Chapter 2 - Atoms, Molecules, and Ions - Exercises - Page 77: 2.58c

Charge: -3

Atoms of elements want to have the electron configuration of the nearest noble gas (in terms of number of electrons) on the periodic table. The closest noble gas to Arsenic (As) is Krypton, so, in order to gain the electron configuration of Krypton, an Arsenic atom must gain 3 electrons, thus making the most common charge of an Arsenic ion -3.