/// <summary>
/// Calculates the valence number as defined by the number of outer shell electrons minus the total number of hydrogens
/// (implicit and explicit)
/// </summary>
/// <param name="atom"></param>
/// <returns></returns>
public static int ValenceNumber(this OBAtom atom)
{
return(atom.NOuterShellElectrons() - (int)atom.TotalNumHydrogens());
}