Complex sentence: Neither the most stable isotope astatine-210, nor the medically useful astatine-211, occur naturally; they can only be produced synthetically, usually by bombarding bismuth-209 with alpha particles.
Split into simpler sentences: Neither the most stable isotope astatine-210 nor the medically useful astatine-211 occur naturally. They can only be produced synthetically, usually by bombarding bismuth-209 with alpha particles.