Atomic Structure and Isotopes

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.