What are microstates in chemistry?

Microstates are basically the many microscopic ways that constitute how we might see a specific observable. The ensemble average gives us what we observe in real life.

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Let's say that we have macrostate #i#, #ii#, and #iii#. Pretend these have the following (arbitrary) requirements:

• #i#: two electrons on energy level #0#, while one must be in energy level #3#.
• #ii#: electrons must occupy energy levels #0#, #1#, and #2#, but none can be in the same energy level.
• #iii#: electrons must occupy energy level #1# only.

In each of these macrostates, we have microstates that describe the many ways to satisfy the above conditions.

Basically...

• If we observe macrostate #i#, three microstates constitute this particular observed macrostate.
• If we observe macrostate #ii#, six microstates constitute this particular observed macrostate.
• If we observe macrostate #iii#, one microstate constitutes this particular observed macrostate.

So in general, microstates are the microscopic arrangements on the molecular and atomic level that constitute what we observe in real life. If we take the ensemble average, that's what we see in real life.

For example, the ensemble average of the kinetic energies for a set of particles (an ensemble of particles) is the kinetic energy we see in real life, and this average was generated from the microstates of the set of particles that we observe.