Atomicraft is all about atoms. In the mod there is an atom for every element of the periodic table. Each atom also has information in its tooltip such as atomic number, atomic mass, and if you hold shift you can access information such as:
What group it is in
Number Of Protons
Number Of Neutrons
Number Of Electrons
There are currently 2 machines in the mod, the atomic centrifuge and the atomic sieve. These models don't rotate properly yet but will do in a later version. The atomic sieve isn't functional in the first version. The purpose of these machines will be to extract certain atoms from certain items or blocks such as the centrifuge, which, when powered can turn an iron ingot into an iron atom.
Now for the second half of the mod. Compoundcraft is included in the mod but is technically a separate mod with its own config file. This is the part of the mod that adds lots of different compounds which can then be used to make other things. I'll give an example, Sedimentary rock can be found underground and when broken has a chance to drop Calcium Carbonate (CaCO3), otherwise known as limestone. This can then be used to make a variety of different decorative blocks that allow for some unique house designs. Compounds are made using the Atomic Construction Chamber which has a grid of slots where atoms can be placed anywhere providing the stack size matches that of the recipe that you are trying to make. For example, Water (H2O) would require 2 hydrogen atoms (in a stack) and one oxygen atom.