Answer:
42
Explanation:
Atoms have a neutral charge overall, which means they have an equal numbers of positively-charged protons and negatively-charged electrons. Here, an atom of arsenic with 33 electrons must also have 33 protons.
The mass of any atom is contained in its <em>nucleus</em>, a part made up of protons and neutrons - particles with no charge. Both protons and neutrons are exactly 1 atomic mass unit each, so to find the atomic mass of an atom, we can use the formula
(# of protons) + (# of neutrons) = atomic mass
or equivalently:
atomic mass - (# of protons) = (# of neutrons).
For arsenic, we have an atomic mass of 75 amu and 33 protons, so using our formula:
# of neutron = 75 - 33 = 42 neutrons
