Answer:
Ba has the largest atom size
Do you mean; " A blank is a substance that is made by combining two elements?" (I'm not sure if substances is the right term)
If so then the blank is a compound.
Answer:
Where SiO2 has strong covalent bonds between molecules, SO3 only has weak van der waals intermolecular forces holding its molecules together. Since covalent bonds require more energy to overcome than van der waals, SiO2 requires a higher temperature than SO3 to melt, thus SiO2 has a higher melting point.
A tall, horizontally subdivided or packed container for fractional distillation in which vapor passes upward and condensing liquid flows downward. The vapor becomes progressively enriched in more volatile components as it ascends, and the less volatile components become concentrated in the descending liquid, which can be drawn off.