As an expert in the field of nuclear chemistry, I can provide a comprehensive answer to your question regarding the reactivity of thorium.

Thorium is a naturally occurring element with the atomic number 90 and is classified as an actinide. It is found in trace amounts in the Earth's crust and is often associated with the mineral monazite. The most common isotope of thorium is thorium-232, which is indeed only slightly radioactive due to its incredibly long half-life of about 14.05 billion years. This half-life is significantly longer than that of uranium-235, which has a half-life of approximately 700 million years, making thorium-232 much less radioactive in comparison.

Now, let's discuss the chemical reactivity of thorium.
Thorium is a metal that is quite reactive chemically. It readily reacts with a variety of substances, including oxygen, hydrogen, the halogens, and sulfur. These reactions can lead to the formation of various thorium compounds, such as thorium oxide (ThO2), thorium nitrate (Th(NO3)4), and thorium sulfide (ThS2), among others.

When exposed to air, thorium metal will oxidize, forming a thin layer of thorium oxide on its surface. This oxide layer can protect the underlying metal from further oxidation to some extent, but it is not as protective as the oxide layer that forms on aluminum, for example. The reaction with oxygen can be represented by the following chemical equation:

\[ 4\text{Th} + 3\text{O}_2 \rightarrow 2\text{ThO}_2 \]

In the presence of water, thorium can react to form thorium hydroxide, although this reaction is not as vigorous as with oxygen. The reaction can be represented as:

\[ \text{Th} + 2\text{H}_2\text{O} \rightarrow \text{Th(OH)}_4 \]

Thorium also reacts with halogens such as fluorine, chlorine, bromine, and iodine. These reactions are typically quite vigorous and can produce various thorium halides. For example, the reaction with chlorine gas can be represented by:

\[ 2\text{Th} + 3\text{Cl}_2 \rightarrow 2\text{ThCl}_3 \]

Similarly, thorium reacts with sulfur to form thorium sulfide:

\[ 2\text{Th} + 3\text{S} \rightarrow \text{Th}_2\text{S}_3 \]

It's important to note that while thorium is reactive, its reactivity is not as high as that of alkali metals or alkaline earth metals. However, it is still considered a hazardous material due to its radioactive properties, and appropriate safety precautions should be taken when handling it.

In summary, thorium is a chemically reactive metal that can react with oxygen, hydrogen, halogens, and sulfur to form various compounds. Its reactivity, while significant, is not as high as that of the more reactive metals in the periodic table. However, due to its radioactive nature, thorium requires careful handling and storage.

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Nearly 100% of thorium found on Earth is thorium-232, which is only slightly radioactive because it has such a long half-life. (Uranium-235's half-life is 700 million years, shorter by a factor of 20.) Thorium is chemically reactive and is attacked by oxygen, hydrogen, the halogens and sulfur.read more >>