cesium 137 vs atomic number 55

cesium 137 and atomic number 55 both are nouns.

Word	Noun	Adjective	Verb	Adverb
cesium 137	Yes	No	No	No
atomic number 55	Yes	No	No	No

As nouns, atomic number 55 is a hypernym of cesium 137; that is, atomic number 55 is a word with a broader meaning than cesium 137:

cesium 137: a radioactive isotope of cesium used in radiation therapy
atomic number 55: a soft silver-white ductile metallic element (liquid at normal temperatures); the most electropositive and alkaline metal

Other hypernyms of cesium 137 include caesium, cesium, Cs.

cesium 137 (noun)	atomic number 55 (noun)
a radioactive isotope of cesium used in radiation therapy	a soft silver-white ductile metallic element (liquid at normal temperatures); the most electropositive and alkaline metal