Valence electron
An atom with a closed shell of valence electrons (corresponding to a noble gas configuration) tends to be chemically inert.
An energy gain can trigger the electron to move (jump) to an outer shell; this is known as atomic excitation.
Or the electron can even break free from its associated atom's shell; this is ionization to form a positive ion.
When an electron loses energy (thereby causing a photon to be emitted), then it can move to an inner shell which is not fully occupied.
In effect, there are possibly seven valence electrons (4s2 3d5) outside the argon-like core; this is consistent with the chemical fact that manganese can have an oxidation state as high as +7 (in the permanganate ion: MnO−4).
(But note that merely having that number of valence electrons does not imply that the corresponding oxidation state will exist.
Thus, although a nickel atom has, in principle, ten valence electrons (4s2 3d8), its oxidation state never exceeds four.
The valence shell is the set of orbitals which are energetically accessible for accepting electrons to form chemical bonds.
[5] As a general rule, a main-group element (except hydrogen or helium) tends to react to form a s2p6 electron configuration.
The heavy group 2 elements calcium, strontium, and barium can use the (n−1)d subshell as well, giving them some similarities to transition metals.
During the formation of an ionic bond, which provides the necessary ionization energy, this one valence electron is easily lost to form a positive ion (cation) with a closed shell (e.g., Na+ or K+).
An alkaline earth metal of group 2 (e.g., magnesium) is somewhat less reactive, because each atom must lose two valence electrons to form a positive ion with a closed shell (e.g., Mg2+).
[citation needed] Within each group of nonmetals, reactivity decreases with each lower row of the table (from a light element to a heavy element) in the periodic table, because the valence electrons are at progressively higher energies and thus progressively less tightly bound.
Valence electrons are also responsible for the bonding in the pure chemical elements, and whether their electrical conductivity is characteristic of metals, semiconductors, or insulators.
Such an element is found toward the right of the periodic table, and it has a valence shell that is at least half full (the exception is boron).
