Hi, I am having trouble determining the correct answer for this question.

Question

Which one of the following decay modes has not been observed? A)neutron emission B)Positron emission C)Alpha emission D) electron capture

I don't know what does it mean be "observed." If I had to guess which one is the answer, I would choose A, because it is the only one where the "Z" or atomic number doesn't change.

I don't know if this is a "trick" question or not. I have read and reread the question looking for a loop hole but I can't find it. The fact is that electron capture is common, alpha emission is common, and positron emission occurs although it isn't that common. In the context of the sentence, I suspect answer A is the correct one for the prof, however, my references list neutron emission from the nucleus for all element of Z=92 and elements above 92. The word observed may be the sticking point. Perhaps we can't observe this because it is splitting of the atom and all we see is a mushroom cloud. :-). My references show this for all transuranium elements.

Why you think a toxic, green gas which is very reactive with other metals and some non-metals is a element or a compound??

I think I am picking at straws, but "a toxic, green gas which is very reactive with other metals...... says to me it is a metal although my first reaction is that it must be chlorine gas (a non-metal). However, since the problem says other metals that means it must be a metal and I don't know of any metals that are a gas (unless of course it is a metal being boiled and we are talking about the vapor and the temperature isn't stated). Even then, however, I think I am batting zero; therefore, I say the "other metal" material must be a compound since it can't be a metal. Frankly, I think the question is flawed, assuming you copied the question exactly as it was given to you.