Question

Question 8: An atom gallium has 31 protons and a mass number of 70. What is the atomic number for an atom of gallium?

A: 70

B: 31

C: 35

D: 39

I was thinking maybe 70, but I am not sure at all or as to why 70.
And could you explain its the answer that it is?

Answer 1

The atomic number is based on the number of protons of the element, if the protons change for example to 34 protons then it is not Ga (gallium) anymore but Selenium.
70 in this case is your atomic weight or mass which is the amount of protons and neutrons so Gallium has 39 neutrons + 31 protons = 70

Answer 2

atomic number = # of protons
atomic mass/weight & mass number are the same thing = # of protons + # of neutrons