Overview
This lecture discusses atomic mass, isotopes, the concept of the mole, calculation of weighted average atomic mass, and the use of mass spectrometry in measuring atomic masses.
Atomic Mass and Isotopes
- Most elements have multiple isotopes, which are atoms of the same element with different numbers of neutrons.
- The atomic number (bottom number) equals the number of protons.
- The mass number (top number) equals the sum of protons and neutrons.
- The atomic mass on the periodic table is the weighted average of all naturally occurring isotopes of an element.
- Percent abundance indicates how common each isotope is in nature.
Common Isotope Examples
- Carbon: C-12 (98.9%), C-13 (1.1%), C-14 (<0.1%).
- Hydrogen: H-1 (protium), H-2 (deuterium), H-3 (tritium).
- Uranium: U-238 (99%), U-235, U-234 (minor abundances).
Calculating Weighted Atomic Mass
- Weighted average atomic mass = Σ (fractional abundance × isotope mass) for all isotopes.
- Convert percent abundance to decimals (e.g., 40% → 0.4) for calculations.
Atomic Mass Unit (amu)
- 1 amu is defined as 1/12 the mass of a carbon-12 atom at rest.
- All isotope masses are compared relative to C-12, which is set at 12.000 amu.
Measuring Atomic Mass: Mass Spectrometry
- Mass spectrometry vaporizes a sample and ionizes the atoms to create positive ions.
- Ions are accelerated and deflected in a magnetic field based on their mass-to-charge ratios.
- Detectors measure the time and abundance of ions, resulting in a mass spectrum with peaks for each isotope.
- The tallest peak (base peak) has the highest relative abundance.
Example: Zirconium and Atomic Mass Calculation
- Use isotopic masses and percent abundances to calculate atomic mass: sum (fractional abundance × isotope mass) for each isotope.
- For zirconium, calculated sum matches the periodic table value.
Problem-Solving Tips
- When given "respectively", match listed values in order to respective isotopes and abundances.
- Compute missing abundances if some are unknown by knowing total percent is 100%.
Key Terms & Definitions
- Isotope — Atoms of the same element with different numbers of neutrons.
- Atomic number — Number of protons in an atom.
- Mass number — Total number of protons and neutrons.
- Atomic mass unit (amu) — 1/12 the mass of a carbon-12 atom.
- Weighted average — Sum of each isotope’s mass times its fractional abundance.
- Mass spectrometry — Technique for measuring atomic or molecular mass and relative abundance.
Action Items / Next Steps
- Practice calculating atomic mass using given isotopic masses and percent abundances.
- Know the names and abundances of common hydrogen isotopes (protium, deuterium, tritium).
- Review how to interpret mass spectrometry data for isotopic analysis.