Electron atomic and molecular orbitals
the arrangement of electrons of an
atom, a molecule, or other physical
It concerns the way
electrons can be distributed in the
orbitals of the given system (atomic or
molecular for instance).
Like other elementary particles, the
electron is subject to the laws of
quantum mechanics, and exhibits both
particle-like and wave-like nature.
Formally, the quantum state of a
particular electron is defined by its
wave function, a
function of space and time. According to the Copenhagen interpretation of quantum mechanics, the position of a
particular electron is not well defined until an act of measurement causes it to be detected. The probability that the
act of measurement will detect the electrons at a particular point in space is proportional to the square of the absolute
value of the wavefunction at that point.
An energy is associated with each electron configuration and, upon certain conditions, electrons are able to move
from one orbital to another by emission or absorption of a quantum of energy, in the form of a photon.
Knowledge of the electron configuration of different atoms is useful in understanding the structure of the periodic
table of elements. The concept is also useful for describing the chemical bonds that hold atoms together. In bulk
materials this same idea helps explain the peculiar properties of lasers and semiconductors.
Shells and subshells
Electron configuration table
Electron configuration was first conceived of under the
Bohr model of the atom, and it is still common to speak of
shells and subshells despite the advances in understanding
of the quantum-mechanical nature of electrons.
An electron shell is the set of allowed states electrons may
occupy which share the same principal quantum number,
(the number before the letter in the orbital label). An atom's
th electron shell can accommodate 2
first shell can accommodate 2 electrons, the second shell
8 electrons, and the third shell 18 electrons. The factor of
two arises because the allowed states are doubled due to
each atomic orbital admits up to two otherwise identical electrons with opposite spin, one with a spin
+1/2 (usually noted by an up-arrow) and one with a spin -1/2 (with a down-arrow).
A subshell is the set of states defined by a common azimuthal quantum number,
, within a shell. The values
= 0, 1,
2, 3 correspond to the
labels, respectively. The maximum number of electrons which can be placed in a
subshell is given by 2(2
+ 1). This gives two electrons in an s subshell, six electrons in a p subshell, ten electrons in