combining weight vs atomic mass

combining weight and atomic mass both are nouns.

Word	Noun	Adjective	Verb	Adverb
combining weight	Yes	No	No	No
atomic mass	Yes	No	No	No

As nouns, atomic mass is a hypernym of combining weight; that is, atomic mass is a word with a broader meaning than combining weight:

combining weight: the atomic weight of an element that has the same combining capacity as a given weight of another element; the standard is 8 for oxygen
atomic mass: (chemistry) the mass of an atom of a chemical element expressed in atomic mass units

Other hypernyms of combining weight include atomic weight, relative atomic mass.

combining weight (noun)	atomic mass (noun)
the atomic weight of an element that has the same combining capacity as a given weight of another element; the standard is 8 for oxygen	(chemistry) the mass of an atom of a chemical element expressed in atomic mass units