Effectiveness is a general term which takes in such criteria as suitability, toxicity, irritancy, etc. For instance, of two similar volatile toxic agents, the one which is toxic at a lower dose can be said to be more effective. Similarly, of two irritant compounds, the one which is irritant at a lower dose can be said to be the more effective. Effectiveness is also dependent on the ability of the population attacked to neutralise or counter the effects of agents once they have been delivered. The duration of effectiveness depends on the physical characteristics of the agent, the amount of agent delivered, the weapon system used and the terrain and weather in the target area at the time the agent is delivered and later.
Nerve agents are organophosphorus esters.
The G agents tend to be non-persistent whereas the V agents are persistent. Sarin is a G-agent and is not as persistent as some of the more modern nerve agents. Nerve gasses on the whole tend not to be gasses at all but a fine suspension of volatile liquids in air. This actually makes them just as effective as a gas but easier to store and to disperse making them a far better weapon.