What is system international unit of atomic radius ?

As far as I know, the picometer is currently being used as the SI unit for atomic radii.

A picometer is equal to ##10^(-12)"m"##.

The more traditional unit used was the angstrom, for which you have

##"1 angstrom" = 10^(-10)"m"##

Keep in mind, however, that angstroms are not SI units.

You'll sometimes see atomic radii expressed in nanometers, for which

##"1 nm" = 10^(-9)"m"##

However, nanometers are not used as often as picometers for atomic radii.