Answer: 243 hours
Explanation:
Expression for rate law for first order kinetics is given by:

where,
k = rate constant
t = age of sample
a = initial amount of the reactant = 
a - x = amount left after decay process = 
a) for completion of half life:
Half life is the amount of time taken by a radioactive material to decay to half of its original value.


b) for reducing the mass to 10.00 % of its original mass



The time taken to reach 10.00 % of its original mass is 243 hours
Answer:
Correc option: 
Explanation:
size of atom : it says somthing about how many shell present in a particular atom or ion and it can also be evaluated on the basis of radius of atom.
Br^- and Kr has highest number of shell as compared to other group of species .
Na ,S , Mg ,P all are from 3rd period but Kr and Br^- in the 4th period so size of species of this group will more,
Size increases on increasring the shell number
Explanation:
<h2>see the attachment. hope it will help you... </h2>