Question

What is the difference among the states of matter at the atomic level?

Answer 1

States of Matter
Gases, liquids and solids are all made up of microscopic particles, but the behaviors of these particles differ in the three phases.
Note that:

Particles in a:
gas are well separated with no regular arrangement.
liquid are close together with no regular arrangement.
solid are tightly packed, usually in a regular pattern.
Particles in a:
gas vibrate and move freely at high speeds.
liquid vibrate, move about, and slide past each other.
solid vibrate (jiggle) but generally do not move from place to place.
Liquids and solids are often referred to as condensed phases because the particles are very close together.
The following table summarizes properties of gases, liquids, and solids and identifies the microscopic behavior responsible for each property.

Some Characteristics of Gases, Liquids and Solids and the Microscopic Explanation for the Behavior
gas liquid solid
assumes the shape and volume of its container
particles can move past one another assumes the shape of the part of the container which it occupies
particles can move/slide past one another retains a fixed volume and shape
rigid - particles locked into place
compressible
lots of free space between particles not easily compressible
little free space between particles not easily compressible
little free space between particles
flows easily
particles can move past one another flows easily
particles can move/slide past one another