Boron forms covalent compounds due to:

Aluminium $(III)$ chloride forms a dimer because ........

Give the physical and chemical properties of orthoboric acid $(H_3BO_3)$.

The element which exists in liquid state for a wide range of temperature and can be used for measuring high temperature is :-

$A$ Lewis acid '$X$' reacts with $LiAlH_4$ in ether medium to give a highly toxic gas,'$Y$'. '$Y$' when heated with $NH_3$ gives a compound known as inorganic benzene. '$Y$' burns in oxygen and gives $H_2O$ and '$Z$'. '$Z$' is

Boron trifluoride on reaction with lithium aluminium hydride in ether gives $LiF$,$AlF_3$ and $X$. $X$ on reaction with $NH_3$ gives $Y$. $Y$ on further heating gives a compound $Z$. The number of $\sigma$-bonds and $\pi$-bonds in $Z$ are $x$ and $y$ respectively. $(x+y)$ is equal to