Glossary

Atomic chemistry glossary — 20 definitions

Definitions of essential terms: atomic number, isotope, orbital, electron configuration, electronegativity, ionisation energy, half-life. IUPAC and CODATA notation.

Atomic number (Z)
Number of protons in the nucleus of an atom. It uniquely defines a chemical element and orders the modern periodic table.
Mass number (A)
Sum of protons and neutrons in the nucleus. Two atoms of the same element with different A values are isotopes.
Isotope
Atom sharing the same Z with another but with a different number of neutrons. Isotopes of an element have nearly identical chemical properties but distinct nuclear properties (stability, half-life).
Standard atomic weight
Average mass of an atom of an element, weighted by the natural abundance of its isotopes. Expressed in unified atomic mass units (u). Published and updated by IUPAC.
Electron configuration
Distribution of the electrons of an atom across its orbitals, written in order of energy filling. Example: [Ne] 3s² 3p¹ for aluminium.
Atomic orbital
Region of space where the probability of finding an electron is non-negligible. Characterised by the quantum numbers n, ℓ, mℓ and the spin ms. The s, p, d, f orbitals have distinctive shapes (sphere, dumbbell, cloverleaf…).
Electron shell
Set of orbitals sharing the same principal quantum number n. Labelled KLMNOPQ for n = 1, 2, 3… Each shell has a maximum capacity of 2n².
Subshell
Subdivision of a shell corresponding to a given azimuthal number ℓ (s for ℓ=0, p for ℓ=1, d for ℓ=2, f for ℓ=3). Each subshell holds at most 2(2ℓ+1) electrons.
Valence electron
Electron in the outer shell, responsible for chemical properties and bonds. Their number determines the valence of the element.
Electronegativity
Ability of an atom to attract the electrons of a chemical bond toward itself. Several scales exist (Pauling, Allen, Mulliken) giving close but not identical values. Fluorine is the most electronegative (3.98 Pauling).
Ionisation energy
Minimum energy required to remove an electron from a neutral atom in the gas phase. One speaks of first, second, third ionisation energy (I₁, I₂, I₃…) depending on the electron removed. Unit: kJ/mol.
Electron affinity
Energy released (positive by convention) when a neutral gaseous atom captures an electron to form a negative ion. Halogens have the highest electron affinities.
Oxidation state
Hypothetical charge of an atom in a compound, obtained by assigning each bond's electrons to the more electronegative atom. Used to balance redox reactions.
Atomic radius
Conventional distance from the nucleus to the 'outer' boundary of the atom. Several definitions coexist: empirical, calculated, covalent, Van der Waals. Unit: picometre (pm).
Ionic radius
Radius of an ion in a crystal. Depends on oxidation state, coordination number and, for anions, the environment. The tabulated values (Shannon-Prewitt) are the standard reference.
Half-life
Time after which half of the nuclei in a radioactive sample have decayed. Characteristic of each radioactive isotope, from nanoseconds to billions of years.
Cross section
Effective area presented by a nucleus to a projectile (typically a neutron). Measured in barns (1 b = 10⁻²⁸ m²). Drives the probability of capture or fission.
s / p / d / f-block
Grouping of elements by the subshell being filled. s-block = groups 1–2; p-block = groups 13–18; d-block = transition metals; f-block = lanthanides and actinides.
Ion
Atom or molecule that has gained (anion) or lost (cation) electrons, and therefore carries an electric charge. Ionic species are ubiquitous in solution chemistry.
pKa
Negative common logarithm of the acidity constant Ka. A quantitative measure of an acid's strength (the lower the pKa, the stronger the acid). pKa(H₂O) ≈ 15.7.