All subjects · Chemistry · WCH11 (Unit 1)

Atoms, isotopes and mass spectrometry

Atoms of the same element can have different masses. A mass spectrometer separates them, and its results let you calculate relative atomic mass.

Particles and isotopes

Protons (relative mass 1, charge +1) and neutrons (1, 0) are in the nucleus; electrons (1/1836, charge -1) surround it. Atomic number Z = protons; mass number A = protons + neutrons. Isotopes have the same number of protons but different numbers of neutrons, so the same chemistry but different masses. In ions, electrons = protons - charge.

Relative masses

Relative atomic mass, Ar, is the weighted mean mass of an atom of an element compared with 1/12 of the mass of a carbon-12 atom. Relative isotopic mass is the mass of one isotope on the same scale. Relative molecular mass (Mr) applies to molecules, relative formula mass to ionic compounds.

Mass spectrometry

In a mass spectrometer, particles are ionised to positive ions, accelerated, separated by mass-to-charge ratio (m/z) and detected. Each peak's height gives relative abundance.

Ar = sum of (isotopic mass x abundance) / total abundance.

A 2+ ion appears at half its mass. The peak with the largest m/z for a molecule (the molecular ion, M+) gives its Mr.

Diatomic molecules

Chlorine has isotopes 35Cl (75%) and 37Cl (25%). Cl2 molecules give peaks at m/z 70 (35-35), 72 (35-37) and 74 (37-37) in the ratio 9 : 6 : 1, from 0.75 x 0.75, 2 x 0.75 x 0.25 and 0.25 x 0.25.

Worked example

Copper is 69.2% copper-63 and 30.8% copper-65. Find Ar(Cu).

  1. Ar = (63 x 69.2 + 65 x 30.8) / 100
  2. = (4359.6 + 2002.0) / 100

Answer: Ar = 63.6

Key idea: Isotopes: same protons, different neutrons. Ar = weighted mean of isotopic masses. Cl2 peaks 70 : 72 : 74 in ratio 9 : 6 : 1.

Check yourself

  1. Magnesium is 79.0% 24Mg, 10.0% 25Mg and 11.0% 26Mg. Find Ar(Mg).
    Show answer

    24.3. (1896 + 250 + 286) / 100 = 24.32, so 24.3.

  2. How many neutrons are in an atom of 37Cl? (Z = 17)
    Show answer

    20. Neutrons = A - Z = 37 - 17 = 20.

  3. How many electrons are in the ion 32S2-? (Z = 16)
    Show answer

    18. A 2- ion has gained 2 electrons: 16 + 2 = 18.

  4. Which m/z values show peaks for Cl2+ ions from chlorine containing 35Cl and 37Cl?
    • 70, 72 and 74
    • 35 and 37 only
    • 70 and 74 only
    • 35, 36 and 37
    Show answer

    70, 72 and 74. Possible pairs: 35+35, 35+37 and 37+37.

  5. What is the ratio of the heights of the peaks at m/z 70, 72 and 74 for Cl2?
    • 3 : 1 : 0
    • 1 : 2 : 1
    • 9 : 6 : 1
    • 3 : 2 : 1
    Show answer

    9 : 6 : 1. 0.752 : 2(0.75)(0.25) : 0.252 = 0.5625 : 0.375 : 0.0625 = 9 : 6 : 1.

  6. A 2+ ion with mass 40 would appear at which m/z value?
    • 20
    • 40
    • 80
    • 42
    Show answer

    20. m/z = 40 / 2 = 20.

  7. Why do isotopes of an element have the same chemical properties?
    • They have the same number of neutrons
    • They have the same mass
    • They have the same electron configuration
    • They have the same number of nucleons
    Show answer

    They have the same electron configuration. Chemistry depends on electrons, and isotopes have the same number and arrangement of electrons.

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Last updated 7 November 2026. Spotted a mistake? Tell us.