Answer
(a) Iodine (I).
(b) Krypton (Kr).
Work Step by Step
(a) Since we are talking about an atom, the number of electrons is equal to the atomic number.
Using a periodic table, we can see that the element with $Z = 53$ is the "Iodine" (Which is a halogen (Group 7A or 17))
(b) "A" represents the atomic weight.
Using a periodic table, we can see that the element with an average atomic weight of approximately 84 is the $Kr$, which is indeed a noble gas (Group 8A or 18)
