Valence electron

In chemistry, valence electrons are the electrons contained in the outermost, or valence, electron shell of an atom. Valence electrons are important in determining how an element reacts chemically with other elements: The fewer valence electrons an atom holds, the less stable it becomes and the more likely it is to react. The reverse is also true, the more full/complete the valence shell is with valence electrons, the more inert an atom is and the less likely it is to chemically react with other chemical elements or with chemical elements of its own type. This is because it takes more transfer of energy (photons) to lose or gain an electron from or into a shell when that shell is more complete/full.

Valence electrons have the ability like electrons in inner shells to absorb or release energy(photons). This gain or loss of energy can trigger an electron to move/jump to another shell or even break free from the atom and its valence shell. When an electron absorbs/gains more energy(photons), then it moves to a more outer shell depending on the amount of energy the electron contains and has gained due to the absorption of 1 or more photons. (Also see: electrons in an excited state)
When an electron releases/loses energy(photons), then it moves to a more inner shell depending on the amount of energy the electron contains and has lost due to the release of 1 or more photons.

The number of valence electrons

Valence Electrons
Helium atom (not to scale)
Helium atom model This helium (He) model displays two valence electrons located in its outermost energy level. Helium is a member of the noble gases and contains two protons, neutrons, and electrons.

The number of valence electrons of an element is determined by its periodic table group (vertical column) in which the element is categorized. With the exception of groups 3–12 (transition metals), the number within the unit's place identifies how many valence electrons are contained within the elements listed under that particular column.

Periodic table group	Valence electrons
Group 1 (I) (alkali metals)	1
Group 2 (II) (alkaline earth metals)	2
Groups 3-12 (transition metals)	1 or 2*
Group 13 (III) (boron group)	3
Group 14 (IV) (carbon group)	4
Group 15 (V) (nitrogen group)	5
Group 16 (VI) (chalcogens)	6
Group 17 (VII) (halogens)	7
Group 18 (VIII or 0) (noble gases)	8**

Valence electrons in chemical reactions

The number of electrons in an atom's outermost valence shell governs its bonding behavior. Therefore, elements with the same number of valence electrons are grouped together in the periodic table of the elements. As a general rule, the fewer electrons in an atom's valence shell, the more reactive it is. Group 1 alkali metals are therefore very reactive, with lithium, sodium, and potassium being the most reactive of all metals.

Every atom is much more stable, or less reactive, with a full valence shell. This can be achieved one of two ways: an atom can either share electrons with neighboring atoms, a covalent bond, or it can remove electrons from other atoms, an ionic bond. Another form of ionic bonding involves an atom giving some of its electrons to another atom; this also works because it can end up with a full valence by giving up its entire outer shell. By moving electrons, the two atoms become linked. This is known as chemical bonding and serves to build atoms into molecules or ionic compounds. Five major types of bonds exist:

Valence electrons and electricity

The valence electrons are also responsible for determining the electrical conductivity nature of an element. The number of valence electrons in the valence shell (the outermost shell) determine if a valence electron in that valence shell can easily break free or not. The valence electrons that move from valence shell to valence shell are called free electrons and are the elementary particles responsible for electric current.
The shells and sub-shells of an atom can contain only a maximum number of electrons per shell. This is the same for the valence shell. In order for free electrons to move easily from the valence shell of an atom to the valence shell of another atom the valence shell needs to contain as few valence electrons as possible, because the more valence electrons a valence shell contains, the more complete/full that valence shell is and the more complete/full a valence shell is with electrons, the harder it will be for a valence electron to break free from the valence shell and act as a free electron. This is because it takes more energy(photons) to break valence electrons loose from a valence shell when that valence shell is nearly complete/full.
The reverse is also true. It takes less energy(photons) to break valence electrons free from a valence shell when that valence shell is nearly empty. (Note: Photons are the elementary particles which are responsible for the electromagnetic force. An electron can absorb or release photons after which it can jump to another shell. Whether or not an electron will jump to another shell depends on the amount of energy the electron has received from absorbing 1 or multiple photons or the amount of energy it loses when releasing 1 or multiple photons.)
So the more incomplete the valence shell of an atom is, the more easily valence electrons can move from the valence shell of one atom to the valence shell of another atom, the easier the material that consists of those type of atoms will conduct electricity.
Conductors are usually atoms that have the property to contain three or fewer valence electrons in their valence shell. Conductors conduct electricity well. Copper, aluminium, silver and gold are relatively good conductors.
Insulators are usually atoms that have the property to contain five or more valence electrons in their valence shell. Insulators conduct electricity relatively poorly in comparison to conductors.
Semiconductors are usually atoms that have 4 valence electrons. Semiconductors conduct electricity, however not as well as conductors, but also not as poorly as insulators. In terms of conductivity semiconductors are between conductors and insulators. Examples of semiconductors are Silicon and Germanium.