Even with atmosphere, the fact that gases are much denser than plasma means that plasma won't penetrate very far through air - think of shooting compressed air underwater.
which is very advanced technology in its own right (and subject to that very same technology being used to disrupt the magnetic bottle of the weapon). Even without atmosphere, it spreads out and becomes useless more than a fraction of an inch from the source, making it useful as a cutting tool but not a (ranged) weapon unless you can practically project a magnetic bottle to contain the plasma until it hits the target. Unfortunately, in real life, plasma's properties make it more than a bit impractical. Naturally some people have thought of weaponizing it.
states of matter, a step up from gas where the electrons become so energized that they break free of their orbits. Plasma is one of the four traditional note If you're interested, non-traditional ones include stuff like Glass, Liquid Crystal, Degenerate matter and several other more exotic forms.