Answer:
Barium-138
Step-by-step explanation:
The mass number is the atomic number + the number of neutrons. ... Identify the atom whose nucleus is composed of 56 protons and 82 neutrons. Barium-138 (the number of protons identifies the atom) Gallium has two naturally occurring isotopes: Ga-69 with a mass of 68.9256 amu and a natural abundance of 60.11% and Ga-71.