Mass Spectra of Elements

Full Notes

Many elements in nature exist as mixtures of isotopes. An isotope is an atom with the same number of protons (same element) but a different number of neutrons, giving it a different mass.

ExampleChlorine gas naturally contains two isotopes - Cl-35 and Cl-37. 75% of the chlorine atoms are the Cl-35 and 25% are Cl-37. With the average relative mass of atoms in a sample being 35.5.

What Is Mass Spectrometry?

Mass spectrometry is a technique used to separate and measure the different isotopes in a sample of an element. It works by ionizing atoms, accelerating them, and then deflecting them based on their mass-to-charge ratio. There are several different types of Mass Spectrometry, however one common example is Time of Flight.

This has been explained in more detail at https://www.chemistrystudent.com/massspectrometrytof.html

Note - you don’t need to know all the details of how a mass spectrometer works however you should be comfortable with the overall idea and purpose of the technique.

Stages in a TOF Mass Spectrometer:

Ionisation

• Sample is turned into ions.
• Two methods:

• Electron impact: High-energy electrons knock out an electron → X → X⁺ + e⁻
• Electrospray ionisation: Sample is dissolved and sprayed, gaining a proton → X + H⁺ → XH⁺

Acceleration

• Ions are accelerated by an electric field so that all ions have the same kinetic energy.

Ion Drift

• Ions pass through a drift region.
• Lighter ions travel faster than heavier ions.

Detection

• Ions hit a detector and generate a current.
• Time taken is used to calculate mass-to-charge ratio (m/z).
• The bigger the current, the greater the abundance.

In AP Chemistry, we only focus on singly charged monatomic ions, meaning each peak in a spectrum represents a different isotope of the same element.

Reading a Mass Spectrum

A mass spectrum is a bar graph where:

• The x-axis shows the mass (often the mass number of each isotope).
• The y-axis shows the relative abundance of each isotope, usually as a percentage.



• Each bar represents an isotope. A taller bar means that isotope is more abundant in nature.

Matt’s Exam Tip

If you are given a mass spectrum with just bars and no percentages, you can treat the bar heights as relative values. Add the total height of all bars, then calculate each bar’s percentage as a fraction of the total.

Calculating Average Atomic Mass

The average atomic mass listed on the periodic table is not a simple average. It is a weighted average that reflects both the mass and natural abundance of each isotope.

Matt’s Exam Tip

To avoid confusion with relative atomic mass, imagine you have 100 atoms of the element. If isotope A has 75% abundance and isotope B has 25%, then: Total mass from A = (mass of A) × 75; Total mass from B = (mass of B) × 25. Add these to get the combined mass of all 100 atoms, then divide by 100 to find the average mass of one atom — which is the relative atomic mass.

Formula:

average atomic mass = (isotope 1 mass × fraction abundance) + (isotope 2 mass × fraction abundance) + …

Summary

A mass spectrum displays the isotopes of an element and their relative abundances. This data allows us to calculate the average atomic mass of the element as a weighted average of the isotope masses. This concept explains why atomic masses on the periodic table are often decimals — they reflect a natural mixture of isotopes.

Key points to remember:

• Isotopes = same element, different mass (different neutrons)
• Mass spectrum = graphical data about isotope masses and abundances
• Average atomic mass = sum of (mass × relative abundance) for each isotope divided by 100