Valence is a measure of the combining power of a particular element. Valence (also called valency) is equal to the number of individual chemical bonds that one atom can form. The valence of an atom is determined by the number of electrons in the outermost (valence) shell. The valence of many elements is determined by their ability to combine with hydrogen or displace it in compounds (hydrogen has a valence of 1). For example, one carbon atom combines with four hydrogen atoms to make methane so the valence of carbon is given as 4. In general, the characteristic valence of an element in Group N of the periodic table is N or (8 - N).
A valence electron is an electron that can be actively involved in chemical change. A valence electron is usually an electron in the shell with the highest value of principal quantum number (n). For example, sodium's ground state electron configuration is 1s2 2s2 2p6 3s1; the 3s electron is the only valence electron in the atom. Germanium (Ge) has the ground state electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p2; the 4s and 4p electrons are the valence electrons.
The valence shell is the shell corresponding to the highest value of principal quantum number in the atom. The valence electrons in this shell are on average farther from the nucleus than other electrons, and are often directly involved in chemical reaction.