Answer: the second and the third represent isotopes of bromine.
Explanation:
1) Isotopes are elements with same atomic number (Z) and different number of neutrons (N)
The atomic number is the number of protons.
2) A is the mass number and it accounts for the sum of the protons (Z) and neutrons (N).
This is A = Z + N.
3) From a periodic table you get the atomic number of bromine, Z = 35.Remember that is the number of protons, and all the isotopes of bromine have the same atomic number.
4) Let's see at the atomic numbers for each choice:
i) Z = 79, A = 196
This Z is not 35, so it is not an isotope of bromine (it is an isotope of gold)
ii) Z = 35, A = 79
This, indeed, is the isotope of bromine with mass number 79. The information indicates:
Z = atomic number = 35 ⇒ element bromine
A = mass number = 79
N = number of neutrons = A - Z = 79 - 35 = 44 neutrons
iii) A = 79, N = 44
Calculate Z = A - N = 79 - 44 = 35, which indicates it is an isotope of bromine.
iv) Z = 44, N = 44
Z = 44, already states it is not bromine. It is another element, which you can find in the periodic table. It is Rutenium.
Answer : There are two isotopes of Br 79 and Br 81.
So for Br-79 Z = number of protons = 35
N = number of neutrons = 44
A = mass number (Z+N) = 79
Now, for Br-81;
Z = number of protons = 35
N = number of neutrons = 46
A = mass number (Z+N) = 81.
