Which of the following species is not stable?

What are allotropes? Sketch the structure of two allotropes of carbon namely diamond and graphite. What is the impact of structure on physical properties of two allotropes?

The order of catenation power is $Ge < Si < C$. What are the $C-C$,$Si-Si$,and $Ge-Ge$ bond energies (in $kJ/mol$) respectively?

Correct statements among $(i)$ to $(iv)$ regarding silicones are:
$(i)$ they are polymers with hydrophobic character
$(ii)$ they are biocompatible
$(iii)$ in general,they have high thermal stability and low dielectric strength
$(iv)$ usually they are resistant to oxidation and used as greases.

Which of the following salts cannot exist?

$A$ major constituent of Portland cement,except lime,is: