mercury vs Hg

mercury and Hg both are nouns.

Word	Noun	Adjective	Verb	Adverb
mercury	Yes	No	No	No
Hg	Yes	No	No	No

As nouns, mercury and Hg are synonyms defined as:

mercury and Hg: a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures

Other synonyms of mercury include atomic number 80, hydrargyrum, quicksilver.

mercury (noun)	Hg (noun)
temperature measured by a mercury thermometer	a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures
a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures