What is the atomic number of radium?

The atomic number of radium is 88. Its symbol is Ra, and it belongs to group 2 of the periodic table, the alkaline-earth metals. The atomic number counts the protons in an atom's nucleus, so every radium atom has 88 protons.

Radium was discovered in 1898 by Marie Curie and Pierre Curie while they investigated pitchblende, a uranium ore. They found that the ore was more radioactive than uranium itself, suggesting that it contained previously unknown elements. Their work led to the identification of both polonium and radium. The name radium comes from the Latin radius, meaning “ray,” referring to its intense radiation.

A common mix-up is confusing radium's atomic number with its mass number. Radium-226, its most stable naturally occurring isotope, has 88 protons and 138 neutrons, giving it a mass number of 226. Radium-226 has a half-life of about 1,600 years and decays into radon. Because radium behaves chemically like calcium, the body can store it in bones, where its radiation can cause serious harm.