Scandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic
d-block element, it has historically been classified as a rare-earth element, together
with yttrium and the lanthanides. It was discovered in 1879 by spectral analysis of the minerals
euxenite and gadolinite from Scandinavia. Scandium is present in most of the deposits
of rare-earth and uranium compounds, but it is extracted from these ores in only a few
mines worldwide. Because of the low availability and the difficulties in the preparation of
metallic scandium, which was first done in 1937, applications for scandium were not developed
until the 1970s. The positive effects of scandium on aluminium alloys were discovered in the
1970s, and its use in such alloys remains its only major application. The global trade
of scandium oxide is about 10 tonnes per year. The properties of scandium compounds are intermediate
between those of aluminium and yttrium. A diagonal relationship exists between the behavior
of magnesium and scandium, just as there is between beryllium and aluminium. In the chemical
compounds of the elements in group 3, the predominant oxidation state is +3.
== Properties ==
=== Chemical characteristics === Scandium is a soft metal with a silvery appearance.
It develops a slightly yellowish or pinkish cast when oxidized by air. It is susceptible
to weathering and dissolves slowly in most dilute acids. It does not react with a 1:1
mixture of nitric acid (HNO3) and 48% hydrofluoric acid (HF), possibly due to the formation of
an impermeable passive layer. Scandium turnings ignite in air with a brilliant yellow flame
to form scandium oxide.
=== Isotopes ===
In nature, scandium is found exclusively as the isotope 45Sc, which has a nuclear spin
of 7/2; this is its only stable isotope. Thirteen radioisotopes have been characterized with
the most stable being 46Sc, which has a half-life of 83.8 days; 47Sc, 3.35 days; the positron
emitter 44Sc, 4 h; and 48Sc, 43.7 hours. All of the remaining radioactive isotopes have
half-lives less than 4 hours, and the majority of these have half-lives less than 2 minutes.
This element also has five nuclear isomers, with the most stable being 44mSc (t1/2 = 58.6
h).The isotopes of scandium range from 36Sc to 60Sc. The primary decay mode at masses
lower than the only stable isotope, 45Sc, is electron capture, and the primary mode
at masses above it is beta emission. The primary decay products at atomic weights below 45Sc
are calcium isotopes and the primary products from higher atomic weights are titanium isotopes.
=== Occurrence === In Earth's crust, scandium is not rare. Estimates
vary from 18 to 25 ppm, which is comparable to the abundance of cobalt (20–30 ppm).
Scandium is only the 50th most common element on Earth (35th most abundant in the crust),
but it is the 23rd most common element in the Sun. However, scandium is distributed
sparsely and occurs in trace amounts in many minerals. Rare minerals from Scandinavia and
Madagascar such as thortveitite, euxenite, and gadolinite are the only known concentrated
sources of this element. Thortveitite can contain up to 45% of scandium in the form
of scandium oxide.The stable form of scandium is created in supernovas via the r-process.
== Production ==
The world production of scandium is in the order of 15 tonnes per year, in the form of
scandium oxide. The demand is about 50% higher, and both the production and demand keep increasing.
In 2003, only three mines produced scandium: the uranium and iron mines in Zhovti Vody
in Ukraine, the rare-earth mines in Bayan Obo, China, and the apatite mines in the Kola
peninsula, Russia; since then many other countries have built scandium-producing facilities,
including 5 tonnes/year (7.5 tonnes/year Sc2O3) by Nickel Asia Corporation and Sumitomo Metal
Mining in the Philippines. In each case scandium is a byproduct from the extraction of other
elements and is sold as scandium oxide.To produce metallic scandium, the oxide is converted
to scandium fluoride and then reduced with metallic calcium.
Madagascar and the Iveland-Evje region in Norway have the only deposits of minerals
with high scandium content, thortveitite (Sc,Y)2(Si2O7) and kolbeckite ScPO4·2H2O, but these are
not being exploited.The absence of reliable, secure, stable, long-term production has limited
the commercial applications of scandium. Despite this low level of use, scandium offers significant
benefits. Particularly promising is the strengthening of aluminium alloys with as little as 0.5%
scandium. Scandium-stabilized zirconia enjoys a growing market demand for use as a high-efficiency
electrolyte in solid oxide fuel cells.
== Price == Because of its rarity, scandium is among the
most expensive elements. Price for pure scandium fluctuates between 4,000 and 20,000 US dollars
per kilogram. Meanwhile, the limited market generates a variety of prices at any given
time. In 2010, at the peak of the rare-earths shortage, the price of scandium rose to over
15,000 US dollars per kilogram, and the widely commercially used scandium oxide (Sc2O3) was
selling above 7 000 US dollars per kilogram. Since then the limited demand coupled with
steady production keeps the price at its 20-year average.
== Compounds ==
Scandium chemistry is almost completely dominated by the trivalent ion, Sc3+. The radii of M3+
ions in the table below indicate that the chemical properties of scandium ions have
more in common with yttrium ions than with aluminium ions. In part because of this similarity,
scandium is often classified as a lanthanide-like element.
=== Oxides and hydroxides === The oxide Sc2O3 and the hydroxide Sc(OH)3
are amphoteric: Sc(OH)3 + 3 OH− → [Sc(OH)6]3− (scandate
ion) Sc(OH)3 + 3 H+ + 3 H2O → [Sc(H2O)6]3+α-
and γ-ScOOH are isostructural with their aluminium hydroxide oxide counterparts. Solutions
of Sc3+ in water are acidic due to hydrolysis.
