What is the atomic number for an element whose mass number is 78, which contains 38 neutrons per atom?

Answer 1

protons = atomic number

mass number = protons + neutrons

atomic number = mass number - neutrons = 78-38 = 40

Answer 2

Hello Mae.

Mass number is composed of the number of proton and neutrons in an atom.

Therefore, we can take the mass number and subtract the number of neutrons to determine the number of protons.

78 - 38 = 40 protons

As you probably know, the number of protons for an atom is the atomic number for that element.