1.2: Mass Spectroscopy and Elements

Learning Targets/ Content Objectives:

I can explain the quantitative relationship between the mass spectrum of an element and the masses of the element's isotopes and their corresponding percentage abundance.
I can use the periodic table to determine the identity of an element given its mass spectrum.
I can calculate the average atomic mass of an element using data represented by the mass spectrum of that element.

Vocabulary:

Isotopes - relative abundance - percentage abundance - atomic mass - atomic number mass spectrometry - mass spectrum - average atomic mass.

Relative abundance and average atomic mass

We already know how that many elements have different isotopes and that isotopes have different atomic masses, but they have the same atomic number as the element. Isotopes can be detected using a spectrometer. A spectrometer can give information about how many different isotopes are in a sample with their percent abundances. It is a powerful technique that is used to determine the number of isotopes and their abundance in a given sample.

Mass spectrometry

We now know how to find average atomic masses by calculating weighted averages from atomic masses and relative abundances of isotopes. But where do those relative abundances come from? For example, how do we know the relative abundances of isotopes of an atoms? Relative abundance of isotopes can be determined experimentally using a technique called mass spectrometry.

How does a mass spectroscopy work?

Watch this animation video to see how a mass spectrometer work

A video that explains how a mass spectrometer works.

How does a mass spectrum look like?

Mass spectrum of Molybdenum

In a mass spectrum the vertical axis (y-axis) is labelled as either "relative abundance" or "relative intensity" and it's related to the number of ions arriving at the detector: the greater the current, the more abundant the ion.

As you will see from the diagram, the most common ion has a mass/charge ratio of 98. Other ions have mass/charge ratios of 92, 94, 95, 96, 97 and 100.

There is a total of 7 peaks in the spectrum and that means that molybdenum consists of 7 different isotopes. Assuming that the ions all have a charge of 1+, that means that the masses of the 7 isotopes on the carbon-12 scale are 92, 94, 95, 96, 97, 98 and 100.

Mass spectrum of Zirconium

A mass spectrum of Zirconium shows 5 peaks which means that Zirconium has 5 isotopes with respective masses of 90, 91, 92, 94 and 95. the most abundant isotope is the one with the biggest percent abundance and that is the isotope with atomic mass equals to 90 amu.

Check for Understanding

Estimating the average atomic mass of Copper

The mass spectrum shows that Copper-63 is more abundant than Copper 65 and that tells us that the average atomic mass of Copper should be closer to 63 amu than 65 amu

How many isotopes does Copper have?

Copper has two isotopes since its mass spectrum show two peaks.

2. What is the average atomic mass of Copper?

The average atomic of Copper is: 63(0.69)+65(0.31)= 63.62 amu

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