The outermost orbital shell, called the valence shell, is most often involved in chemical bonding. Oxygens valence shell has six electrons but it can hold up to eight electrons. Therefore, oxygen will often combine with other elements that allow it to share or lose two electrons. Atoms with four or more electrons in the valence shell like to gain electrons to fill the shell. Atoms with three or less electrons in their valence shell like to lose electrons to reveal the full inner shell. Atoms with full valence shells will not combine with other elements.