In a one-electron system, the radius of the shortest orbit is given...

The radius of the $n$-th orbit for a hydrogen-like species is given by:

$r_n = \dfrac{52.9 n^2}{Z} \text{ pm}$

For the shortest orbit, $n = 1$. Given that the radius is $18 \text{ pm}$, we can find the atomic number $Z$:

$18 = \dfrac{52.9 (1)^2}{Z}$

$Z = \dfrac{52.9}{18} \approx 2.94$

The closest integer value is $Z = 3$, which corresponds to Lithium. Since it is a one-electron system, the species must be $\text{Li}^{++}$.