=== Halides and pseudohalides === The halides ScX3, where X= Cl, Br, or I, are
very soluble in water, but ScF3 is insoluble. In all four halides, the scandium is 6-coordinated.
The halides are Lewis acids; for example, ScF3 dissolves in a solution containing excess
fluoride ion to form [ScF6]3−. The coordination number 6 is typical for Sc(III). In the larger
Y3+ and La3+ ions, coordination numbers of 8 and 9 are common. Scandium triflate is sometimes
used as a Lewis acid catalyst in organic chemistry.
=== Organic derivatives ===
Scandium forms a series of organometallic compounds with cyclopentadienyl ligands (Cp),
similar to the behavior of the lanthanides. One example is the chlorine-bridged dimer,
[ScCp2Cl]2 and related derivatives of pentamethylcyclopentadienyl ligands.
=== Uncommon oxidation states === Compounds that feature scandium in the oxidation
state other than +3 are rare but well characterized. The blue-black compound CsScCl3 is one of
the simplest. This material adopts a sheet-like structure that exhibits extensive bonding
between the scandium(II) centers. Scandium hydride is not well understood, although it
appears not to be a saline hydride of Sc(II). As is observed for most elements, a diatomic
scandium hydride has been observed spectroscopically at high temperatures in the gas phase. Scandium
borides and carbides are non-stoichiometric, as is typical for neighboring elements.Lower
oxidation states (+2, +1, 0) have also been observed in organoscandium compounds.
== History == Dmitri Mendeleev, who is referred to as the
father of the periodic table, predicted the existence of an element ekaboron, with an
atomic mass between 40 and 48 in 1869. Lars Fredrik Nilson and his team detected this
element in the minerals euxenite and gadolinite in 1879. Nilson prepared 2 grams of scandium
oxide of high purity. He named the element scandium, from the Latin Scandia meaning "Scandinavia".
Nilson was apparently unaware of Mendeleev's prediction, but Per Teodor Cleve recognized
the correspondence and notified Mendeleev.Metallic scandium was produced for the first time in
1937 by electrolysis of a eutectic mixture of potassium, lithium, and scandium chlorides,
at 700–800 °C. The first pound of 99% pure scandium metal was produced in 1960. Production
of aluminium alloys began in 1971, following a US patent. Aluminium-scandium alloys were
also developed in the USSR.Laser crystals of gadolinium-scandium-gallium garnet (GSGG)
were used in strategic defense applications developed for the Strategic Defense Initiative
(SDI) in the 1980s and 1990s.
==== Red giant stars near the Galactic Center ====
In early 2018, evidence was gathered from spectrometer data of significant scandium,
vanadium and yttrium abundances in red giant stars in the Nuclear Star Cluster (NSC) in
the Galactic Center. Further research showed that this was an illusion caused by the relatively
low temperature (below 3,500 K) of these stars masking the abundance signals, and that this
phenomenon was observable in other red giants.
== Applications ==
The addition of scandium to aluminium limits the grain growth in the heat zone of welded
aluminium components. This has two beneficial effects: the precipitated Al3Sc forms smaller
crystals than in other aluminium alloys, and the volume of precipitate-free zones at the
grain boundaries of age-hardening aluminium alloys is reduced. Both of these effects increase
the usefulness of the alloy. However, titanium alloys, which are similar in lightness and
strength, are cheaper and much more widely used.The alloy Al20Li20Mg10Sc20Ti30 is as
strong as titanium, light as aluminium, and hard as ceramic.The main application of scandium
by weight is in aluminium-scandium alloys for minor aerospace industry components. These
alloys contain between 0.1% and 0.5% of scandium. They were used in the Russian military aircraft,
specifically the MiG-21 and MiG-29.Some items of sports equipment, which rely on high-performance
materials, have been made with scandium-aluminium alloys, including baseball bats and bicycle
frames and components. Lacrosse sticks are also made with scandium. The American firearm
manufacturing company Smith & Wesson produces semi-automatic pistols and revolvers with
frames of scandium alloy and cylinders of titanium or carbon steel.Dentists use erbium-chromium-doped
yttrium-scandium-gallium garnet (Er,Cr:YSGG) lasers for cavity preparation and in endodontics.The
first scandium-based metal-halide lamps were patented by General Electric and initially
made in North America, although they are now produced in all major industrialized countries.
Approximately 20 kg of scandium (as Sc2O3) is used annually in the United States for
high-intensity discharge lamps. One type of metal-halide lamp, similar to the mercury-vapor
lamp, is made from scandium triiodide and sodium iodide. This lamp is a white-light
source with high color rendering index that sufficiently resembles sunlight to allow good
color-reproduction with TV cameras. About 80 kg of scandium is used in metal-halide
lamps/light bulbs globally per year.The radioactive isotope 46Sc is used in oil refineries as
a tracing agent. Scandium triflate is a catalytic Lewis acid used in organic chemistry.
== Health and safety == Elemental scandium is considered non-toxic,
though extensive animal testing of scandium compounds has not been done. The median lethal
dose (LD50) levels for scandium chloride for rats have been determined as 4 mg/kg for intraperitoneal
and 755 mg/kg for oral administration. In the light of these results, compounds of scandium
should be handled as compounds of moderate toxicity.
== See also == Rare-earth element