In the usual classification (the fourteen elements that follow actinium), the last member of the actinoid series is

The actinoids are the fourteen elements after actinium in which $5f$ is filled, from Th (90) to Lr (103) (Ac itself is placed with them in some listings). Lawrencium, Z = 103, has $5f^{14}$ and closes the series; Rf (104) begins the 6d series.