Hg vs mercury

Hg and mercury both are nouns.

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

As nouns, Hg and mercury are synonyms defined as:

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

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

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