Holmium is a chemical element with symbol
Ho and atomic number 67.
Part of the lanthanide series, holmium is
a rare-earth element.
Holmium was discovered by Swedish chemist
Per Theodor Cleve.
Its oxide was first isolated from rare-earth
ores in 1878.
The element's name comes from Holmia, the
Latin name for the city of Stockholm.
Elemental holmium is a relatively soft and
malleable silvery-white metal.
It is too reactive to be found uncombined
in nature, but when isolated, is relatively
stable in dry air at room temperature.
However, it reacts with water and corrodes
readily and also burns in air when heated.
Holmium is found in the minerals monazite
and gadolinite and is usually commercially
extracted from monazite using ion-exchange
techniques.
Its compounds in nature and in nearly all
of its laboratory chemistry are trivalently
oxidized, containing Ho(III) ions.
Trivalent holmium ions have fluorescent properties
similar to many other rare-earth ions (while
yielding their own set of unique emission
light lines), and thus are used in the same
way as some other rare earths in certain laser
and glass-colorant applications.
Holmium has the highest magnetic permeability
of any element and therefore is used for the
polepieces of the strongest static magnets.
Because holmium strongly absorbs neutrons,
it is also used as a burnable poison in nuclear
reactors.
== Characteristics ==
=== 
Physical properties ===
Holmium is a relatively soft and malleable
element that is fairly corrosion-resistant
and stable in dry air at standard temperature
and pressure.
In moist air and at higher temperatures, however,
it quickly oxidizes, forming a yellowish oxide.
In pure form, holmium possesses a metallic,
bright silvery luster.
Holmium oxide has some fairly dramatic color
changes depending on the lighting conditions.
In daylight, it has a tannish yellow color.
Under trichromatic light, it is fiery orange-red,
almost indistinguishable from the appearance
of erbium oxide under the same lighting conditions.
The perceived color change is related to the
sharp absorption bands of holmium interacting
with a subset of the sharp emission bands
of the trivalent ions of europium and terbium,
acting as phosphors.Holmium has the highest
magnetic moment (10.6 µB) of any naturally
occurring element and possesses other unusual
magnetic properties.
When combined with yttrium, it forms highly
magnetic compounds.
Holmium is paramagnetic at ambient conditions,
but is ferromagnetic at temperatures below
19 K.
=== 
Chemical properties ===
Holmium metal tarnishes slowly in air and
burns readily to form holmium(III) oxide:
4 Ho + 3 O2 → 2 Ho2O3Holmium is quite electropositive
and is generally trivalent.
It reacts slowly with cold water and quite
quickly with hot water to form holmium hydroxide:
2 Ho (s) + 6 H2O (l) → 2 Ho(OH)3 (aq) +
3 H2 (g)Holmium metal reacts with all the
halogens:
2 Ho (s) + 3 F2 (g) → 2 HoF3 (s) [pink]
2 Ho (s) + 3 Cl2 (g) → 2 HoCl3 (s) [yellow]
2 Ho (s) + 3 Br2 (g) → 2 HoBr3 (s) [yellow]
2 Ho (s) + 3 I2 (g) → 2 HoI3 (s) [yellow]Holmium
dissolves readily in dilute sulfuric acid
to form solutions containing the yellow Ho(III)
ions, which exist as a [Ho(OH2)9]3+ complexes:
2 Ho (s) + 3 H2SO4 (aq) → 2 Ho3+ (aq) +
3 SO2−4 (aq) + 3 H2 (g)Holmium's most common
oxidation state is +3.
Holmium in solution is in the form of Ho3+
surrounded by nine molecules of water.
Holmium dissolves in acids.
=== Isotopes ===
Natural holmium contains one stable isotope,
holmium-165.
Some synthetic radioactive isotopes are known;
the most stable one is holmium-163, with a
half-life of 4570 years.
All other radioisotopes have ground-state
half-lives not greater than 1.117 days, and
most have half-lives under 3 hours.
However, the metastable 166m1Ho has a half-life
of around 1200 years because of its high spin.
This fact, combined with a high excitation
energy resulting in a particularly rich spectrum
of decay gamma rays produced when the metastable
state de-excites, makes this isotope useful
in nuclear physics experiments as a means
for calibrating energy responses and intrinsic
efficiencies of gamma ray spectrometers.
== History ==
Holmium (Holmia, Latin name for Stockholm)
was discovered by Jacques-Louis Soret and
Marc Delafontaine in 1878 who noticed the
aberrant spectrographic absorption bands of
the then-unknown element (they called it "Element
X").
