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Answer:
If I'm not mistaken the horizontal rows are periods and the vertical columns are familes.
Explanation:
Boron trichloride has a simple molecular structure,
whereas sodium chloride has a giant ionic lattice
structure. The ionic bonding between oppositely
charged ions in NaCl is stronger than the covalent
intramolecular bonding in BCl3. Therefore, less energy
is required to break the intramolecular covalent bonds in
BCl3, hence a lower melting temperature.
The answer is 4.
Gases have low densities, because of the increased space between hight-energy particles.