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Formation
Valence electrons are formed when the inner electron shells are filled and the last few electrons are placed on the outermost shell. For example, oxygen has eight total electrons. The first two electrons are placed in the inner shell or the lowest energy shell, because it can hold only two electrons. The second, or outermost, shell in oxygen holds six electrons. These six electrons are oxygen's valence electrons.
Periodic Table
How an element's valence electrons are formed will determine where the element is on the periodic table. Within the periodic table are "periods" and "groups." Periods are the horizontal rows of elements, while the groups are the vertical columns. While all the elements in a period have the same number of electron orbitals, the elements in a group have the same number of valence electrons in that outermost orbital. The number of valence electrons increases from left to right on the periodic table, but stay the same going up or down in a group.
Transition Metals
The electrons contained in the transition metals behave differently than the other elements. At times, these electrons act like typical valence electrons, while at other times they act like shielding electrons. Shielding electrons are those electrons on the energy levels between the nucleus and the valence electrons. They are called shielding electrons because they protect the actual valence electrons from the positive charge in the nucleus. The valence electrons in transition metals can be found in more than one electron shell. These valence electrons' outer two electron shells have the ability to bond with other metals. Because the outer two shells in these transition metals are not holding the maximum amount of electrons, these metals have the ability to bond with several elements in a variety of shapes. This allows those two outer shells to become full, thus making a stable compound.
Valence Electron Octet
Elements in Group 8 of the periodic table have the eight-electron limit in the outermost electron shell, which follows the "octet rule." The octet rule states that elements must gain or lose electrons to form an outer electron shell that has eight valence electrons. However, because these Group 8 elements already have the eight valence electrons, they are not a reactive group when it comes to forming compounds.