ratni -rat- gismu

x1 is an atom of element/atomic number x2 of isotope number/atomic weight x3.

(cf. kantu for a basic unit of a property or activity; selci for a basic physical subunit, generally of a mass)

In notes:

kantu
x1 is a quantum/ray/elementary particle/smallest measurable increment of property/activity x2.
selci
x1 is a cell/atom/unit/molecule of x2; x1 is an indivisible, most basic subunit of x2.
cmuxu'i
x1 is an element of kind x2 with purity x3.
ratmidju
x1=m1 is the nucleus of atom x2=m2=r1.
xumsle
s1 is a molecule of chemical x2.