Why are solids generally denser than liquids and gases? Are there any exceptions to this rule?

Solids are generally denser than liquids and gases because their particles are tightly packed and held in fixed positions, occupying the least amount of space for a given mass. This close packing results in a higher mass per unit volume. 

Gases, with their widely spaced particles, are the least dense. Liquids have particles closer than gases but less organized than solids. An important exception to this rule is water: ice (solid water) is less dense than liquid water, which is why ice floats on water.