3. What is the meaning of coordination number?

Explanation:

Its simple

This is the number of atoms, ions, or molecules that a central atom or ion holds as its nearest neighbours in a complex or coordination compound or in a crystal.

for example: Sodium chloride (NaCl) or table salt is an ionic compound. It means that each cation will have exactly one anion associated with it, which means the cation Na+ and the anion Cl− have the same number of coordinates.

It may be really confusing at first

I recommend you go online and watch tutorial  videos on the topic

explanation:

0.07 g/cm³

1.37 g/cm³