Terbium

What is Terbium?

Terbium is a solid, silvery metal when it is purified. Like most lanthanide metals, however, it does not exist on Earth in its pure form. It has only one stable isotope and it is found as a minor component of minerals that contain other lanthanide metals. There is no known biological role for terbium and its compounds can be moderately toxic. It is used in a variety of technological devices used in the energy and lighting industries.

Terbium’s Place in the Periodic Table

Terbium is found in period 6 and is a member of the lanthanide group of elements. Lanthanides do not have group numbers but separate group 1 through 3 from the transition metals on the periodic table. Their atomic numbers range from 57 to 71. They are also known as f-block elements because their electrons fill f-orbitals. In 1843, terbium was discovered as a component of the mineral, yttria, by a Swedish chemist, Carl Gustaf Mosander. Its name was derived from the name, Ytterby, which is a Swedish town where yttria was discovered.

Atomic number: 65
Atomic Radius: 194 picometers
Atomic mass: 158.93
Symbol: Tb
Group: N/A
Period: 6
Number of Protons: 65
Number of Electrons: 65
Number of Neutrons: ~ 94
Number of Isotopes: 1 stable isotopes

Properties of Terbium

Pure terbium is a silvery-white, soft metal. So soft that it can be cut with a knife. It is malleable and ductile, which means it can be easily bent and pulled into wires. It is an intermediate conductor of heat and a poor conductor of electricity. There are only 15 elements with lower electrical conductivities. Terbium can be burned in air to form oxides, the most common being terbium (III) oxide. Terbium oxides act as weak bases. It reacts slowly with cold water to form hydroxides, also with a +3 oxidation state. Solid terbium can be dissolved in dilute sulfuric acid. Although +3 is the most common oxidation state it can also form compounds as terbium (I), (II), and (IV).

Physical Properties

Terbium is solid at room temperature. It melts at 1356°C, which is similar to gadolinium and silicon. It boils at 3123°C, which is similar to nickel and gold. It has an intermediate density similar to neodymium and plutonium.

Melting Point: 1356°C.
Boiling Point: 3123°C.
Density of Solid Terbium: 23 g cm^-3
Phase at Room Temperature: solid

Chemical Properties

Terbium has 2 valence electrons in its 4s shell. However, the 4f shell is not completely full, and has 9 additional valence electrons. This is true for all but the last lanthanide elements, which are also known as the f-block elements. As the atomic number increases across period 6, the 14 lanthanides fill the 4f orbitals with up to 14 electrons. Two main factors contribute to the reactivity of terbium: 1) its size, 2) electron shielding. Terbium is among the 20 elements with the lowest ionization energies on the periodic table. The larger the size of an atom, the further the distance of the valence electrons to the nucleus. This distance decreases the strength of the attraction between protons and electrons. The large number of lower energy level electrons also act as a shield against attraction to the nucleus.

Oxidation states: +1, +2, +3, +4
Specific Heat: 0.450 J g^-1K^-1
Electronegativity: 1.83 (Pauling scale)
Heat of Fusion: 13.81 kj mol^-1
Heat of Vaporization: 340 kj mol^-1
Electron Configuration: [Xe] 4f⁹6s²

Isotopes

Terbium-159 is the only stable isotope that occurs in nature. The term, nuclide, is a noun for a nucleus with a specific number of protons and neutrons. Elements with only one stable isotope are known as monomuclidic, meaning there is only one nuclide. Because all isotopes of an element have the same number of protons, different nuclides of an element are determined by their number of neutrons. Thirty six radioactive isotopes have been generated in laboratories. The most stable radioactive isotope, terbium-158, has a half life of 180 years. Radioactive decay of terbium can produce dysprosium and gadolinium. There are no major uses for terbium isotopes.

Alloys and Allotropes

Terbium is added to alloys of iron with dysprosium and iron. This alloy of iron, called Terfenol-D, has unique magnetic properties. It expands and contracts when a magnetic field is applied. This process is known as magnetostriction. Terfenol-D is the alloy with the highest magnetostriction, providing the largest range of expansion and contraction for a given magnetic field strength. When the amount of terbium and dysporium is increased, the magnet retain its properties to temperatures as low as -200°C. When decreased, the opposite effect is true, allowing the magnet to retain its properties up to 200°C. This has made terbium alloys useful in different environments.

