The "Chemistry`" context contains mostly internal functions used by the built-in System Chemistry functionality. These functions are undocumented and may change from version to version.
A new and documented context has been created for extra chemistry functions - from minor utilities that don't quite belong in the System context to prototype functions that may some day become part of the language. The ChemistryFunctions paclet is now available from the Wolfram Language Paclet Repository.
To install the paclet use
PacletInstall["WolframChemistry/ChemistryFunctions"]
To load the paclet in a kernel session use
Needs["WolframChemistry`ChemistryFunctions`"]
Doing this will make all of the functions in the paclet available.
Some of the interesting functions include StereoisomersQ, MoleculeManipulate3D, AtomSelector, and EmpiricalFormula. New functionality will be added regularly.