Valence electron help in forming the chemical bonds. They are usually placed in the outermost shell of an atom( see image below). 2 atoms form a single covalent bond by sharing one valence electron from each resulting in forming a shared pair. The chemical properties of an element are determined by its ability to form valence electrons. On the contrary, an atom that has a closed shell of valence electrons is chemically inert as there is no freedom of exchange of electrons. On the contrary, atoms with one or two valence electrons are highly reactive than those that have closed shells. Similar to an electron, in an inner shell, a valence electron has the ability to absorb or release energy in the form of a photon. This form of energy gain can trigger the electrons to move (jump) to an outer shell; this is known as atomic excitation. On the contrary, when an electron loses it`s energy (thereby causing a photon to be emitted) so, it can move to an inner shell that is not fully occupied.
Why valency and how to find it?
Valence electrons are mainly used to predict the chemical properties of elements in the periodic table. It is also important to recall what Mendeleev used to organize the periodic table with over 100 known elements. The number of valence electrons in the periodic table can easily be identified by looking at the electron arrangement of a given element. Moreover, you can obtain the valence electron number by looking at the group number of the element in question. For example, the valence electron for sodium with electron configuration of 1s22s22p63s1 is one, and boron, aluminium, gallium, indium, and thallium have three valence electrons. It is important to determine the number of electrons for various reasons such as these help in drawing the accurate dot structures. The number of valence electrons of an element can be found through the vertical column of the periodic table where the element is categorized except in the groups 3–12 (the transition metals). The units (digits) of the group number identifies how many valence electrons are associated with a neutral atom of an element listed under that particular column. The number of electrons present in the outermost shell of an atom governs its bonding behaviour. This is the reason, elements in the periodic table are grouped in such a way that atoms that have the same number of valence electrons are put together at a particular column of the table. As a general rule, the main group element (except hydrogen or helium) tends to react to form a closed shell, corresponding to the electron configuration s2p6 and this tendency is called the octet rule. Sodium or potassium are said to be the most reactive of metallic element because atoms in these elements have only a single valence electron as a result of their ability to lose one valence electron. Such a loss of electron helps to form a positive ion (cation).
Formation of chemical bonds
Chemical bond refers to the force of attraction holding the atoms or ions together in a chemical substance. The elements can react by sharing or transferring an electron(s) from one species to another. In case the atoms react by sharing electrons, they will form a covalent bond. On the other hand, if they combine by transferring electron(s), then an ionic bond will be formed. Scientists have put forth a great number of theories suggesting how chemical bonds are formed. The theories include; electronic theory, valence bond theory, molecular orbital theory, and the valence shell electron pair repulsion theory. The formation of chemical bonds is accredited by the tendency of attaining stability by an element. If you take for example noble gases, they are referred to be the most inert because they have a completely filled outermost orbital with no free electrons to release. It is true that unstable elements have unfilled outermost orbitals and are therefore highly reactive. For this reason, an atom of an element can, therefore, react by each other in order for them to attain the octet or duplet stable configuration.
Valence bond theory
It was proposed by Heitler and London based on the concepts of atomic orbital, electronic configuration of elements, overlapping of atomic orbitals and hybridization of atomic orbitals. The theory describes the electronic structure of molecules where the structure is formed the movement of electrons to fill the atomic orbitals of an atom within a molecule.