The three-dimensional shape of a molecule depends on the number of electron groups around the central atom. Because like charges repel, the molecule adopts a shape that allows the electron groups to be as far apart as possible. Very often, a two-dimensional dot structure does not accurately represent what the molecule would look like in three dimensions. Match each two-dimensional structure to its correct three-dimensional description.

it is the minimum amount of material needed to sustain a fission reaction.

critical mass is the minimum amount of fissionable material that must be assembled for a sustained fission reaction to take place. although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. if the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.   at the point where the chain reaction can become self-sustaining, this is referred to as critical mass.