Assertion: The interstitial compounds formed by transition metals are hard and rigid.

Reason: The non-metals like hydrogen, carbon, nitrogen are taken up in vacant spaces (voids) in the lattices of transition metals. 

Assertion and Reason both are correct statements and reason is the correct explanation for assertion.

Transition metals have unique ability to react with small atoms of nonmetals to produce interstitial compounds which are hard and rigid because the nonmetals like hydrogen, carbon, nitrogen are taken up in vacant spaces in the lattices of transition metals.