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.
