The Periodic Table of Elements is very useful in SpaceChem as a reference guide for determining what processes to use to solve a puzzle. The name of each element is at the bottom of each box, with the element symbol in large letters above the name. In the upper left corner is the atomic number which is useful in fission and fusion applications. The number of maximum bonds is in the upper right corner, and this is useful when creating various molecules.
In chemistry, the maximum number of bonds for an atom type (chemical element) is dictated by its number of outer electrons (valence electrons).
This is possible to tell by the column number inside the Periodic Table. This is why atom types in the same column often have a similar number of maximum bonds.
Last column, on the right, are noble gases / inert gases. They usually do not make any bonds and do not take part in chemical reactions. Thus their name: inert = inactive.