Unit 1 · Atomic Structure and Properties

Mole ● (core concept) — The SI unit for amount of substance; one mole of any substance contains 6.022 × 10²³ (Avogadro's number) of its particles.

Avogadro's number ● (core concept) — 6.022 × 10²³ mol⁻¹; the number of particles in one mole of a substance.

Molar mass ● (core concept) — The mass of one mole of a substance, in grams per mole (g/mol); numerically equal to the average atomic or formula mass in amu.

Atomic mass unit (amu) ● (core concept) — A unit for the mass of individual atoms or molecules; a substance whose average particle mass is x amu has a molar mass of x g/mol.

Dimensional analysis ○ — A method of converting between units (e.g., grams to moles to particles) by multiplying by conversion factors such as molar mass or Avogadro's number.

Formula unit ● (core concept) — The lowest whole-number ratio of ions in an ionic compound; used as the countable unit when converting moles of ionic substances to particles.

Mass spectrum ● (core concept) — A plot of ion abundance versus mass for a sample. For a single element, each peak corresponds to an isotope and its height shows the isotope's relative abundance.

Isotope ● (core concept) — Atoms of the same element that have different numbers of neutrons, and therefore different masses.

Relative abundance ● (core concept) — The percentage of each isotope of an element that occurs in nature; read from the heights of the peaks in a mass spectrum.

Average atomic mass ● (core concept) — The weighted average of the masses of an element's isotopes, using their relative abundances as the weights.

Law of definite proportions ● (core concept) — A pure compound always contains its constituent elements in the same fixed ratio by mass.

Empirical formula ● (core concept) — The chemical formula showing the lowest whole-number ratio of atoms of each element in a compound.

Molecular formula ○ — The formula giving the actual number of atoms of each element in one molecule of a compound.

Elemental analysis ● (core concept) — The experimental determination of the relative numbers of atoms of each element in a substance; used to identify composition and purity.

Pure substance ● (core concept) — Matter containing only one type of atom, molecule, or formula unit; mixtures contain two or more types of particles.

Mixture ● (core concept) — A sample containing two or more types of particles whose proportions can vary; can be homogeneous or heterogeneous.

Elemental composition of a mixture ● (core concept) — The mass percent of an element in a mixture can be determined from the masses of the components, or used to find how much of a pure component a mixture contains.

Proton, neutron, and electron ○ — The three subatomic particles: protons (+1, in the nucleus), neutrons (neutral, in the nucleus), and electrons (−1, occupying shells around the nucleus).

Electron configuration ● (core concept) — The arrangement of electrons in shells (energy levels) and subshells (s, p, d, f), written by filling the lowest-energy subshells first.

Aufbau principle ● (core concept) — Electrons occupy the lowest-energy available subshells first when writing an electron configuration.

Coulomb's law ● (core concept) — The force between charged particles: F is proportional to q₁q₂/r². In an atom, greater nuclear charge and shorter electron–nucleus distance mean stronger attraction.

Valence electrons ● (core concept) — Electrons in the outermost shell; they determine an element's chemical behavior. Inner electrons are core electrons.

Ionization energy ● (core concept) — The energy required to remove an electron from an atom or ion in the gas phase. It can be estimated from Coulomb's law using the electron's distance from the nucleus and the effective nuclear charge it feels.

Effective nuclear charge (shielding) ● (core concept) — The net positive charge felt by an electron after repulsion from inner (core) electrons is taken into account; inner electrons shield outer electrons from the full nuclear charge.

Photoelectron spectroscopy (PES) ● (core concept) — A technique that measures the binding energy of electrons in an atom. Each peak's position gives the energy needed to remove an electron from a subshell, and its height shows how many electrons occupy it.

Binding energy ● (core concept) — The energy required to remove an electron from its subshell; shown by the position of a PES peak.

Periodicity ● (core concept) — The repeating patterns of chemical properties across periods and down groups, explained by repeating electron configurations.

Atomic radius trend ● (core concept) — Atomic radius decreases across a period (rising effective nuclear charge pulls electrons in) and increases down a group (new shells are added).

Ionic radius ● (core concept) — Cations are smaller than their parent atoms (lost shells/valence electrons) and anions are larger than their parent atoms (extra electron repulsion).

Electron affinity ● (core concept) — The energy change when an atom gains an electron; a periodic trend that can be explained by Coulomb's law.

Electronegativity ● (core concept) — A measure of an atom's tendency to attract shared electrons in a bond. It increases across a period and decreases down a group, since the attractive force on bonding electrons depends on nuclear charge and distance.

Shell model ● (core concept) — The model of the atom in which electrons occupy shells (energy levels) and subshells; with Coulomb's law and shielding, it explains periodic trends in atomic properties.

Predicting ionic charge ● (core concept) — Main-group elements typically form ions by gaining or losing electrons to match a noble-gas configuration; the resulting charge can be predicted from valence-electron count and position on the periodic table.

Analogous compounds ● (core concept) — Elements in the same group form compounds with similar formulas and structures because they have the same number of valence electrons.