Ytterbium, symbol Yb, is a member of the lanthanoid group of elements, which were originally called the rare earths. Its atomic number is 70, meaning it has 70 protons in its nucleus. It has many isotopes, that is, many species it appears in. These species are ytterbium because they have 70 protons but because of extra neutrons have varying atomic weights. It has three allotropes, that is, three forms in which the atoms are arranged differently in molecules. To explain by example, diamond is a different allotrope than graphite, although both are forms of carbon. Ytterbium's three allotropes are formed in the solid state at different temperatures. Like the allotropes of carbon, they differ in the way their chemical bonds hold them together. Ytterbium melts at 819 degrees Celsius. It boils at 1196 degrees C.
The lanthanoid group contains all the elements with atomic numbers between 57 and 71. They were called rare earths, because they were believed to occur rarely, but it turns out they're common enough, or most of them are, but they commonly occur in forms that once weren't easily recognized. The Word lanthanoid is from the Greek for "to lie hidden". Lanthanoids occur together, usually, and used to be very difficult to separate one from the other, until modern ion exchange and solvent extraction methods were devised in the mid-twentieth century.
Ytterbium itself is a silver-white metallic element. It's soft and ductile. It oxidizes (rusts) in air, and combines quickly with water. Therefore it is usually stored covered. It is essentially never found in nature in a free state, but is almost always combined with other lanthanoids in ore. Its ores include monazite sand (the commonest commercial source), and also gadolinite (the ore in which these elements were first discovered), and xenotime. China is the largest producer of ytterbium.
Jean Charles Galissard de Marignac, a Swiss chemist, discovered this element in 1878. He found it in the rare earth source then called erbia, and called it ytterbia after the Swedish town near the quarry where it was found. All of the rare earths were then believed to come from a small area of Sweden. He thought that his ytterbia was a chemical compound of the element he named ytterbium. Another chemist, Georges Urbain, separated ytterbia into neoytterbia and lutecia. Lutecia became known as the element lutetium and neoytterbia became known as ytterbium.
The addition of small amounts of ytterbium improves stainless steel for some applications, making it finer grained and stronger. Ytterbium is also used in some lasers, as are others of the lanthanoid group, and in doping certain semiconductors, especially for solar applications.