Answer:
The difference between a solid and a liquid at the molecular level lies in the strength of the intermolecular attraction.
Step-by-step explanation:
Solids and liquids have similar densities, so the tightness of the packing is not the difference.
The difference lies in the strength of the intermolecular (between different molecules) attraction. In solids, the intermolecular attractions are strong enough to hold the same molecules together with their neighbors, despite the random thermal vibrations. The same group of nearest neighbor molecules and second nearest and third nearest, and so on out to very large numbers, stays together. So the molecules are stationary, except for vibrations around an average position. That's solid.
the intermolecular forces are strong enough in liquids to keep nearest neighbors together only for short times before the thermal vibrations break apart the groupings. Thus the molecules are always close to each other, but don’t stay next to any particular other molecules, so the whole collection of it can easily flow, but with a specific volume. That’s a liquid.
In a gas, the intermolecular forces are too weak to hold the molecules near each other and the thermal random motion quickly splits up any short-term pairs. So it flows easily with no specific volume - the molecules are all approximately independent from each other.
In ionic solids, there are no specific molecules, but replace “molecules” above with “ions of opposite charge”