Li++ The radius of the nth orbit in one electron system is given by rn​=Zn2a0​​ Here, a0​=53pm For the shortest orbit,n=1For hydrogen,Z=1∴ Radius of the first state of hydrogen atom =53pm For deuterium,Z=1∴ Radius of the first state of deuterium atom =53pm For He+ Z=2∴ Radius of He+ atom =253​pm=26.5pm For Li++ Z=3∴ Radius of Li++ atom =353​pm=17.66pm≈18pm The given one-electron system having radius of the shortest orbit to be 18pm may be Li++ .