What element has the symbol 'Rb'?

The story behind the answer

Rb is the symbol for Rubidium, a soft, highly reactive alkali metal (atomic number 37). The other choices have different symbols: Rhenium = Re, Radon = Rn, Ruthenium = Ru.

Rubidium was discovered in 1861 by Bunsen and Kirchhoff using early spectroscopy; its name comes from the Latin rubidus, “deep red,” for its bright red spectral lines. It burns with a lilac-violet flame and reacts vigorously with water, so it’s stored under oil. Rubidium’s isotopes are used in atomic clocks and research. Memory tip: Rubidium contains a “b,” so its symbol keeps that letter—Rb—and think “ruby-red rubidium” to recall its name and color.