Mass Spectra of Elements (College Board AP® Chemistry): Flashcards
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Define mass spectrum.
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Define mass spectrum.
A mass spectrum is a graph produced by a mass spectrometer that displays the relative abundance of each isotope of an element plotted against its mass-to-charge ratio (m/z).
What two pieces of information does a mass spectrum provide about an element's isotopes?
A mass spectrum provides the mass (m/z value) and the relative abundance of each isotope present in the sample.
True or False?
A mass spectrometer separates isotopes by their charge alone.
False.
A mass spectrometer separates isotopes by their mass-to-charge ratio (m/z), not charge alone. Both mass and charge determine where an isotope appears on the spectrum.
On a mass spectrum, the x-axis shows the .......... and the y-axis shows the .......... of each isotope.
On a mass spectrum, the x-axis shows the mass-to-charge ratio (m/z) and the y-axis shows the relative abundance of each isotope.
Why does the average atomic mass of an element lie closer to the mass of the most abundant isotope?
Average atomic mass is a weighted average, meaning each isotope's mass is multiplied by its relative abundance before summing. The most abundant isotope therefore contributes most to the total, pulling the average toward its mass value.
True or False?
The terms relative abundance and percent abundance are interchangeable.
True.
Both terms describe the proportion of a given isotope in a sample. Some textbooks use them interchangeably, though relative abundance is often expressed as a decimal and percent abundance as a percentage.
An element has one highly abundant isotope at mass 64 amu and two less abundant isotopes at 66 amu and 68 amu.
Why is its average atomic mass greater than 64 amu?
The isotopes at 66 amu and 68 amu — although less abundant — still contribute positively to the weighted average atomic mass. Their combined contribution shifts the weighted average above 64 amu, even though the 64 amu isotope dominates.
Define isotope.
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons, giving them different mass numbers.
Average atomic mass = sum of (isotope mass × .......... ) for each naturally occurring isotope.
Average atomic mass = sum of (isotope mass × relative abundance) for each naturally occurring isotope.
Why must percent abundance be converted to a decimal before calculating average atomic mass?
The formula requires relative abundances to sum to 1, so using percent values directly (which sum to 100) makes the result 100 times too large. Dividing by 100 converts each percent to its decimal equivalent so the weighted sum gives the correct average atomic mass in amu.
True or False?
The sum of all isotope relative abundances in decimal form equals 1.
True.
Relative abundances in decimal form represent fractions of the whole sample. Since every atom belongs to one isotope, the fractions must sum to exactly 1.
Define average atomic mass.
Average atomic mass is the weighted average of the masses of all naturally occurring isotopes of an element, where each isotope mass is weighted by its relative abundance.
Boron-10 (10.01 amu) has 20% abundance and boron-11 (11.01 amu) has 80% abundance.
What is the average atomic mass of boron?
average atomic mass = (10.01 × 0.20) + (11.01 × 0.80)
= 2.002 + 8.808
= 10.81 amu
True or False?
An element's average atomic mass can fall outside the mass range of its lightest and heaviest isotopes.
False.
Average atomic mass is a weighted average of the isotope masses, so it must always lie between the masses of the lightest and heaviest naturally occurring isotopes.
To solve for isotope abundances algebraically, a student writes x + y = 1, where x and y are the abundances expressed in .......... form.
To solve for isotope abundances algebraically, a student writes x + y = 1, where x and y are the abundances expressed in decimal form.
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