Uranium-238 decays very slowly. its half-life is 4.47 billion years.
what percentage of a sample of uranium-238 will remain after 13.4 billion years?
i have to test after 3 !

12.53 % of a sample of Uranium-238 will remain after 13.4 billion years.

Explanation:

The half life of the uranium-238 = =4.47 billion years

All the radioactive reaction are of first order kinetics. The rate constant and t half of the reaction are related as:

where,  

k = rate constant =

t = time taken during radio decay  = 4.47 billion years

= initial amount of the reactant =

[A] = amount left left after time t.

12.53 % of a sample of Uranium-238 will remain after 13.4 billion years.

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

it forms an anion that has a larger radius. anions have a negative charge and are larger than the original atom.