The atomic number of the element formerly known as Ununpentium is 115.
Ununpentium was the temporary systematic name for element 115, formed from numerical roots meaning “one-one-five.” After its discovery was confirmed, the element received the permanent name moscovium and the symbol Mc. Atomic number counts the protons in an atom’s nucleus, so every moscovium atom has 115 protons.
Moscovium is synthetic and highly radioactive. It was first produced in 2003 by Russian and American researchers at the Joint Institute for Nuclear Research in Dubna, Russia. IUPAC officially recognized the discovery in 2015 and approved the name moscovium in 2016, honoring Moscow Oblast. It belongs to group 15 of the periodic table, alongside nitrogen, phosphorus, arsenic, antimony, and bismuth.
A common mix-up is confusing atomic number with mass number. Moscovium-290, for example, has 115 protons but 175 neutrons; the isotope’s mass number is 290, not its atomic number.