AP Chem Unit 1

Mole

The SI unit for amount of substance.

Avogadro's Number

6.022 × 10²³ mol⁻¹; the number of particles in one mole of a substance.

Molar Mass

The mass in grams of one mole of a substance; numerically equal to the atomic or molecular mass in amu.

Atomic Mass Unit (amu)

Unit of mass equal to 1/12 the mass of a carbon-12 atom; the average mass of one atom in amu equals the molar mass in g/mol.

Isotope

Atoms of the same element with the same number of protons but different numbers of neutrons, and therefore different masses.

Mass Spectrum

A graph showing relative abundance of isotopes vs. their mass-to-charge ratio; used to identify isotopes and calculate average atomic mass.

Weighted Average Atomic Mass

Average atomic mass calculated by multiplying each isotope's mass by its fractional abundance and summing.

Law of Definite Proportions

A pure compound always contains the same elements in the same ratio by mass, regardless of the sample size.

Empirical Formula

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

Molecular Formula

The actual number of atoms of each element in one molecule; may be a multiple of the empirical formula.

Percent Composition

The mass percent of each element in a compound; used to determine empirical formula.

Elemental Analysis

Experimental method to determine the relative amounts of each element in a substance, used to confirm purity or determine formula.

Pure Substance

Matter with a uniform and definite composition; consists of only one type of atom, molecule, or formula unit.

Mixture

Matter composed of two or more substances in variable proportions; can be separated by physical means.

Homogeneous Mixture

A mixture with uniform composition throughout (a solution).

Heterogeneous Mixture

A mixture where composition varies from point to point.

Electron Configuration

A notation showing the distribution of electrons among the shells and subshells of an atom.

Aufbau Principle

Electrons occupy the lowest available energy orbitals first when filling an atom's electron configuration.

Core Electrons

Electrons in the inner shells of an atom that are not involved in bonding.

Valence Electrons

Electrons in the outermost shell; determine chemical reactivity and bonding behavior.

Shell (Energy Level)

The principal energy level of an electron, labeled n = 1, 2, 3, etc.; higher n means greater distance from nucleus and higher energy.

Subshell (Sublevel)

A subdivision of an energy level corresponding to s, p, d, or f orbitals.

Photoelectron Spectroscopy (PES)

A technique that measures the energy needed to remove electrons from different subshells; peaks correspond to subshells, peak height corresponds to number of electrons in that subshell.

Ionization Energy

The energy required to remove an electron from a gaseous atom or ion; increases across a period, decreases down a group.

Effective Nuclear Charge (Z_eff)

The net positive charge experienced by an electron after accounting for shielding by inner electrons.

Shielding (Screening)

The reduction of the nuclear charge felt by outer electrons due to the presence of inner electrons.

Coulomb's Law (atomic context)

The attractive or repulsive force between charged particles is proportional to the product of their charges and inversely proportional to the square of the distance between them; F ∝ q₁q₂/r².

Periodic Trend

A recurring pattern in element properties as atomic number increases across a period or down a group.

Atomic Radius

The size of an atom; decreases across a period (increasing Z_eff), increases down a group (more shells).

Ionic Radius

The size of an ion; cations are smaller than the parent atom; anions are larger.

Electron Affinity

The energy change when an electron is added to a neutral gaseous atom; generally more negative across a period.

Electronegativity

The tendency of an atom in a molecule to attract electrons toward itself; increases across a period, decreases down a group.

Periodicity

The repeating pattern of chemical and physical properties of elements as a function of atomic number, explained by electron configurations.