The following year, Per Teodor Cleve independently
discovered the element while he was working
on erbia earth (erbium oxide).Using the method
developed by Carl Gustaf Mosander, Cleve first
removed all of the known contaminants from
erbia.
The result of that effort was two new materials,
one brown and one green.
He named the brown substance holmia (after
the Latin name for Cleve's home town, Stockholm)
and the green one thulia.
Holmia was later found to be the holmium oxide,
and thulia was thulium oxide.
In Henry Moseley's classic paper on atomic
numbers, holmium was assigned an atomic number
of 66.
Evidently, the holmium preparation he had
been given to investigate had been grossly
impure, dominated by neighboring (and unplotted)
dysprosium.
He would have seen x-ray emission lines for
both elements, but assumed that the dominant
ones belonged to holmium, instead of the dysprosium
impurity.
== Occurrence and production ==
Like all other rare earths, holmium is not
naturally found as a free element.
It does occur combined with other elements
in gadolinite (the black part of the specimen
illustrated to the right), monazite and other
rare-earth minerals.
No holmium-dominant mineral has yet been found.
The main mining areas are China, United States,
Brazil, India, Sri Lanka, and Australia with
reserves of holmium estimated as 400,000 tonnes.Holmium
makes up 1.4 parts per million of the Earth's
crust by mass.
This makes it the 56th most abundant element
in the Earth's crust.
Holmium makes up 1 part per million of the
soils, 400 parts per quadrillion of seawater,
and almost none of Earth's atmosphere.
Holmium is rare for a lanthanide.
It makes up 500 parts per trillion of the
universe by mass.It is commercially extracted
by ion exchange from monazite sand (0.05%
holmium), but is still difficult to separate
from other rare earths.
The element has been isolated through the
reduction of its anhydrous chloride or fluoride
with metallic calcium.
Its estimated abundance in the Earth's crust
is 1.3 mg/kg.
Holmium obeys the Oddo–Harkins rule: as
an odd-numbered element, it is less abundant
than its immediate even-numbered neighbors,
dysprosium and erbium.
However, it is the most abundant of the odd-numbered
heavy lanthanides.
The principal current source are some of the
ion-adsorption clays of southern China.
Some of these have a rare-earth composition
similar to that found in xenotime or gadolinite.
Yttrium makes up about 2/3 of the total by
mass; holmium is around 1.5%.
The original ores themselves are very lean,
maybe only 0.1% total lanthanide, but are
easily extracted.
Holmium is relatively inexpensive for a rare-earth
metal with the price about 1000 USD/kg.
== Applications ==
Holmium has the highest magnetic strength
of any element, and therefore is used to create
the strongest artificially generated magnetic
fields, when placed within high-strength magnets
as a magnetic pole piece (also called a magnetic
flux concentrator).
Since it can absorb nuclear fission-bred neutrons,
it is also used as a burnable poison to regulate
nuclear reactors.Holmium is used in yttrium-iron-garnet
(YIG) and yttrium-lanthanum-fluoride (YLF)
solid-state lasers found in microwave equipment
(which are in turn found in a variety of medical
and dental settings).
Holmium lasers emit at 2.1 micrometres.
They are used in medical, dental, and fiber-optical
applications.Holmium is one of the colorants
used for cubic zirconia and glass, providing
yellow or red coloring.
Glass containing holmium oxide and holmium
oxide solutions (usually in perchloric acid)
have sharp optical absorption peaks in the
spectral range 200–900 nm.
They are therefore used as a calibration standard
for optical spectrophotometers and are available
commercially.The radioactive but long-lived
166m1Ho (see "Isotopes" above) is used in
calibration of gamma-ray spectrometers.In
March 2017, IBM announced that they have developed
a technique to store one bit of data on a
single Holmium atom set on a bed of magnesium
oxide.
== Biological role ==
Holmium plays no biological role in humans,
but its salts are able to stimulate metabolism.
Humans typically consume about a milligram
of holmium a year.
Plants do not readily take up holmium from
the soil.
Some vegetables have had their holmium content
measured, and it amounted to 100 parts per
trillion.
=== Toxicity ===
Large amounts of holmium salts can cause severe
damage if inhaled, consumed orally, or injected.
The biological effects of holmium over a long
period of time are not known.
Holmium has a low level of acute toxicity.
== See also ==
Holmium compounds
