AP Chemistry Unit 1: Atomic Structure, Moles, and Periodicity

Mole (mol)

A quantity of substance containing exactly Avogadro's number of particles.

Avogadro's Number

$6.022 \times 10^{23} \text{ particles/mol}$.

Molar Mass

The mass of one mole of a substance, expressed in grams per mole (g/mol).

Mass to Moles Conversion

Formula: n = \frac{\text{mass (g)}}{\text{Molar Mass (g/mol)}}.

Particles to Moles Conversion

Formula: \text{Particles} = n \times (6.022 \times 10^{23}).

Gas Volume at STP

1 mole of any ideal gas occupies 22.4 Liters at Standard Temperature and Pressure (0°C and 1 atm).

Percent Composition

Describes percentage by mass of each element in a compound.

Empirical Formula

The simplest whole-number ratio of atoms in a compound.

Molecular Formula

The actual number of atoms in a molecule.

Isotopes

Atoms of the same element with different numbers of neutrons.

Average Atomic Mass

The weighted average of atomic masses of the naturally occurring isotopes of an element.

Mass Spectrometry

An analytical technique to determine the relative abundance and mass of isotopes.

Coulomb’s Law

FE \propto \frac{q1 q_2}{r^2}, describes the force of attraction/repulsion between charged particles.

Aufbau Principle

Electrons fill the lowest energy orbitals first.

Hund’s Rule

Electrons fill degenerate orbitals singly before pairing up.

Pauli Exclusion Principle

No two electrons can have the same quantum state.

Binding Energy in PES

The energy needed to remove an electron from an atom.

Effective Nuclear Charge (Z_e)

The net positive charge experienced by valence electrons.

Atomic Radius Trend

Decreases across a period and increases down a group.

Ionic Radius Trend

Cations are smaller and anions are larger than their parent atoms.

Ionization Energy (IE)

The energy required to remove the outermost electron from a gas-phase atom.

Electronegativity

The ability of an atom to attract shared electrons in a molecule.

Mass Number vs. Average Atomic Mass

Mass Number is Protons + Neutrons (integer), Average Atomic Mass is a weighted average (decimal).

Diatomic Molar Mass

The molar mass of a diatomic molecule, such as $N_2$, is not the same as the atomic mass of a single atom.

Removing Electrons from Transition Metals

Electrons are removed from the s-orbital before the d-orbital.

Example of Percent Composition Calculation

% N in $Ca(NO3)2$: \% N = (\frac{28.02}{164.10}) \times 100 = 17.07\%.

PES Peak Interpretation

Peak height indicates the number of electrons in that subshell.

Isoelectronic Series

A group of ions with the same electron configuration; the ion with the most protons is the smallest.

Common Error with Ionization Energy

IE for Boron is lower because the $2p$ electron is shielded by the $2s$ electron.

Common Error with PES

Ensure correct interpretation of x-axis; may vary between graphs.

Fractional Abundance

The proportion of each isotope in a naturally occurring sample.

Kinetic Energy in PES

The energy of an electron after being ejected, calculated from photon energy minus binding energy.

Periodic Trends Summary

Incorporates trends like atomic radius, electronegativity, and ionization energy.

Shielding Effect

Reduction in effective nuclear charge on the electron cloud, due to other electrons.

Mass Spectrometry Peaks

Number of peaks indicates number of isotopes, and height indicates abundance.

Difference in Electron Configurations of Transition Metals

In transition metals, electrons from the 4s subshell are removed before those from the 3d subshell.

Formula for Average Atomic Mass

\text{Avg Mass} = \sum (\text{Isotope Mass} \times \text{Fractional Abundance}).

Role of Neutrons

Neutrons contribute to the mass number but do not affect the charge of the atom.

Real-world application of Molar Mass

Used in stoichiometric calculations to convert between moles and mass.

Converting Volume to Moles at STP

Formula: n = \frac{\text{Volume (L)}}{22.4 \text{ L/mol}}.

Example of a Common Transition Metal Removal Error

For Iron (Fe), $[Ar] 4s^2 3d^6 \rightarrow Fe^{2+}: [Ar] 3d^6$.

Mass Spectrometry vs. PES

Mass spectrometry identifies isotopes and their abundances; PES gives energy levels of electrons.

Identifying element using PES peaks

Compare mean binding energies shown in PES spectra to known values for identification.

Abundance of heavier isotopes

Heavier isotopes generally have lower natural abundances compared to lighter isotopes.

Conversion of Mass Percentage to Empirical Formulas

Requires conversion to moles and simplification to the smallest ratio.

Effects of shielding on ionization energy

Increased shielding leads to decreased ionization energy.

Electronegativity Trend Exceptions

Noble gases tend not to engage in bonding, hence not typically assigned electronegativity values.

Calculating Electronegativity Difference

Used to predict bond type: ionic, polar covalent, or nonpolar covalent.

Insight into Electron Configurations

Transition metals can exhibit variable oxidation states.

Chemical significance of Percent Composition

Useful in determining the formula of compounds and their empirical formulas.