determine what type of decay occurs when thorium-231 undergoes radioactive decay to form protactinium-231.

²³¹₉₀Th  → ²³¹ ₉₁Pa + ?

 since the atomic number increases  by one while the mass number remain unchanged, the above decay is beta decay, and new equation is as follows

²³¹₉₀Th  →  ²³¹ ₉₁Pa + ⁰_-1e