The chemical symbol for polonium is Po.
Polonium is element 84 on the periodic table, and its symbol comes from the first and third letters of its English name. Like other official element symbols, it uses a capital first letter followed by a lowercase second letter. That makes “Po” correct, while “P” is not available for polonium and “Pl” is not an official element symbol.
Marie and Pierre Curie discovered polonium in 1898 while investigating the radioactivity of pitchblende, a uranium ore. Marie Curie named it after Poland, her homeland, which at the time was under foreign rule. The discovery helped establish the idea that radioactivity was an atomic property rather than merely a chemical reaction.
Polonium is highly radioactive, and polonium-210 is its best-known isotope. It occurs naturally in tiny quantities in uranium ores and has also been used in specialized industrial devices. It should not be confused with potassium, whose symbol is K, or phosphorus, whose symbol is P.