For what property you do not get units?
In[97]:= ElementData["Silicon", "AtomicMass"]
Out[97]= Quantity[28.0855, "AtomicMassUnit"]
In[98]:= ElementData["Silicon", "AtomicMass", "Units"]
Out[98]= "AtomicMassUnit"
In[103]:= ChemicalData["Silicon", "MolarMass"]
Out[103]= Quantity[28.0855, ("Grams")/("Moles")]
In[104]:= ChemicalData["Silicon", "MolarMass", "Units"]
Out[104]= "GramsPerMole"