As a subject matter expert in materials science, I am well-versed in the properties and characteristics of various elements and compounds. Among these, thorium is a particularly interesting element due to its unique properties and potential applications.

Thorium is indeed a metal, belonging to the group of actinides in the periodic table. It is a naturally occurring element that is found in trace amounts in the Earth's crust. The atomic number of thorium is 90, and it is denoted by the symbol Th. It is a radioactive element, which means it undergoes natural decay, emitting alpha particles, beta particles, and gamma radiation.

When pure, thorium exhibits a silvery-white metallic luster. It is relatively soft and can be cut with a knife. However, it is important to note that thorium is not found in its pure form in nature; it is typically found in the form of thorium minerals, such as thorite (ThSiO4) and monazite ((Ce,La,Nd,Th)PO4).

The statement that thorium is "air-stable" is partially correct. While pure thorium metal can retain its luster for several months when exposed to air, it is not indefinitely stable. Over time, it reacts with oxygen and other elements in the air, leading to the formation of a thin oxide layer on its surface. This oxide layer can cause the metal to tarnish and change color, as mentioned in the provided information.

The formation of thorium oxide is an important aspect to consider. Thorium oxide, ThO2, is a black powdery substance with a very high melting point, reported to be around 3300 degrees Celsius. This makes it the oxide with the highest melting point of all known oxides. The high melting point of thorium oxide is a testament to the strong ionic bonds within its crystal lattice.

In terms of applications, thorium has been considered for use in nuclear reactors as an alternative to uranium. The reason for this is that thorium can undergo a nuclear reaction to produce energy, similar to uranium. However, thorium-based reactors are not yet widely used due to various technical and safety challenges.

Another application of thorium is in the production of high-temperature ceramics and refractory materials. The high melting point of thorium oxide makes it suitable for use in environments where extreme heat is encountered.

It is also worth mentioning that thorium has been used in the past in various other applications, such as gas mantles for lighting, where its radioactive decay could produce a bright glow. However, due to the health risks associated with radiation exposure, such uses have been largely discontinued.

In summary, thorium is a metal with unique properties that make it valuable for certain applications. Its high melting point, radioactive nature, and potential use in nuclear energy production are some of the key characteristics that define this element. However, handling thorium requires careful consideration of its radioactive properties and the safety measures necessary to protect against radiation exposure.

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When pure, thorium is a silvery white metal which is air-stable and retains its lustre for several months. When contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and finally black. Thorium oxide has a melting point of 3300--C, the highest of all oxides.read more >>