How many neutrons are in an arsenic atom? I belive there are 42, but better safe than sorry!

Arsenic is Element Number 33 ... it has 33 protons in its nucleus.

You're correct that the stable isotope of the atom is the one with

42 neutrons, for a total Atomic Weight of 75.

Atoms of Arsenic also exist with 41 neutrons and 40 neutrons

in their nucleus, but those are unstable.

The nucleus with 41 neutrons (Atomic Weight = 74) has a 50% chance

of falling apart and becoming a nucleus of Germanium within 80 days.

The nucleus with 40 neutrons (Atomic Weight = 73) has a 50% chance

of falling apart and becoming a nucleus of Germanium within 18 days.