An element whose $IUPAC$ name is ununtrium $(Uut)$ belongs to

Why does the reactivity of nitrogen differ from phosphorus?

Phosphonic acid $(H_3PO_3)$ is:

Rationalize the given statements and provide chemical reactions:
$1$. Lead $(II)$ chloride reacts with $Cl_2$ to give $PbCl_4$.
$2$. Lead $(IV)$ chloride is highly unstable towards heat.
$3$. Lead is known not to form an iodide,$PbI_4$.

Which statement is wrong for $NO$?

How can you explain the higher stability of $BCl_3$ as compared to $TlCl_3$?