Erbium atomic radius

Also as a result of the lanthanide contraction, yttrium has an ionic radius comparable to that of the heavier REE species in the holmium-erbium region. If the effective ionic radius (Shannon 1976) of is plotted (0.90 A)., it plots in between element 67 (Ho) and 68 (Er). Scandium (effective ionic radius is 0.745 A), plots outside of the Lanthanide series. As also the outermost electronic arrangement of yttrium is similar to the heavy rare earths, the element behaves chemically like the heavy rare earths. It concentrates during (geo)chemical processes with the heavier REEs, and is difhcult to separate from the heavy REEs. Scandium, on the other hand, has a much smaller atomic radius, and the trivalent ionic size is much smaller than that of the heavy rare earths. Therefore, scandium does not occur in rare earth minerals, and in general has a chemical behavior that is significantiy different from the other rare earth elements (Gupta and Krishnamurthy 2005). 

The lanthanide or rare earth elements (atomic numbers 57 through 71) typically add electrons to the 4f orbitals as the atomic number increases, but lanthanum (4f°) is usually considered a lanthanide. Scandium and yttrium are also chemically similar to lanthanides. Lanthanide chemistry is typically that of + 3 cations, and as the atomic number increases, there is a decrease in radius for each lanthanide, known as the lanthanide contraction. Because bonding within the lanthanide series is usually predominantly ionic, the lanthanide contraction often determines the differences in properties of lanthanide compounds and ions. Lanthanide compounds often have high coordination numbers between 6 and 12. see also Cerium Dysprosium Erbium Europium Gadolinium Holmium Lanthanum Lutetium Praseodymium Promethium Samarium Terbium Thulium Ytterbium. 

The difference in activity among the lanthanide salts was further demonstrated by the fact that para-nitrobenzaldehyde was not acetalized after 20 hr in the presence of erbium chloride, but was completely converted when ytterbium chloride was the catalyst. This is consistent with the observation that acetalization yields increased with increasing atomic number (decreasing ionic radius), a phenomenon related to the Lewis acidity (or degree of hardness) of the cations. The role of the lanthanide catalyst is not well-defined, however. 

---