Isotopes & Mass Spectrometry

You’ll be able to

Distinguish atomic number, mass number, and isotopes
Compute average atomic mass from isotopic abundances
Read a mass spectrum to identify isotopes and their abundances

Same element, different mass

An element is defined by its number of protons (its atomic number, Z). But the number of neutrons can vary, producing isotopes — atoms of the same element with different mass numbers (A = protons + neutrons). Carbon-12 and carbon-13 are both carbon; they just carry a different neutron count.

Why atomic masses are decimals

The periodic table lists carbon as 12.011, not a whole number, because that value is a weighted average of every naturally occurring isotope, weighted by how common each one is. A rare heavy isotope nudges the average up only slightly.

Average atomic mass

avg mass = Σ (isotope mass × fractional abundance)

Fractional abundance is the percentage written as a decimal (e.g. 75% → 0.75).

Worked example

Chlorine is 75.8% ³⁵Cl (34.97 u) and 24.2% ³⁷Cl (36.97 u). Find its average atomic mass.

1.Convert percentages to fractions: 0.758 and 0.242.
2.Weight each isotope: (0.758 × 34.97) + (0.242 × 36.97).
3.= 26.51 + 8.95.

Answer: 35.46 u — matching the periodic-table value for chlorine

Checkpoint

An element has two isotopes: mass 10 (20% abundant) and mass 11 (80% abundant). Its average atomic mass is closest to:

Reading a mass spectrum

A mass spectrometer ionizes atoms, accelerates them, and bends their paths with a magnetic field — lighter ions bend more. The detector produces a spectrum: each peak is an isotope, its horizontal position is the mass, and its height is the relative abundance. Tall peak = common isotope.

Checkpoint

A mass spectrum shows a tall peak at mass 63 and a shorter peak at mass 65. What can you conclude?

Tip

Sanity check every average-mass answer: it must land between the lightest and heaviest isotope, and closer to the more abundant one. If it doesn’t, you flipped an abundance.

Answer the 2 checkpoints as you read.