The atomic number of polonium, the element discovered by Marie Curie, is 84. That number means every polonium atom has 84 protons in its nucleus.
Marie Curie and Pierre Curie announced polonium in 1898 while investigating pitchblende, a uranium ore whose radioactivity was stronger than its uranium content could explain. Marie named the new element polonium after Poland, her birthplace, which was then partitioned and absent as an independent state.
Polonium is symbolized Po and belongs to the chalcogen group, alongside oxygen, sulfur, selenium and tellurium. It is extremely radioactive and has no stable isotopes. Its best-known isotope, polonium-210, has a half-life of about 138 days.
A common mix-up is confusing polonium’s atomic number with radium’s: radium is 88. Atomic number is determined by proton count, not by an element’s mass or radioactivity.