Which element has the highest atomic number?
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.
Hesperium
Hesperium (also known as esperium; atomic symbol Es) was the name assigned to the element with atomic number 94, now known as plutonium.
It was named in Italian "Esperio" after a Greek name of Italy, Hesperia, "the land of the West".
The same team assigned the name ausonium to element 93, after Ausonia, a poetic name of Italy. By comparison, uranium, the heaviest of the primordial elements, has atomic number 92.