Thallium-201, a radioactive isotope used to image the heart, has a half life of 3.05 days. how long would it take for a sample of thallium-201 to decay to 18% of its original activity?

= 7.55 days

Half life is the time taken by a radioactive isotope to decay by half its original amount.

To calculate the new mass or amount;

New mass = Original mass × (1/2)^n ; where n is the number of half lives.

New mass = 18%, while the original mass = 100 %, half life = 3.05 days

18 = 100 × (1/2)^n

(1/2)^n = 0.18

introducing logarithms;

n log 0.5 = log 0.18

n = log 0.18/ log 0.5

  = 2.474

But 1 half life is 3.05 days

Hence; time taken will be;

= 3.05 × 2.474

= 7.55 days

it's filtration, but this is an assumption so my apologies if i got this

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.