity of the element?
- Cu
- Zn
- Ga
- Ni
Answer:
- The average atomic mass of an element must fall between the masses of its lightest and heaviest isotopes
- The element cannot be Ni (58.69 amu)
- The average atomic mass will most likely be close in value to the mass of the isotope with the highest abundance
- The element cannot be Ga (69.72 amu)
- The average atomic mass of the element will be near 64 amu, but the element also has isotopes of significant abundance with masses of 66 and 68
- This will skew the average atomic mass to be greater than 64 amu
- So, the element is Zn (65.38 amu), not Cu (63.55 amu)
Exam Tip
Note that some textbooks will use the terms relative abundance and percent abundance interchangeably.
