Radon.
The radioactive gas with 136 neutrons is Radon-220, also known as thoron. It is a naturally occurring radioactive gas that is part of the decay chain of uranium-232. Thoron is produced by the decay of radium-224 and has a half-life of about 55 seconds.
If the gas is neutral this would mean the atom would also contain 136 protons, thus its atom number would be 136. Such an element has never been made (it does not occur naturally) and it would probably be unstable.
Oxygen
oxygen
The only element in nitrogen family with 16 neutrons is the element Phosphorus (symbol: P). However it is a solid at room temperature and not gas.
Oxygen is the only element with 8 protons.
Oxygen isotope 16
The element with 18 protons and 22 neutrons is argon. Argon has an atomic number of 18, which corresponds to the number of protons, and an atomic mass of 40 (approx.), which is the sum of protons and neutrons.
Krypton.
Ununoctium has 118 protons, 118 electrons and 176 neutrons.
Atomic Mass is the count of how many protons and neutrons an element contains. So, there is not really an answer to your question. It would depend upon what elements the gas is made up of.
The element is argon and the isotope is 18Ar40