AP Chemistry Unit 1 Essentials: Key Terms & Big Ideas

Big Idea 1

The mole connects the atomic scale to the lab scale

Atoms and molecules are far too small to count individually, so chemists use the mole — 6.022 × 10²³ particles — as a bridge between the invisible particle world and measurable quantities like grams and liters. Every stoichiometry calculation in this course, from Unit 1 through Unit 9, runs through this conversion.

Mole Molar Mass Avogadro's Number

Big Idea 2

Electron configuration is not just memorized — it's measured

Photoelectron spectroscopy (PES) provides direct experimental evidence for electron configuration: each peak's position shows binding energy (how tightly an electron is held) and each peak's height shows the relative number of electrons in that subshell. This is the strongest evidence AP Chemistry gives you that the "shells and subshells" model of the atom isn't just a convenient story — it's testable.

PES Electron Configuration Binding Energy

Big Idea 3

Periodic trends are all Coulomb's law in disguise

Atomic radius shrinks, ionization energy rises, and electronegativity increases across a period for the same underlying reason: increasing effective nuclear charge pulls valence electrons in more tightly as you add protons without adding a new shell. Down a group, the trends reverse because a new principal energy level adds distance (and shielding) between the nucleus and the valence electrons. Once you see Coulomb's law underneath every trend, you stop memorizing arrows and start predicting them.

Periodic Trends Effective Nuclear Charge Coulomb's Law

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Mole

The SI base unit for amount of substance. One mole contains exactly 6.022 × 10²³ particles (Avogadro's number) — atoms, molecules, or ions.

Moles

Molar mass

The mass in grams of one mole of a substance, numerically equal to the substance's atomic or molecular mass in amu. Used to convert between grams and moles.

Moles

Avogadro's number

6.022 × 10²³ — the number of particles in one mole of any substance.

Moles

Mass spectrometry

An analytical technique that ionizes a sample and separates the resulting ions by their mass-to-charge ratio, producing a spectrum that reveals isotopes and their relative abundances.

Mass Spectrometry

Isotope

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

Mass Spectrometry

Average atomic mass

The weighted average of the masses of all naturally occurring isotopes of an element, weighted by their relative abundance. This is the number on the periodic table.

Mass Spectrometry

Percent composition

The percentage by mass that each element contributes to a compound's total mass.

Composition

Empirical formula

The simplest whole-number ratio of atoms in a compound, derived from percent composition or mass data.

Composition

Chromatography

A separation technique that exploits differences in how strongly mixture components interact with a stationary phase versus a mobile phase, used to determine mixture composition.

Mixtures

Electron configuration

The distribution of an atom's electrons among orbitals, written using shell number, subshell letter, and electron count (e.g., 1s² 2s² 2p⁶).

Atomic Structure

Pauli exclusion principle

No two electrons in the same atom can have identical sets of quantum numbers — practically, this means each orbital holds at most two electrons, and they must have opposite spins.

Atomic Structure

Hund's rule

Electrons fill degenerate (equal-energy) orbitals singly before any orbital receives a second electron, minimizing electron-electron repulsion.

Atomic Structure

Valence electrons

The electrons in an atom's outermost principal energy level. These are the electrons involved in bonding and largely determine an element's chemical behavior.

Atomic Structure

Photoelectron spectroscopy (PES)

A technique that ejects electrons from atoms using high-energy photons and measures their kinetic energy to determine binding energy — direct experimental evidence for electron configuration.

PES

Binding energy

The energy required to remove an electron from a particular subshell. Electrons closer to the nucleus (lower n, less shielded) have higher binding energy.

PES

Effective nuclear charge (Zeff)

The net positive charge experienced by a valence electron, equal to the actual nuclear charge minus the shielding effect of inner electrons. Increases across a period.

Periodic Trends

Shielding

The reduction in attractive force felt by outer electrons due to the repulsion of inner-shell electrons, which partially block the pull of the nucleus.

Periodic Trends

Atomic radius

A measure of the size of an atom. Decreases across a period (increasing Zeff) and increases down a group (added principal energy levels).

Periodic Trends

Ionization energy

The energy required to remove an electron from a gaseous atom or ion. Increases across a period and decreases down a group — the inverse trend of atomic radius.

Periodic Trends

Electronegativity

A measure of an atom's ability to attract shared electrons in a chemical bond. Increases across a period and decreases down a group; fluorine is the most electronegative element.

Periodic Trends

Ionic compound

A compound formed when atoms transfer electrons to achieve stable (often noble-gas) electron configurations, producing oppositely charged ions held together by electrostatic attraction.

Ionic Compounds

Coulomb's law

Describes the electrostatic force between two charged particles: force increases with the magnitude of the charges and decreases with the square of the distance between them. The underlying explanation for nearly every periodic trend.

Periodic Trends