Question

Isotope A contains 56 protons and 80 neutrons. Isotope B contains 55 protons and 81 neutrons. Isotope C contains 57 protons and 80 neutrons. Isotope D contains 56 protons and 74 neutrons. Which isotopes have the same mass number?

Answer 1

mass number is defined as sum of number of protons and neutrons

so here for Isotope A

Isotope A contains 56 protons and 80 neutrons.

Mass number will be given as

$A = 56 + 80 = 136$

For Isotope B

Isotope B contains 55 protons and 81 neutrons.

$A = 55 + 81 = 136$

For Isotope C

Isotope C contains 57 protons and 80 neutrons.

$A = 57 + 80 = 137$

For Isotope D

Isotope D contains 56 protons and 74 neutrons.

$A = 56 + 74 = 130$

So here Isotope A and Isotope B has same mass number