The ability to count correctly the number of electrons around any given atom is central to being able to write a plausible reaction mechanism in organic chemistry. In an atom, any charge that is present may easily be determined by the difference between the number of electrons that are accommodated in the orbitals around it and the number of protons within the nucleus. A molecular orbital, like an atomic orbital, can accommodate only two electrons. The charge on any given atom can be calculated according to the following formula. First, find the number of electrons that would surround the uncharged, isolated atom in its valence shell. Then, subtract the total number of electrons that are actually in the valence shell of that atom within the molecule. Finally, add half the number of electrons that atom shares with any other atom in the molecule. The result is the charge on that atom within the molecule.