The interesting thing about arsenic is that it is considered to be a monoisotopic element, which is a term used to describe an element that has only one stable isotope.
To get the identity of this stable isotope, pull up a periodic table and look for arsenic,
The number written below the chemical symbol represents the element's average atomic mass. For elements that have more than one isotope, this number tells you the weighted average of the atomic masses of its stable isotopes.
But since arsenic is monoisotopic, that number will tell you directly the atomic mass of its isotope.
This means that arsenic's stable isotope has an atomic mass of
In this case, you have
#74.992 ~~ color(green)(75)#
Therefore, the stable arsenic isotope is arsenic-75,