Mass Spectrometer

A Mass Spectrometer is an instrument used to measure the masses of various isotopes present in an atom.

In 1919, a chemist, Francis. W. Aston designed an instrument called a mass spectrometer for separating ions of different masses.

• When a beam of positively charged chlorine ions was passed through a magnetic field in a mass spectrometer, two distinct beams were observed instead of one.
• One beam corresponded to mass 35 (stronger beam) and the other to mass 37 (weaker beam).
• This indicates that chlorine exists as two isotopes:
  1. Cl-35 → more abundant
  2. Cl-37 → less abundant
• The relative atomic mass of chlorine (35.5) is a weighted average of these isotopes based on their natural abundances.

Conclusion: Chlorine is not made of identical atoms — it’s a mixture of isotopes. The mass spectrometer allows us to see the different masses and relative proportions of these isotopes.