Element showing the phenomenon of allotropy is

In the following reaction,assign the correct category for the underlined atom in the product of complete hydrolysis at $R.T.$:
$H_3\underset{-}{P}O_5 + H_2O \longrightarrow H_3PO_4 + H_2O_2$

Mention the starting material to produce silicones.

$A$ solid element $Y$ conducts electricity and forms two types of chlorides: $YCl_n$ (a colorless volatile liquid) and $YCl_{n-2}$ (a colorless solid). To which group does element $Y$ belong?

Non-oxide ceramics can be

$P_4O_{10}$ has short and long $P-O$ bonds. The number of short $P-O$ bonds in this compound is