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Anion  -  An ion with a net negative charge.
Atomic orbital  -  An orbital, associated with only one particular atom, in which electrons reside. Though they are called orbitals, atomic orbitals should not be conceived as akin to the orbits of planets rather around a star. Instead, orbitals describe a locus of space in which an electron is likely to reside. Each orbital can hold up to two electrons.
Aufbau principle  -  German for "building up", a systematic procedure for determining the electron configuration of any atom. Incorporates the Pauli Exclusion Principle and Hund's Rule.
Cation  -  An ion with a net positive charge.
degenerate orbitals  -  Orbitals with identical energies.
Electron  -  A negatively charged elementary particle of mass 9.109390x10-31 . Electrons of an unbonded atom move around the atomic nucleus in orbitals. Those electrons in the orbitals furthest from the nucleus are the highest in energy, play a crucial role in chemical processes such as bonding, and are called valence electrons.
Electron affinity  -  The energy change in an atom when it gains an electron.
Electronegativity  -  A measure of the ability of an atom to attract electrons to itself. Incorporates the atom's ionization energy and electron affinity.
Hund's Rule  -  A rule which says that, when choosing between orbitals, electrons prefer to go in separate orbitals of the same energy. In this way, every orbital within a particular shell (or subshell when the orbitals are not degenerate) will be ha lf-filled before any single one orbital becomes completely filled.
Ion  -  Any atom or molecule with a net charge.
Ionization energy  -  The energy it takes to remove an electron from an atom.
Isoelectronic  -  Description for two elemental species with the same electronic configuration.
Isotope  -  Atoms with the same number of protons (i.e. same atomic number) but a different number of neutrons.
Neutron  -  An uncharged atomic particle of mass 1.67493x10-27 . It resides in the nucleus.
Nucleus  -  The small, dense central region of an atom around which electrons orbit. The nucleus is made up of protons and neutrons.
Octet rule  -  The cardinal rule of bonding. The octet rule states that atoms gain stability when they have a full complement of 8 electrons in their valence shells.
Pauli Exclusion Principle  -  States that no two electrons in an atom or molecule can have the same set of four quantum numbers.
Proton  -  A positively charged particle of mass 1.6726x10-27 . Protons reside in the nucleus.
Quantum Numbers  -  The four numbers that define each particular electron of an atom. The Principle Quantum Number (n) describes the electrons' energy and distance from the nucleus. The Angular Momentum Quantum Number (l) describes the shape of the orbital in which the electron resides. The Magnetic Quantum Number (m describes the orientation of the orbital in space. The Spin Quantum number describes whether the spin of the electron is positive or negative.
Shell  -  A group of subshells of similar energy levels. For example, 2s and 2p subshells occupy the same shell. Indicated by the principle quantum number.
Shielding  -  When the attraction from the nucleus felt by one electron is lessened or blocked by intermediate electrons. Shielding can split degenerate orbitals. For example, since s-orbital electrons shield for p-orbital electrons and rece ive little shielding themselves, s-orbitals are usually of lower energy level than p-orbitals of the same shell.
Splitting  -  Through shielding, the breaking of degenerate orbitals within a shell in multi-electron atoms.
Subshell  -  Orbitals of the same subshell are of the same shape and energy. p-orbitals are of the same subshell, while s-orbitals are of a separate subshell. Indicated by the angular momentum quantum number.
Uncertainty Principle  -  A tenet of quantum mechanics that says that the position and momentum of any particle cannot both be known precisely at the same time.
Valence electrons  -  The electrons in the outermost energy shell of an atom. The configuration of these electrons determine the chemical properties of the element.
Valence shell  -  The highest energy shell in an atom, containing valence electrons. All interactions between atoms take place through the electrons of the valence shell.