Overview
This lecture explains what average atomic mass is, how it differs from mass number, and why a weighted average is used to represent atomic masses on the periodic table.
Atomic Number and Atomic Mass
- The atomic number (top number) is the number of protons in an atom.
- The number with decimals at the bottom of the periodic table is the average atomic mass, not the mass number.
Atomic Mass Unit (AMU)
- Masses of individual atoms are extremely small when measured in grams.
- Scientists use the atomic mass unit (AMU or amu) to express the mass of atoms.
- One atomic mass unit is based on carbon-12.
Isotopes and Relative Abundance
- Elements exist as mixtures of isotopes (atoms with same protons but different neutrons).
- A typical sample contains different isotopes in different amounts (e.g., carbon-12, carbon-13, carbon-14).
- Relative abundance is the percentage of each isotope in a sample (e.g., carbon-12 ≈ 98.89%, carbon-13 ≈ 1.1%).
Weighted Average Concept
- Average atomic mass is a weighted average of all the naturally occurring isotopes.
- Weighted average means more abundant isotopes contribute more to the average mass.
- The calculation requires knowing each isotope’s mass and its relative abundance.
Mass Number vs. Actual Atomic Mass
- Mass number is the total number of protons and neutrons in an isotope.
- Actual atomic mass is measured in amu and is not always a whole number.
Key Terms & Definitions
- Atomic Number — Number of protons in an atom.
- Average Atomic Mass — The weighted average mass of all naturally occurring isotopes of an element.
- Atomic Mass Unit (amu) — Unit for expressing atomic and molecular masses; based on carbon-12.
- Isotope — Atoms of the same element with different numbers of neutrons.
- Relative Abundance — Percentage of each isotope present in a natural sample.
- Weighted Average — An average where each component contributes according to its proportion in the whole.
- Mass Number — The total number of protons and neutrons in an atom’s nucleus.
Action Items / Next Steps
- Watch the next video for detailed steps on calculating average atomic mass.