When atoms gain energy, electrons jump to higher orbits and the entire atom moves around more energetically. At set energy levels, the atom changes its physical state of matter from solid to liquid to gas to plasma.
The atom as a whole does not gain energy; all energy is stored in the electrons, which orbit the nucleus in shells. Electrons that are excited or energized move outward from one shell to the next, disrupting the crystalline structure of solids, the least energetic structure, and allows atoms to flow freely in a liquid state. This liquid state is not easily compressible, as it still retains close contact between atoms. When an atom gains more energy, it becomes a gas, which unlike a liquid can be compressed.
In the fourth state of matter, plasma, the electrons in the gaseous atom become so energetic they escape the atom entirely. This hot state of matter is composed entirely of energetic free electrons and ions. The unique properties of plasmas led to the development of plasma televisions, and researchers are investigating the possibilities of using plasma hydrogen, a field of free electrons and bare protons, in nuclear fusion reactors to create clean, abundant energy sources.