electron gas
/ ee-LEK-tron gas /
Inside a metal, the conduction electrons are so numerous and so loosely tied that physicists treat them as a gas — not a gas of air molecules, but a gas of electrons sloshing freely through the rigid scaffold of atoms. The atoms hold still; the electron gas flows around them, fills the metal, and carries current and heat.
Unlike ordinary air, this is a quantum gas: its particles refuse to share states, so they stack up to the Fermi energy instead of all slowing down when cold. That single rule makes the electron gas behave nothing like the air in a balloon — it's dense, energetic even at absolute zero, and barely warms when you heat it. Physicists call this special kind a Fermi gas.
It matters because "electron gas" is the working object of the whole free-electron model — when people compute a metal's conductivity, heat capacity, or magnetism, they're really doing arithmetic on this gas. The honest caveat: the word "gas" implies the electrons ignore each other, but real electrons repel one another strongly; that they still act almost like a simple gas is a deep and partly lucky fact, later explained by Fermi liquid theory.
A grain-of-sand-sized speck of copper holds roughly a hundred billion billion conduction electrons, all sloshing as one electron gas. It's denser than any ordinary gas you could squeeze into a bottle, yet it flows through the metal almost without friction until something scatters it.
Even a sand-grain of copper holds a colossal electron gas, far denser than ordinary air.
When the electrons' mutual repulsion really can't be ignored, physicists upgrade the name to "electron liquid" to flag the strong interactions. The free-electron model is the "gas" idealisation, where those interactions are swept under the rug.