A hypothetical compound is a chemical compound that has not been made or found by chemists. Hypothetical compounds can have their chemistry predicted by simulations, or by extrapolation or interpolation from researching related compounds.
Common types of hypothetical compounds are extreme ends of a family of known compounds. An example is the azines: pyridine, all three diazines, all three triazines, and 1,2,4,5-tetrazine are all known, but the other two tetrazines as well as pentazine and hexazine are hypothetical. Scientists can predict the properties of these compounds based on the other azines and their derivatives, but the compounds themselves have not been made.
A hypothetical element is a chemical element that has not been made or found. Most hypothetical compounds do not have hypothetical elements, but are instead made of known elements with a hypothetical structure. Many hypothetical compounds in organic chemistry are made of the four most common organic elements.
Historically, most elements were found as compounds before the element was purified. These non-hypothetical compounds, usually ores of transition metals or rare earths containing lanthanides, needed hypothetical elements in order to explain their properties. Until those elements were purified, these were non-hypothetical compounds of hypothetical elements. Nowadays, all compounds of hypothetical elements are also hypothetical, because the remaining hypothetical elements are not expected to exist in nature.