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CHEM Atomic Structure

Terms

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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.
degenerate orbitals
Orbitals with identical energies.
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.
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.
Nucleus
The small, dense central region of an atom around which electrons orbit. The nucleus is made up of protons and neutrons.
Isoelectronic
Description for two elemental species with the same electronic configuration.
Ionization energy
The energy it takes to remove an electron from an atom.
Isotope
Atoms with the same number of protons (i.e. same atomic number) but a different number of neutrons.
Shielding
When the attraction from the nucleus felt by one electron is lessened or blocked by intermediate electrons.
Neutron
An uncharged atomic particle of mass 1.67493x10-27. It resides in the nucleus.
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.
Proton
A positively charged particle of mass 1.6726x10-27. Protons reside in the nucleus.
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.
Cation
An ion with a net positive charge.
Splitting
Through shielding, the breaking of degenerate orbitals within a shell in multi-electron atoms.
Valence electrons
The electrons in the outermost energy shell of an atom. The configuration of these electrons determine the chemical properties of the element.
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.
Ion
Any atom or molecule with a net charge.
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.
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.
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.
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.
Pauli Exclusion Principle
States that no two electrons in an atom or molecule can have the same set of four quantum numbers.
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.

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