Which non-metals have high melting and boiling points? State them.

Explain the following:
$(a)$ $CO_2$ is a gas whereas $SiO_2$ is a solid.
$(b)$ Silicon forms $SiF_6^{2-}$ ion whereas corresponding fluoro compound of carbon is not known.

The element whose hydride contains the maximum number of hydrogen atoms per atom of the element is

An oxide of an element is a gas and dissolves in water to give an acidic solution. The element belongs to:

The number of six-membered carbon rings and five-membered carbon rings present in Buckminsterfullerene are $x$ and $y$ respectively. The sum of $x$ and $y$ is:

The plague of tin is the: