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.
dircynavni
x1 is a quantity of/contains/is made of radon (Rn).
fangynavni
x1 is a quantity of/contains/is made of xenon (Xe).
laznynavni
x1 is a quantity of/contains/is made of argon (Ar).
mipnavni
x1 is a quantity of/contains/is made of krypton (Kr).
ninynavni
x1 is a quantity of/contains/is made of neon (Ne).
ratmidju
x1=m1 is the nucleus of atom x2=m2=r1.
ratyjunla
x1=j1 is an atomic clock measuring time units x2=j2 to precision x3=j3 by using atoms of element/atomic number x4=r2 of isotope number/atomic weight x5=r3
ratypo'i
r1=p1 undergoes nuclear fission
xumsle
s1 is a molecule of chemical x2.
rakle
x1 is an atomic element in group x2 [usually, vertical column; denotes electron configuration and, thereby, chemical similarity with vertical neighbors] and period x3 [usually, horizontal row; denotes similarity in size with horizontal neighbors, as well as having the same number and type of electron shells as them] and belonging to other 'class'/'category'/'type'/having other properties x4 according to scheme/organization pattern/standard/periodic table x5.
de'i'a (exp!)
in the Nth century.
ratniklesi
x1 is an atomic element in group x2 [usually, vertical column; denotes electron configuration and, thereby, chemical similarity with vertical neighbors] and period x3 [usually, horizontal row; denotes similarity in size with horizontal neighbors, as well as having the same number and type of electron shells as them] and belonging to other 'class'/'category'/'type'/having other properties x4 according to scheme/organization pattern/standard/periodic table x5.
ratrni
x1 is a bound state of constituents x2 with overall/emergent/systemic properties/qualities/configuration (of the bound state) x3
rartni
x1 is a bound state of constituents x2 with overall/emergent/systemic properties/qualities/configuration (of the bound state) x3