Adding attributes to Atom class

[pcpatfire]

Hi,
I need to add some more attributes to the Atom class. How can I do it, without changing your Atom and Structures classes?
For instance, I need to add the atomic mass information. I was using settattr to add it to each Atom instance, as follows:

for i in range(atom_num):
    # some code here...
    setattr(atoms[i], 'mass', mass_value)

s = Structure(atoms, lattice=lattice, title=title)

This solution is partially working, because the information is saved in the Atom instance, and I can get it as, for instance, s[5].mass (where 5 is the index of the 6th atom in the list), but s[5:7].mass generates the following error:

AttributeError: 'Structure' object has no attribute 'mass'

Is there a better to fix this issue?
Thank you