What is the atomic number for an element whose mass number is 78, which contains 35 neutrons per atom

Since the sum of neutrons and protons are equal to the mass number but the number of protons is equal to the atomic number of an element. Since the element has 35 neutrons and we’re trying to find protons we’re going to use the given mass number and neutron number to find the number of protons.

Mass number = neutrons + protons
78 = 35 + protons
78 - 35 = -35 + protons
43 = protons

Protons = atomic number
The atomic number of the element is 43