identify the missing isotope in the nuclear reaction below:

from the ²²⁷₈₉Ac → ? + ⁴ ₂He 

an alpha decay will result in lost of four unit from the atomic mass and 2 from the nuclear charge, therefore the resulting atom  will an atomic number of 87 and this corresponds to and atomic mass of 223, the nucleus is francium-223.

²²⁷₈₉Ac → ²²³ ₈₇Fr + ⁴ ₂He 

 the missing nucleus is ²²³ ₈₇Fr.