Source code:
@Grab(group='org.openscience.cdk', module='cdk-bundle', version='2.9')
import org.openscience.cdk.interfaces.*;
import org.openscience.cdk.*;
IAtom atom = new Atom("C");
atom.setMassNumber(13)
atom.setNaturalAbundance(1.07)
atom.setExactMass(13.00335484)
Output: