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.
Chemistry: A Volatile History
Because this is the number of protons, the atomic number must be a whole number – there cannot be any fractional values. Moseley realised it was the atomic number, not the atomic weight that determines the order of the elements. What's more, because the atomic number increases in whole numbers from one element to the next there can be no extra elements between Hydrogen (atomic number 1) and Uranium (atomic number 92) – there can only be 92 elements, there is no room for any more.