building block vs chain

building block and chain both are nouns.

Word	Noun	Adjective	Verb	Adverb
building block	Yes	No	No	No
chain	Yes	No	Yes	No

As nouns, chain is a hyponym of building block; that is, chain is a word with a more specific, narrower meaning than building block:

building block: a single undivided natural thing occurring in the composition of something else
chain: (chemistry) a series of linked atoms (generally in an organic molecule)

Other hyponyms of building block include chemical chain, couple, unit cell, chemical group, group, radical, molecule.

building block (noun)	chain (noun)
a block of material used in construction work	a series of (usually metal) rings or links fitted into one another to make a flexible ligament
a single undivided natural thing occurring in the composition of something else	a necklace made by stringing objects together
	anything that acts as a restraint
	a linked or connected series of objects
	a series of things depending on each other as if linked together
	(business) a number of similar establishments (stores or restaurants or banks or hotels or theaters) under one ownership
	(chemistry) a series of linked atoms (generally in an organic molecule)
	a series of hills or mountains
	a unit of length

building block (verb)	chain (verb)
	fasten or secure with chains
	connect or arrange into a chain by linking