Odd though it seems at first, gravity is pulling the cat down while the floor is pushing the cat up - in equal amounts. Forces are absolutely acting on the cat but they balance - so there is no net force.
Proceed with caution before crossing
Since you are referring to the TI-203 and TI-205, you need to know the actual masses of these two isotopes. TI-203 has 202.9723 amu and TI-205 has 204.9744 amu. Since you are concluding that this Thallium have 29.5% (Ti-203) and 70.5% (Ti-205), you need to multiply the percentage to the actual masses of the isotopes. With that, you should be able to get 204.3833 amu
