∞ generated and posted on 2016.12.18 ∞
Means by which atoms participate in chemical bonds.
These are the electrons found in an atoms outer electron shell and therefore which are available to interact with the valence electrons associated with other atoms.
Note that not all of the valence electrons necessarily will participate in bonding but instead, in many cases, it is just a subset of these electrons that will. With oxygen, for example, though there are six valence electrons, but typically only two of those electrons will participate in a chemical bond. Thus, the number of valence electrons that an atom possess is usually not identical to the valence that the atom displays.
Figure legend: Shown is a depiction, of sorts, of water. Oxygen possess six valence electrons, all in the atom's outer electron shell (and as shown in white). Each hydrogen atom possesses a single valence electron (also white). Two of oxygen's valence electrons and one valence electron from each of the hydrogens are participating in a total of two covalent bonds. Note that oxygen's inner electrons are not valence electrons and are not participating in chemical bonds. Note also that oxygen possesses eight protons (P) and eight neutrons (N) while hydrogen possess only a single proton.
The number of valence electrons for carbon is 4 (as so too is carbon's valence; noting that 4 + 4 = 8). Oxygen, as noted, as six valence electrons but, also as noted, has a valence of just 2 (and 6 + 2 also equals 8). Hydrogen, by contrast, has only a single valence electron and also a valence of 1 (which adds up to 2, which for hydrogen works as well as adding up to 8 works for both carbon and oxygen).
For additional discussion, see [Bioport].