Question: What is the abundance in ppm of the most reactive lanthanide among the rare-earth elements ?

text: Rare-earth element
List
A table listing the 17 rare-earth elements , their atomic number and symbol , the etymology of their names , and their main usages ( see also Applications of lanthanides ) is provided here . Some of the rare-earth elements are named after the scientists who discovered or elucidated their elemental properties , and some after their geographical discovery .
A rare-earth element (REE) or rare-earth metal (REM), as defined by the International Union of Pure and Applied Chemistry, is one of a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides, as well as scandium and yttrium. Scandium and yttrium are considered rare-earth elements because they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties, but have different electronic and magnetic properties. Rarely, a broader definition that includes actinides may be used, since the actinides share some mineralogical, chemical, and physical (especially electron shell configuration) characteristics. The 17 rare-earth elements are cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb), and yttrium (Y). They are often found in minerals with thorium (Th), and less commonly uranium (U). Despite their name, rare-earth elements are - with the exception of the radioactive promethium - relatively plentiful in Earth's crust, with cerium being the 25th most abundant element at 68 parts per million, more abundant than copper. However, because of their geochemical properties, rare-earth elements are typically dispersed and not often found concentrated in rare-earth minerals; as a result economically exploitable ore deposits are less common. The first rare-earth mineral discovered (1787) was gadolinite, a mineral composed of cerium, yttrium, iron, silicon, and other elements. This mineral was extracted from a mine in the village of Ytterby in Sweden; four of the rare-earth elements bear names derived from this single location.
Z Symbol Name Etymology Selected applications Abundance ( ppm )
21 Sc Scandium from Latin Scandia ( Scandinavia ) Light aluminium-scandium alloys for aerospace components , additive in metal-halide lamps and mercury-vapor lamps , radioactive tracing agent in oil refineries 22
39 Y Yttrium after the village of Ytterby , Sweden , where the first rare earth ore was discovered Yttrium aluminium garnet ( YAG ) laser , yttrium vanadate ( YVO 4 ) as host for europium in television red phosphor , YBCO high-temperature superconductors , yttria-stabilized zirconia ( YSZ ) , yttrium iron garnet ( YIG ) microwave filters , energy-efficient light bulbs ( part of triphosphor white phosphor coating in fluorescent tubes , CFLs and CCFLs , and yellow phosphor coating in white LEDs ) , spark plugs , gas mantles , additive to steel , cancer treatments 33
57 La Lanthanum from the Greek lanthanein , meaning to be hidden High refractive index and alkali-resistant glass , flint , hydrogen storage , battery-electrodes , camera lenses , fluid catalytic cracking catalyst for oil refineries 39
58 Ce Cerium after the dwarf planet Ceres , named after the Roman goddess of agriculture Chemical oxidizing agent , polishing powder , yellow colors in glass and ceramics , catalyst for self-cleaning ovens , fluid catalytic cracking catalyst for oil refineries , ferrocerium flints for lighters , robust intrinsically hydrophobic coatings for turbine blades 66.5
59 Pr Praseodymium from the Greek prasios , meaning leek-green , and didymos , meaning twin Rare-earth magnets , lasers , core material for carbon arc lighting , colorant in glasses and enamels , additive in didymium glass used in welding goggles , ferrocerium firesteel ( flint ) products 9.2
60 Nd Neodymium from the Greek neos , meaning new , and didymos , meaning twin Rare-earth magnets , lasers , violet colors in glass and ceramics , didymium glass , ceramic capacitors , electric motors of electric automobiles 41.5
61 Pm Promethium after the Titan Prometheus , who brought fire to mortals Nuclear batteries , luminous paint 1 × 10
62 Sm Samarium after mine official , Vasili Samarsky-Bykhovets Rare-earth magnets , lasers , neutron capture , masers , control rods of nuclear reactors 7.05
63 Eu Europium after the continent of Europe Red and blue phosphors , lasers , mercury-vapor lamps , fluorescent lamps , NMR relaxation agent 2
64 Gd Gadolinium after Johan Gadolin ( 1760-1852 ) , to honor his investigation of rare earths High refractive index glass or garnets , lasers , X-ray tubes , computer memories , neutron capture , MRI contrast agent , NMR relaxation agent , magnetostrictive alloys such as Galfenol , steel additive 6.2
65 Tb Terbium after the village of Ytterby , Sweden Additive in Neodymium based magnets , green phosphors , lasers , fluorescent lamps ( as part of the white triband phosphor coating ) , magnetostrictive alloys such as terfenol-D , naval sonar systems , stabilizer of fuel cells 1.2
66 Dy Dysprosium from the Greek dysprositos , meaning hard to get Additive in Neodymium based magnets , lasers , magnetostrictive alloys such as terfenol-D , hard disk drives 5.2
67 Ho Holmium after Stockholm ( in Latin , Holmia ) , native city of one of its discoverers Lasers , wavelength calibration standards for optical spectrophotometers , magnets 1.3
68 Er Erbium after the village of Ytterby , Sweden Infrared lasers , vanadium steel , fiber-optic technology 3.5
69 Tm Thulium after the mythological northern land of Thule Portable X-ray machines , metal-halide lamps , lasers 0.52
70 Yb Ytterbium after the village of Ytterby , Sweden Infrared lasers , chemical reducing agent , decoy flares , stainless steel , stress gauges , nuclear medicine , monitoring earthquakes 3.2
71 Lu Lutetium after Lutetia , the city that later became Paris Positron emission tomography - PET scan detectors , high-refractive-index glass , lutetium tantalate hosts for phosphors , catalyst used in refineries , LED light bulb 0.8