Atomic Structure & Periodic Trends

Overview

This lecture covers the foundational concepts of atomic structure and properties for AP Chemistry, including moles, molar mass, mass spectra, empirical formulas, mixtures, atomic structure, periodic trends, and ionic compound formation.

1.1 Moles and Molar Mass

• One mole contains 6.022 x 10²³ entities (Avogadro’s number), linking formula mass in amu to mass in grams.
• The formula weight (FW) is the sum of atomic weights in a chemical formula.
• Molar mass (M) is the mass of one mole of a substance in grams, numerically equal to FW in amu.
• To relate moles, mass, and molar mass: n = m / M.
• Mole fraction is the ratio of moles of a component to total moles in a solution.

1.2 Mass Spectra of Elements

• Mass number = protons + neutrons (electrons do not contribute to mass).
• Mass spectrometry identifies isotopes, their atomic masses, and relative abundance.
• Average atomic mass is a weighted mean of isotope masses.
• Exam questions focus on interpreting spectra, not operating the spectrometer.

1.3 Elemental Composition of Pure Substances

• Molecular formula gives actual atom numbers; empirical formula gives simplest ratio.
• Empirical formula is calculated by converting masses/percentages to mole ratios.
• Analysis methods: gravimetric, compositional, and combustion analysis.
• Example: empirical formula from combustion or percent composition calculations.

1.4 Composition of Mixtures

• Mixtures’ purity and composition are determined by separation methods: solubility, precipitation, or distillation.
• Calculations often require determining individual masses or moles of components in mixtures.

1.5 Atomic Structure & Electron Configuration

• Coulomb’s Law: force is proportional to product of charges and inversely to square of distance.
• Electron configuration follows Aufbau principle (fill from lowest energy up), Pauli Exclusion Principle (no identical quantum states), and Hund’s Rule (maximum unpaired electrons).
• Effective nuclear charge (Z_eff) is less than actual nuclear charge due to shielding by inner electrons.
• Atomic radius, ionization energy, and electronegativity trends across periods/groups are governed by changes in Z_eff and shell number.

1.6 Photoelectron Spectroscopy (PES)

• PES uses high-energy photons to eject electrons, measuring binding energies to deduce electron configurations.
• Peak heights in PES spectra indicate number of electrons per orbital set.
• Energy trends in binding energies reflect nuclear charge and electron distance from nucleus.

1.7 Periodic Trends

• Ionization energy generally decreases down a group (greater distance) and increases across a period (greater Z_eff).
• Atomic/ionic radius increases down a group and decreases across a period.
• Electron affinity is more negative for smaller atoms with high Z_eff.
• Electronegativity increases across a period and decreases down a group.

1.8 Valence Electrons & Ionic Compounds

• Type of chemical bond depends on difference in electronegativity between atoms, ranging from ionic to covalent.
• Atoms tend to gain/lose electrons to achieve noble gas configuration, affecting their chemical and physical properties.
• Periodic trends explain similarities in properties among group or period members.

Key Terms & Definitions

• Mole (mol) — amount of substance with 6.022 x 10²³ entities.
• Molar Mass (M) — mass of one mole of a substance in grams.
• Empirical Formula — simplest whole-number ratio of elements in a compound.
• Mass Spectrum — graph showing isotopic masses and abundances.
• Electron Configuration — arrangement of electrons among orbitals.
• Effective Nuclear Charge (Z_eff) — net charge felt by valence electrons.
• Photoelectron Spectroscopy (PES) — technique to measure electron binding energies.
• Ionization Energy — energy needed to remove an electron from an atom.
• Electronegativity — ability of an atom to attract shared electrons in a bond.

Action Items / Next Steps

• Practice mole, mass, and empirical formula calculations.
• Review sample mass spectra and identify isotopic composition.
• Work on periodic trend problem sets.
• Complete all assigned practice problems and FRQs from each section.