Unit 1: Topic 2 - Mass Spectroscopy of Elements Flashcards
What are isotopes?
Isotopes are variants of the same element with different numbers of neutrons in their nuclei. While these atoms have the same atomic number (the number of protons in an atom’s nucleus), they have different atomic masses.
What is mass spectrometry, and what is its purpose?
Mass spectrometry is an analytical technique that can determine the atomic mass of an element by providing each isotope’s relative abundance or the percentage of atoms with that specific atomic mass found in a naturally occurring sample of the element.
The results are displayed in a mass spectrum where the x-axis represents the atomic mass of each isotope, and the y-axis represents the relative abundance. As a general rule, the higher the relative abundance of a certain isotope, the closer the average atomic mass will be to that specific atomic mass the isotope has.
What is average atomic mass, and how do we calculate it?
The average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of that element.
To calculate the average atomic mass of an element, first convert the percent abundances to decimals, then multiply the atomic mass of each isotope by its respective abundance, and finally add up all of the values resulting from the previous step.
How do we find the average atomic mass of an element when given a mass spectrum?
Since the mass spectrum of an element shows us the atomic masses and relative abundances of its isotopes, we can find the average atomic mass of the given element.
In the example shown, there are three isotopes with atomic masses of 24, 25, and 26 amu. The atomic mass of 24 amu corresponds with the relative abundance of 79%, 25 amu with 10%, and 26 amu with 11%. To find the element’s average atomic mass, first convert the abundances to decimals by dividing each by 100, resulting in 0.79, 0.10, and 0.11. Next, multiply 24 amu by its abundance of .79, 25 by 0.10, and 26 by 0.11. The values we get are 18.96, 2.5, and 2.86. Finally, adding up all these values gives us an average atomic mass of 24.32 amu.
After finding the average atomic mass, we could also determine the element’s identity by looking at the periodic table. In this case, the element would be magnesium, as our calculated average atomic mass is closest to 24.305 amu, shown underneath Mg in the periodic table.
Additionally, we know that our calculated value makes sense because if we look at the mass spectrum, the relative abundance of 24 amu is very high, indicating that the average atomic mass should be close to 24 amu.
