What is the atomic number of polonium?

The atomic number of polonium is 84. Every polonium atom has 84 protons in its nucleus, and that proton count is what defines the element.

Polonium was discovered in 1898 by Marie Curie and Pierre Curie while studying pitchblende, a uranium ore. They found evidence of a highly radioactive substance that was chemically distinct from known elements. Marie Curie named it polonium after Poland, her homeland. Its discovery helped establish that radioactivity could reveal previously unknown elements.

Polonium belongs to the chalcogen group, alongside oxygen, sulfur, selenium, and tellurium. It is extremely rare and has no stable isotopes. The best-known isotope, polonium-210, has a half-life of about 138 days and decays by emitting an alpha particle into stable lead-206.

The number 84 is often confused with nearby heavy elements: lead is 82, bismuth is 83, astatine is 85, and radium is 88. Atomic number counts protons; an isotope’s mass number counts protons plus neutrons, so different polonium isotopes still all have atomic number 84.