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Atomic Structure and Isotopes

Dec 5, 2025

Overview

  • Lecture: Chem 121 Unit 3 Part 2 — Atoms and Isotopes (Winter 2019).
  • Instructor: Kristine Petesch.
  • Focus: atomic structure, atomic number, isotopes, atomic mass unit, and calculating average atomic mass.

Atomic Mass Unit (amu)

  • Definition: amu is a unit created to express atomic and subatomic masses.
  • Purpose: simplifies comparison of masses of atoms and isotopes.
  • Reference: 1 amu ≈ 1/12 the mass of a carbon-12 atom (implied by common chemistry conventions).

Atomic Number and Element Identity

  • Atomic Number (Z): equals the number of protons in the nucleus.
  • Key point: every atom of a given element always has the same number of protons.
  • Chemical identity: defined by atomic number, not by mass.

Structure Of An Atom (Summary)

  • Nucleus: contains protons (positive) and neutrons (neutral).
  • Electrons: occupy surrounding space; number of electrons equals protons in neutral atoms.
  • Table summary: protons, neutrons, and electrons are the primary subatomic particles defining atom structure.

Isotopes

  • Definition: isotopes are atoms of the same element with different numbers of neutrons.
  • Chemical behavior: isotopes of an element have the same chemical properties.
  • Natural occurrence: most elements exist as a mixture of isotopes.

Atomic Mass Characteristics

  • Atomic mass listed on the periodic table is a weighted average of isotopic masses.
  • Weighted average accounts for each isotope’s mass and natural percent abundance.
  • Formula (concept): average atomic mass = Σ(isotope mass × fractional abundance).

Example: Chlorine Average Atomic Mass

  • Two natural isotopes of chlorine are used as an example.
  • Given abundances: 75.78% and 24.22% convert to fractions 0.7578 and 0.2422.
  • Calculation approach: multiply each isotope mass by its fractional abundance, then sum results to get chlorine’s atomic mass.

Key Terms and Definitions

  • Atomic Mass Unit (amu): unit for atomic/subatomic masses.
  • Atomic Number (Z): number of protons; defines element identity.
  • Isotope: same element, different neutron count.
  • Average (Atomic) Mass: weighted mean of isotopic masses based on abundance.

Action Items / Next Steps (if studying)

  • Practice: calculate average atomic mass from isotopic masses and percent abundances.
  • Review: identify protons, neutrons, electrons for given nuclide notations.
  • Memorize: relationship between atomic number, element identity, and isotopes.