2021 O Level Chem P1 Q5 — Atomic Structure / Isotopes Solution

Answer: B. Isotopes of the same element have the same number of protons but different numbers of neutrons, and therefore different nucleon (mass) numbers. The atom given is \(^{81}_{35}\text{Br}\): 35 protons and nucleon number 81, so 46 neutrons. A different isotope of bromine must still have 35 protons but a different nucleon number.

Row D has 44 protons, so it is not bromine at all. Row C (35 protons, nucleon number 81) is the same isotope as the one given, not a different one. Row A is impossible: 35 protons + 44 neutrons should give a nucleon number of 79, not 44.

Row B has 35 protons and 44 neutrons, giving nucleon number \(35 + 44 = 79\). This is a genuine different isotope of bromine, \(^{79}_{35}\text{Br}\) (bromine-79).