Which metal is protected by a layer of its own oxide?

Aluminium reacts with caustic soda to form

The number of $P-H$ bond$(s)$ in $H_3PO_2, H_3PO_3$ and $H_3PO_4$,respectively,is

Two elements $X$ and $Y$ combine separately with a highly electropositive metal to form binary compounds,which upon hydrolysis yield borane and silane,respectively. What are $X$ and $Y$?

Explain the following:
$(1)$ Aluminum forms $[AlF_6]^{3-}$ ion but boron does not form $[BF_6]^{3-}$ ion.
$(2)$ $PbX_2$ is more stable than $PbX_4$.

The acidic strength of boron trihalides follows the order: