Atomic Structure & Periodicity — 60 Terms Defined

Alkali Metals Atomic Structure & Periodicity Dimensionless The group 1 elements — lithium through francium — which have a single valence s electron. ns¹

Alkaline Earth Metals Atomic Structure & Periodicity Dimensionless The group 2 elements — beryllium through radium — which have two valence s electrons. ns²

Atomic Emission Spectrum Atomic Structure & Periodicity nm The pattern of discrete wavelengths emitted when excited atoms of an element return to lower energy states. ΔE = hν

Atomic Number Atomic Structure & Periodicity Dimensionless The number of protons in the nucleus of an atom, which uniquely identifies the element. Z = number of protons

Atomic Orbital Atomic Structure & Periodicity Dimensionless A mathematical function describing the region around a nucleus where an electron of given energy is most likely to be found. |ψ|² = probability density

Atomic Radius Atomic Structure & Periodicity pm A measure of the size of an atom, usually taken as half the distance between the nuclei of two bonded identical atoms. r = d(X–X) / 2

Aufbau Principle Atomic Structure & Periodicity Dimensionless The rule that electrons occupy the lowest-energy orbital available before filling higher-energy ones. fill in order of increasing (n + l)

Azimuthal Quantum Number Atomic Structure & Periodicity Dimensionless The quantum number l that determines the shape of an orbital and the magnitude of the electron's orbital angular momentum. l = 0 to (n − 1)

Bohr Model Atomic Structure & Periodicity Dimensionless The 1913 model in which electrons orbit the nucleus in fixed circular paths of quantised energy and angular momentum. mvr = nh / 2π

Chalcogens Atomic Structure & Periodicity Dimensionless The group 16 elements — oxygen, sulfur, selenium, tellurium and polonium — collectively known as the ore-forming elements. ns² np⁴

Core Electron Atomic Structure & Periodicity Dimensionless An electron occupying a completely filled inner shell that does not take part in chemical bonding.

d-Block Elements Atomic Structure & Periodicity Dimensionless The elements of groups 3 to 12, whose valence electrons fill d orbitals of the penultimate shell. (n−1)d¹⁻¹⁰ ns⁰⁻²

de Broglie Relation Atomic Structure & Periodicity m The relation stating that every particle has an associated wavelength inversely proportional to its momentum. λ = h / mv

Degenerate Orbitals Atomic Structure & Periodicity Dimensionless Two or more orbitals in the same atom that have exactly the same energy.

Diagonal Relationship Atomic Structure & Periodicity Dimensionless The similarity in properties between an element of period 2 and the element one place to its lower right in period 3.

Diamagnetism Atomic Structure & Periodicity BM Weak repulsion from a magnetic field, shown by substances in which all electrons are spin-paired. μ = 0 BM

Effective Nuclear Charge Atomic Structure & Periodicity Dimensionless The net positive charge actually experienced by a particular electron after allowing for shielding by other electrons. Z_eff = Z − σ

Electron Atomic Structure & Periodicity u A negatively charged fundamental particle that occupies orbitals around the nucleus and governs chemical bonding. m_e = 9.109 × 10⁻³¹ kg

Electron Affinity Atomic Structure & Periodicity kJ/mol The energy change when one mole of gaseous atoms each gains an electron to form gaseous negative ions. X(g) + e⁻ → X⁻(g)

Electron Configuration Atomic Structure & Periodicity Dimensionless The notation describing how an atom's electrons are distributed among its available orbitals. 1s² 2s² 2p⁶ …

Electronegativity Atomic Structure & Periodicity Dimensionless The relative tendency of an atom in a covalent bond to attract the shared pair of electrons towards itself. ΔEN determines bond polarity

f-Block Elements Atomic Structure & Periodicity Dimensionless The lanthanides and actinides, in which electrons fill the f orbitals of the antepenultimate shell. (n−2)f¹⁻¹⁴

Group Atomic Structure & Periodicity Dimensionless A vertical column of the periodic table whose members share the same valence electron configuration.

Halogens Atomic Structure & Periodicity Dimensionless The group 17 elements — fluorine through astatine — which are one electron short of a full outer shell. ns² np⁵