Thank you for the opportunity to help you with your question!

The element Arsenic (As) has an atomic # of 33, Atomic mass of 75 (74.92 if you need the non-rounded version), 33 protons, 42 neutrons (if you take the atomic mass and subtract the atomic number, you get the number of neutrons), and 36 electrons (in a normally balanced element that is not positive or negative, there are the same number of electrons as there are protons. To make a negative charge, you add the number of electrons that the charge says, so I added three electrons. If it were a positive ion, we would subtract from the number of electrons.)

The following link may help in the future:
http://education.jlab.org/qa/pen_number.html

Please let me know if you need any clarification. I'm always happy to answer your questions.