From the given observations,
You can see that as the concentration is doubled, half-life is halved.
That is,half-life is inversely proportional to concentration
As t( half-life) ~ 1/a^(n-1)
For this case n = 2,second order reaction.
R = k X a^n
Using the above formula you will get the rate and rate constant.
Answer:
I don't know 100% but im pretty sure its electrons, if im wrong im really sorry let me know in the comments ill change it
explanation
the nucleus has more weight but its more compact but the electrons are spread apart circling the nucleus and therefor take up more space
