Ununpentium is the temporary name of a synthetic

Synthetic element

In chemistry, a synthetic element is a chemical element that is too unstable to occur naturally on Earth, and therefore has to be created artificially. So far 30 synthetic elements have been discovered—that is, synthesized...

 superheavy element in the periodic table

Periodic table

The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...

 that has the temporary symbol Uup and has the atomic number

Atomic number

In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...

 115.

It is placed as the heaviest member of group 15 (VA) although a sufficiently stable isotope is not known at this time that would allow chemical experiments to confirm its position.
It was first observed in 2003 and only about 30 atoms of ununpentium have been synthesized to date, with just 4 direct decays of the parent element having been detected.