Electron shell
In atomic physics, an electron shell, also known as a main energy level, is a group of
atomic orbitals with the same value of the principal quantum number
n. Electron shells are made up of one or more electron subshells, or sublevels, which have two or more orbitals with the same angular momentum quantum number
l. Electron shells make up the
electron configuration of an
atom. It can be shown that the number of
electrons that can reside in a shell is equal to .
The existence of electron shells was first observed experimentally in Charles Barkla's and
Henry Moseley's
X-ray absorption studies.
Encyclopedia
In atomic physics, an
electron shell, also known as a
main energy level, is a group of
atomic orbitals with the same value of the principal quantum number
n. Electron shells are made up of one or more
electron subshells, or
sublevels, which have two or more orbitals with the same angular momentum quantum number
l. Electron shells make up the
electron configuration of an
atom. It can be shown that the number of
electrons that can reside in a shell is equal to .
The existence of electron shells was first observed experimentally in Charles Barkla's and
Henry Moseley's
X-ray absorption studies. Barkla labelled them with the letters
K,
L,
M, etc. These letters were later found to correspond to the
n-values 1, 2, 3, etc. They are used in the spectroscopic Siegbahn notation.
The name for electron shells originates from the
Bohr model, in which groups of electrons were believed to orbit the nucleus at certain distances, so that their orbits formed "shells".
Valence shell
The
valence shell is the outermost shell of an
atom, which contains the
electrons most likely to account for the nature of any
reactions involving the atom and of the
bonding interactions it has with other atoms. Electrons in the valence shell are referred to as
valence electrons. The
chemist Gilbert Newton Lewis was responsible for much of the early development of the theory of the participation of valence shell electrons in chemical bonding.
Linus Pauling later generalized and extended the theory while applying insights from
quantum mechanics.
In a noble gas, an atom tends to have 8 electrons in its outer shell . This serves as the model for the
octet rule which is mostly applicable to main group
elements of the second and third periods. In terms of
atomic orbitals, the electrons in the valence shell are distributed 2 in the single
s orbital and 2 each in the three
p orbitals.
For coordination complexes containing
transition metals, the valence shell consists of electrons in these
s and
p orbitals, as well as up to 10 additional electrons, distributed as 2 into each of 5
d orbitals, to make a total of 18 electrons in a complete valence shell for such a compound. This is referred to as the
eighteen electron rule.
Subshells
Electron subshells are identified by the letters s, p, d, f, g, h, i, etc., corresponding to the
azimuthal quantum numbers 0, 1, 2, 3, 4, 5, 6, etc. Each shell can hold up to 2, 6, 10, 14, 18, 22 and 26 electrons respectively, or 2 electrons in each subshell. The notation 's', 'p', 'd', and 'f' originate from a now-discredited system of categorizing
spectral lines as "sharp", "principal", "diffuse", or "fundamental", based on their observed fine structure. When the first four types of orbitals were described, they were associated with these spectral line types, but there were no other names. The designations 'g', 'h', and so on, were derived by following alphabetical order.
See also
References
- Tipler, Paul & Ralph Llewellyn . Modern Physics . New York: W. H. Freeman and Company. ISBN 0-7167-4345-0
