AP Chemistry Unit 1 Study Notes

AP Chemistry 1.2: Mass spectra of elements

Interpret isotope peaks and calculate weighted average atomic mass.

Aligned to Atomic Structure and Properties from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.

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Overview 2.1 Protons, Neutrons, and Electrons 2.2 Atomic Number 2.3 Mass Number 2.4 Isotope Notation 2.5 What Is Mass Spectrometry? 2.6 Reading a Mass Spectrum 2.7 Peak Position 2.8 Peak Height 2.9 Finding Average Atomic Mass From a Mass Spectrum 2.10 Sanity Check Topic 2 Common Mistakes

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These detailed Unit 1 notes were organized from the provided study document. For further study, visit Khan Academy. All Khan Academy content is available for free at www.khanacademy.org.

Overview Open

  • Khan Academy's second topic covers isotopes, mass spectra, relative abundance, and identifying elements from their spectra.

2.1 Protons, Neutrons, and Electrons Open

  • Before mass spectrometry, you need to understand atomic structure.

Particle Charge Location Approx. mass
Proton +1 nucleus 1 amu
Neutron 0 nucleus 1 amu
Electron −1 outside nucleus nearly 0 amu

  • Almost all atomic mass comes from the nucleus .

2.2 Atomic Number Open

  • The atomic number (Z) tells you the number of protons.

    • Example:

  • Oxygen has atomic number 8.

    • Therefore:

    • every oxygen atom has 8 protons .

  • If an atom suddenly had 9 protons, it would no longer be oxygen. It would be fluorine.

Key idea

  • Number of protons determines elemental identity.

2.3 Mass Number Open

  • The mass number (A) equals:

    • protons + neutrons

    • So:

    • A = p + n

    • and:

    • neutrons = mass number − atomic number

Example

  • Carbon-14:

    • Atomic number = 6

    • Mass number = 14

  • Neutrons:

    • 14 − 6 = 8

2.4 Isotope Notation Open

  • An isotope may be written as:

    • carbon-14

    • or nuclear notation showing:

    • mass number

    • atomic number

    • chemical symbol

  • The number after the element name is the mass number , not the atomic mass from the periodic table.

  • Carbon-14 therefore contains:

    • 6 protons

    • 14 − 6 = 8 neutrons

2.5 What Is Mass Spectrometry? Open

  • Mass spectrometry is an experimental technique that can separate particles based on their mass-to-charge behavior.

  • For AP Chemistry Unit 1, the important result is that a mass spectrum can reveal:

    • which isotopes exist

    • their approximate masses

    • their relative abundances

  • Khan Academy uses these data to estimate average atomic mass and identify unknown elements.

2.6 Reading a Mass Spectrum Open

  • A simplified elemental mass spectrum usually has:

    • x-axis: isotope mass

    • y-axis: relative abundance

  • Each peak generally corresponds to a different isotope.

    • Suppose the spectrum shows:

    • 24 amu → 79%

    • 25 amu → 10%

    • 26 amu → 11%

    • That tells us:

    • there are three major isotopes represented

    • mass 24 is the most abundant isotope

    • mass 25 is less common

    • mass 26 is also less common

    • This kind of spectrum appears in Khan Academy's current practice.

2.7 Peak Position Open

  • The x-position of the peak tells you approximately how massive the isotope is.

    • Peak at 24 → isotope mass ≈ 24 amu

    • Peak at 25 → isotope mass ≈ 25 amu

    • Peak at 26 → isotope mass ≈ 26 amu

2.8 Peak Height Open

  • The height or intensity represents relative abundance.

    • Tall peak = more common isotope

    • Short peak = less common isotope

  • Do not confuse:

    • x-coordinate → isotope mass

    • with:

    • y-coordinate → abundance

2.9 Finding Average Atomic Mass From a Mass Spectrum Open

  • Using:

    • 24 amu at 79%

    • 25 amu at 10%

    • 26 amu at 11%

  • Calculate:

    • 24(0.79) + 25(0.10) + 26(0.11)

    • = 18.96 + 2.50 + 2.86

    • = 24.32 amu

  • The periodic table element with an atomic mass near 24.3 is magnesium.

    • Therefore, the unknown element is likely Mg .

2.10 Sanity Check Open

  • The weighted average must fall somewhere between the smallest and largest isotope masses.

  • If isotopes are:

    • 24, 25, and 26

    • then average mass must be:

    • between 24 and 26

    • It cannot be 20 or 30.

    • And because 24 is overwhelmingly most common, the average should be much closer to 24 than 26.

Topic 2 Common Mistakes Open

  • Mistake 1

  • Thinking each isotope is a different element.

  • No.

  • Isotopes have the same protons , so they are the same element.

  • Mistake 2

  • Thinking average atomic mass equals the most common isotope.

  • Not necessarily.

    • It will often be close, but it is a weighted average .

  • Mistake 3

  • Counting peaks incorrectly.

  • Three isotope peaks means three isotopes represented.

  • Mistake 4

  • Treating percent abundance as a whole number.

  • Convert:

    • 79% → 0.79