Problem 1
The text (from the screenshot you posted) mentions that
"The atoms of covalent materials are bound tightly to each other in stable molecules, but those molecules are generally not very strongly attracted to other molecules in the material"
Put another way, we can think of each atom as a person, and each molecule as a family or a collection of persons. This is because any molecule is a collection of atoms. In covalent molecules, the connection between any two families isn't that strong since each person is focused more on their respective family, instead of other people outside the family. This is one way to think of it, but you may have a more effective way to remember.
In short, the exterior bonds between different covalent molecules isn't that strong, which means it's easier to break those bonds.
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Problem 2
The text also says
"The atoms (ions) in ionic materials show strong attractions to other ions in their vicinity"
This is due to the positively and negatively charged ions that attract to one another, and this causes a stronger chain so to speak. The stronger the chain, the harder it is to break when you apply temperature to it. Keep in mind that the only difference between a solid and a liquid is that the solid is a rigid structure while a liquid is more loose and free flowing. The ionic materials having a stronger inner bond like this is why ionic materials have higher melting points. You need to apply more temperature than normal to get the solid to melt.
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Problem 3
Again we turn to the text and it says
"For example, the molecule carbon tetrachloride is a non-polar covalent molecule, CCl4. It's melting point is -23 degrees Celsius. By contrast, the ionic solid NaCl has a melting point of 800 degrees Celsius"
In short, the difference between the two is that carbon tetrachloride has covalent bonds and NaCl (sodium chloride, aka table salt) has ionic bonds. It's much easier to melt the carbon tetrachloride than it is to melt the table salt.
Side note: carbon tetrachloride was historically used in fire extinguishers, but have been phased out due to safety concerns. Exposure to this toxin can lead to central nervous system problems, kidney failure, or liver problems. Due to its very low melting point, carbon tetrachloride is a liquid at STP.
