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Atomic structure definitions

Every term the lessons in this unit define, lesson by lesson, worded as the lessons use them. Each lesson title links to its notes.

36 terms

Protons, neutrons, electrons and isotopes

Nucleon: A proton or a neutron: either of the two particles found in the nucleus.
Proton: A particle in the nucleus with a relative mass of 1 and a relative charge of +1.
Neutron: A particle in the nucleus with a relative mass of 1 and no charge.
Electron: A particle outside the nucleus with a relative mass of 1/1840 and a relative charge of −1.
Atomic number: The number of protons in the nucleus of an atom, also called the proton number, Z.
Mass number: The total number of protons and neutrons in the nucleus of an atom, A.
Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons.
Ion: An atom or group of atoms that carries an overall charge because it has lost or gained electrons.
Isoelectronic: Having the same number of electrons, as Na⁺, Ne and F⁻ do.

Relative isotopic mass: The mass of an atom of an isotope relative to one twelfth of the mass of an atom of carbon-12.
Relative atomic mass: The weighted mean mass of an atom of an element relative to one twelfth of the mass of an atom of carbon-12, Ar.
Relative molecular mass: The weighted mean mass of a molecule relative to one twelfth of the mass of an atom of carbon-12, Mr.
m/z: The mass of an ion divided by its charge, equal to the relative mass of the ion when its charge is +1.
Electron impact ionisation: Ionisation in which high-energy electrons from an electron gun knock an electron off each gaseous particle, forming a 1+ ion.
Electrospray ionisation: Ionisation in which a solution of the sample is sprayed from a needle at a high positive voltage, so that each molecule gains a proton, forming XH⁺.
Molecular ion: The ion formed when a whole molecule loses one electron, M⁺, whose m/z gives the relative molecular mass.
M+1 peak: A small peak one m/z unit above the molecular ion peak, from molecules that contain one carbon-13 atom.
M+2 peak: A peak two m/z units above the molecular ion peak, from molecules that contain chlorine-37 or bromine-81 in place of the lighter isotope.

Shell: A main energy level in an atom, numbered n = 1, 2, 3 outwards from the nucleus, holding up to 2n² electrons.
Subshell: A division of a shell made of orbitals of equal energy, labelled s, p, d or f.
Orbital: A region of space around the nucleus where there is a high probability of finding an electron; it holds up to two electrons, with opposite spins.
Electron configuration: The arrangement of the electrons of an atom or ion in shells and subshells, written for example as 1s² 2s² 2p⁶.
Pauli principle: No orbital holds more than two electrons, and two electrons in the same orbital must have opposite spins.
Hund's rule: Electrons fill the orbitals of a subshell singly, with parallel spins, before any orbital takes a pair.
Block: A region of the periodic table in which the highest-energy electron of each element is in the same type of subshell, s, p, d or f.
Principal quantum number: The number n of the shell an electron occupies, 1, 2, 3 and so on outwards from the nucleus.
Ground state: The lowest-energy electron configuration of an atom or ion, with each electron in the lowest available orbital.
Excited state: A configuration in which one or more electrons occupy a higher-energy orbital than in the ground state.

First ionisation energy: The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Second ionisation energy: The energy required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions.
Successive ionisation energies: The energies needed to remove the electrons of an atom one at a time, first, second, third and so on.
nth ionisation energy: The energy required to remove one electron from each ion in one mole of gaseous (n − 1)+ ions to form one mole of gaseous n+ ions.
Nuclear charge: The total positive charge of the nucleus, equal to the number of protons in it.
Atomic radius: Half the distance between the nuclei of two bonded atoms of the same element.
Shielding: The repulsion of an outer electron by the electrons in inner shells, which reduces the attraction of the nucleus it feels.
Spin-pair repulsion: The repulsion between two electrons in the same orbital, which makes one of them easier to remove than an unpaired electron.