noun Chemistry.
a silver-white, metallic, active element resembling potassium, used in photoelectric cells and radio vacuum tubes. Symbol: Rb; atomic weight: 85.47; atomic number: 37; specific gravity: 1.53 at 20°C.

1860–65; < Neo-Latin, equivalent to Latin rūbid(us) red (in allusion to the two red lines in its spectrum) + -ium -ium

rubidic, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
rubidium (ruːˈbɪdɪəm)
a soft highly reactive radioactive element of the alkali metal group; the 16th most abundant element in the earth's crust (310 parts per million), occurring principally in pollucite, carnallite, and lepidolite. It is used in electronic valves, photocells, and special glass. Symbol: Rb; atomic no: 37; atomic wt: 85.4678; half-life of 87Rb: 5 × 1011 years; valency: 1, 2, 3, or 4; relative density: 1.532 (solid), 1.475 (liquid); melting pt: 39.48°C; boiling pt: 688°C
[C19: from New Latin, from Latin rubidus dark red, with reference to the two red lines in its spectrum]

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Medical Dictionary

rubidium ru·bid·i·um (rōō-bĭd'ē-əm)
Symbol Rb
A soft metallic element of the alkali group. Atomic number 37; atomic weight 85.47; melting point 39.31°C; boiling point 688°C; specific gravity (solid) 1.532; valence 1, 2, 3, 4.

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Science Dictionary
rubidium   (r-bĭd'ē-əm)  Pronunciation Key 
Symbol Rb
A soft, silvery-white metallic element of the alkali group. It ignites spontaneously in air and reacts violently with water. Rubidium is used in photoelectric cells, in making vacuum tubes, and in radiometric dating. Atomic number 37; atomic weight 85.47; melting point 38.89°C; boiling point 688°C; specific gravity (solid) 1.532; valence 1, 2, 3, 4. See Periodic Table.
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Encyclopedia Britannica


chemicalchemical element of Group 1 (also called Group Ia) in the periodic table, the alkali metal group. Rubidium is the second most reactive metal and is very soft, with a silvery-white lustre. A brief treatment of rubidium follows. For full treatment, see alkali metal.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Cool a gas of rubidium atoms to one-hundred-millionth of a degree above absolute zero or less and something strange happens.
What kinds of chemical reactions does rubidium have when you place it in water.
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