alkaline-earth metal vs alkaline earth

alkaline-earth metal and alkaline earth both are nouns.

Word	Noun	Adjective	Verb	Adverb
alkaline-earth metal	Yes	No	No	No
alkaline earth	Yes	No	No	No

As nouns, alkaline-earth metal and alkaline earth are synonyms defined as:

alkaline-earth metal and alkaline earth: any of the bivalent metals of group II of the periodic table (calcium or strontium or barium or magnesium or beryllium)

alkaline-earth metal (noun)	alkaline earth (noun)
any of the bivalent metals of group II of the periodic table (calcium or strontium or barium or magnesium or beryllium)	any of the bivalent metals of group II of the periodic table (calcium or strontium or barium or magnesium or beryllium)