Silvery heavy radioactive metal m.p. 1132.3±0.8 ºC; 2070.1±1.4 ºF b.p. 2818 ºC; 5104 ºF density ~18.95 g/cc; ~1183.01 pound/cubic foot	memory peg
	1789 Martin H. Klaproth, Germany
	Uranus, planet, discovered 1781 and named after the Greek god of the Sky and father of the Titans.

. The German chemist Martin Klaproth (1743-1817) discovered in 1789 a new element in pitchblende ores from Johanngeorgenstadt in the Saxonian Erzgebirge (Metalliferous Mts.) and Joachimsthal in Bohemia (note). Pitchblende was thought to be a Zinc, Iron or Tungsten ore. First he wanted to name the element Klaprothium, after himself. But he resisted the temptation and proposed, "until a better name was found," to call his element Uranium after the last planet to have been discovered, the planet Uranus. This planet was discovered eight years before, in 1781, by William Herschel. Herschel was a musician who had emigrated to England and had become both the director of the orchestra at the celebrated spa, Bath, and a first class astronomer. His fame was crowned by the discovery of a new planet, named Uranus after Urania, the muse of astronomy and geometry.

Since Mendeleyev's periodic system did not yet exist, Klaproth would not have realized that Uranium, named after the ultimate planet, would for many years be also the ultimate element. When elements 93 and 94 were artificially made many years later, the names Neptunium and Plutonium were chosen on the analogy with the solar system.

The French scientist Henri Becquerel (1852-1908) discovered the radioactive property of Uranium in 1896. Soon many different substances were discovered emanating radiation. Before the nature of radiation was understood, before it was understood that an element can be transformed into a different element, and before the existence of isotopes was proposed, the discovery of these substances created massive confusion. Each source of radiation was around 1900 seen as a separate element. In 1900 William Crookes found that Uranium contained a substance soluble in ammonium hydroxide and ammonium carbonate, which he called Uranium X. Others found there were two Uraniums which they called Uranium 1 and Uranium 2. Uranium X₁ formed Uranium X₂. Uranium X₁ was found to produce two kinds of beta rays resulting in Uranium X₂ and Z. In 1911 Uranium Y was found. Eventually they were placed in a sequential series:

Uranium 1 → Uranium X₁ → Uranium X₂ → Uranium Z → Uranium 2 → Uranium Y.

Later was understood that this concered a decay series and that each of the Uraniums was an isotope of a different element. In 1902 Frederick Soddy and Ernest Rutherford proposed two of these decay series, one starting with Uranium ending with Lead, and the other starting with Thorium (Ionium) and also ending in Lead (note). (See the tables at all of the elements from Thallium (#81) to Uranium (#92).

In 1913 Frederick Soddy proposed that an element emitting an α-particle is transmuted into the element two spaces to the left on the periodic table, whereas an element emitting a β-particle is transmuted into the element immediately to the right (note). For example, Uranium 1 (now ²³⁵U, atomic number 92) emits an α-particle and becomes Uranium X₁ (= ²³⁴Th, atomic number 90), and this emits a β-particle and becomes Uranium X₂ (= ²³⁴Pa, atomic number 91). The rules provide a way to understand the decay series and led to Soddy's proposal of isotopes to explain differing atomic weights for samples of the same element produced by different decay modes.

In the 19th century a native Czech name was proposed: nebesník, probably derived from "nebe" = sky.

• James B. Calvert, "Uranium" 2002 (on-line).
• Winfried Kölzer, Radioaktivität, Strahlenexposition, Strahlenwirkung. Bonn: Informationskreis Kernenergie, 2000 (PDF file on-line).
• Philippe Hillion, La radioactivité : une épopée - Histoire - chronologie (on-line).
• Bertrand Goldschmidt, Uranium's Scientific History 1789-1939. World Nuclear Association (on-line).
• Uran und Isotope. Gmelins Handbuch der anorganische Chemie, 8. Aufl.; System-Nummer 55 (1936).
• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 266-271.