gram atom vs gram-atomic weight

gram atom and gram-atomic weight both are nouns.

Word	Noun	Adjective	Verb	Adverb
gram atom	Yes	No	No	No
gram-atomic weight	Yes	No	No	No

As nouns, gram atom and gram-atomic weight are synonyms defined as:

gram atom and gram-atomic weight: the quantity of an element whose weight in grams is numerically equal to the atomic weight of the element

gram atom (noun)	gram-atomic weight (noun)
the quantity of an element whose weight in grams is numerically equal to the atomic weight of the element	the quantity of an element whose weight in grams is numerically equal to the atomic weight of the element