Question

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

Answer 1

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.