def getFormulaAtoms(atomS, atomN):
    """
    transform a compound list and number in string
    >>> AtomsToCompound(['H','O'],[2,1])
    'H 2 O 1' 
    """
    S = ''
    if len(atomS) == len(atomN):
        for i in range(len(atomS)):
            S = S + ' ' + str(atomS[i]) + ' ' + str(atomN[i])
    return S.strip()