Compounds of Terbium

Terbium sulfate is a colorless solid. However, it fluoresces green when it is exposed to ultraviolet light. Another compound that fluoresces green light is terbium (III) nitrate. When voltage is be applied to sodium terbium borate, it produces light in lasers.

Interesting Facts about Terbium

In 2018, on a small coral island in the Pacific Ocean, one of the largest sources of terbium was discovered. It is estimated that the quantity present would be sufficient to meet the demand for terbium for centuries. This island is no more than 30 feet above sea level, and the ocean drops to more than 3000 feet deep. Marcus Island, or Minami Torishima, belongs to Japan, and they were searching for deposits of rare earth metals after China limited their export.
Terbium is used to combat counterfeits. The Euro currency is printed with fluorescent rare earth elements. When exposed to UV light, their authenticity can be verified. Terbium is used for green fluorescence, europium for red fluorescence, and thulium for blue fluorescence.
Terbium-containing magnets expand and contract when a magnetic field is applied. So much so that they can be used to produce sound. A thin film of a terbium magnetic material can be placed on a flat surface to be used as a speaker on a store-front window, or to hide the source of music during flash mob routine, perhaps.

Occurrence and Abundance of Terbium

Terbium is rare in the Earth’s crust with an abundance that is intermediate to other elements. At 0.000093% of the Earth’s crust, its abundance is similar to other f-block elements, such as lutetium and europium. Among all of the elements in the universe, terbium is the 10^thleast abundant. It is found as a minor fraction of minerals that contain lanthanides, such as monazite, xenotime, and euxenite.

Uses of Terbium

Most Notable Uses in General

A variety of methods can be used to produce light. One of the oldest is incandescent lighting, where an electrical current is passed through a wire with high resistance. This wire then produced heat and light. The light produced by incandescent bulbs is a continuous range of wavelengths, and a lot of energy used is lost to heat. To design a more efficient way to produce light, engineers looked to biology for answers. We know that humans perceive light using specific cells in our eyes that perceive a specific wavelength of light. There are three types of these cells that perceive red, green, or blue light (RGB). So rather than using the continuous range of wavelengths, the images we perceive with our eyes are derived from only three colors. Harnessing this knowledge, engineers developed electronics that produced RGB light to preserve the perceived image but reduce the loss of electricity to heat and irrelevant wavelengths. A closer look at monitors and televisions we use every day reveals that each pixel is composed of a cluster of circuits that produce red, green, and blue light when voltage is applied. These circuits contain terbium in the green conductors. Red and blue are produced using different forms of another lanthanide, europium.

Most Notable Uses in Science

Magnets are common components of audio speakers because they can be used to transform electrical signals to sound. Likewise, they can be used to produce sound to electrical signals in microphones. Standard magnets suffice for the electronic devices we use every day. However, what if we wanted to produce or detect ultrasound, which is inaudible to the human ear? Standard magnets pose a problem. When magnets are placed in a magnetic field, they generate heat and vibration. Those vibrations can be perceived as sound, so standard magnets cannot be used. To detect and produce ultrasound, terbium alloys with unique magnetic properties are used (Terfenol-D). Ultrasound has been used to map the sea floor, study marine animal communication, and detect obstacles to military submarines under low light and low visibility.

Discovery of Terbium

The discovery of terbium is a good case to illustrate the difficulty of isolating elements from the minerals that contain other elements with similar properties. In 1843, Carl Gustaf Mosander separated the yttria into three fractions that he called yttria for yttrium, terbia for terbium, and erbia for erbium. The terbia fraction was pink, and the erbium fraction was colorless. However, it was later discovered that erbium forms a pink solution. So, in fact, the original fraction of terbia was pink because of the presence of erbium. And terbium was present in the colorless fraction called erbia.

Terbium in the Future

As described for its current uses, terbium remains of great interest for its chemical and luminescent properties. Researchers are developing technologies that detect microorganisms by harnessing terbium fluorescence. It has been shown that when terbium is bound to chemicals on the surface of bacteria, it would fluoresce when exposed to ultraviolet light. Researchers hope that similar methods can be developed for fields like food safety, hospital sanitation, and even trips to outer space.