Interstitial compounds is not formed by 

Interstitial compounds are formed when small atoms such as hydrogen (H), carbon (C), or nitrogen (N) occupy the voids or spaces within the crystal lattice of metals. These compounds are typically associated with transition metals due to their unique crystalline structure and high atomization energy.

Transition metals possess the ability to form interstitial compounds because their lattice structures allow smaller atoms to fit into the interstitial spaces without significantly disrupting the overall crystal framework. This leads to enhanced hardness, strength, and other desirable properties in the resulting compounds.

Why Calcium (Ca) Does Not Form Interstitial Compounds

Calcium (Ca), unlike transition metals, is an alkaline earth metal. Its atomic structure lacks the closed-packed crystalline arrangement typically required for the formation of interstitial compounds. Additionally, calcium has lower atomization energy compared to transition metals, making it unsuitable for forming such compounds. Consequently, calcium does not exhibit the ability to trap smaller atoms like H, C, or N in its lattice structure.

Thus, the correct answer to the question "Interstitial compounds is not formed by" is calcium (Ca).