A chemical element, symbol As, atomic number 33. Arsenic is found widely distributed in nature (approximately 5 × 10⁻⁴ of the Earth's crust). It is one of the 22 known elements composed of only one stable nuclide, ₃₃⁷⁵As; the atomic weight is 74.92158. There are 17 other radioactive arsenic nuclides known.