Oganesson
Oganesson has the highest atomic number and highest atomic mass of all known elements. The radioactive oganesson atom is very unstable, and since 2005, only five (possibly six) atoms of the nuclide Og have been detected. Although this allowed very little experimental characterization of its properties and possible compounds, theoretical calculations have resulted in many predictions, including some surprising ones. For example, although oganesson is a member of group 18 – the first synthetic element to be so – it may be significantly reactive, unlike all the other elements of that group (the noble gases). It was formerly thought to be a gas under normal conditions but is now predicted to be a solid due to relativistic effects. On the periodic table of the elements it is a p-block element and the last one of the 7th period.
Henry Moseley
In addition, Moseley showed that there were gaps in the atomic number sequence at numbers 43, 61, 72, and 75. These spaces are now known, respectively, to be the places of the radioactive synthetic elements technetium and promethium, and also the last two quite rare naturally occurring stable elements hafnium (discovered 1923) and rhenium (discovered 1925). Nothing was known about these four elements in Moseley's lifetime, not even their very existence. Based on the intuition of a very experienced chemist, Dmitri Mendeleev had predicted the existence of a missing element in the Periodic Table, which was later found to be filled by technetium, and Bohuslav Brauner had predicted the existence of another missing element in this Table, which was later found to be filled by promethium. Henry Moseley's experiments confirmed these predictions, by showing exactly what the missing atomic numbers were, 43 and 61. In addition, Moseley predicted the existence of two more undiscovered elements, those with the atomic numbers 72 and 75, and gave very strong evidence that there were no other gaps in the Periodic Table between the elements aluminium (atomic number 13) and gold (atomic number 79).