def _GetPrincipleQuantumNumber(atNum):
    """
    Get the principle quantum number of atom with atomic
    number equal to atNum 
    """
    if atNum<=2:
        return 1
    elif atNum<=10:
        return 2
    elif atNum<=18:
        return 3
    elif atNum<=36:
        return 4
    elif atNum<=54:
        return 5
    elif atNum<=86:
        return 6
    else:
        return 7