Which of the following does not describe nor contain electrons? 1. a subatomic particle having a mass of 0.00055 u 2. a cathode ray 3. a subatomic particle having a charge of −1 4. a γ ray 5. a β particle

Cathode ray - a beam of electrons emitted from the cathode of a high-vacuum tube.

Beta particle - A beta particle, sometimes called beta ray is a high-energy, high-speed electron or positron emitted in the radioactive decay of an atomic nucleus.

It is a subatomic particle indeed with given mass and charge.

So the answer is 4. a Gamma ray
Gamma ray - Gamma rays (also called gamma radiation) are penetrating electromagnetic radiation of a kind arising from the radioactive decay of atomic nuclei. It consists of photons in the highest observed range of photon energy.

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The air is going to come out because of the low pressure.