Electrovalent compounds are

Which of the following compounds contains an electrovalent (ionic) bond?

$HCl$ gas is covalent and $NaCl$ is an ionic compound. This is because

The melting point of $RbBr$ is $682\,^{\circ}C$ while that of $NaF$ is $988\,^{\circ}C$. The principal reason for this fact is

In the formation of $NaCl$ from $Na$ and $Cl$,which of the following occurs?

An atom of an element $A$ has three electrons in its outermost shell and that of $B$ has six electrons in the outermost shell. The formula of the compound between these two will be