Discovery of radiometric dating

Thorium has a characteristic terrestrial isotopic composition, with atomic weight 232.0377(4).It is one of only three radioactive elements (along with protactinium and uranium) that occur in large enough quantities on Earth for a standard atomic weight to be determined.Uranium–thorium dating is commonly used to determine the age of calcium carbonate materials such as speleothem or coral, because uranium is more soluble in water than thorium and protactinium, which are selectively precipitated into ocean-floor sediments, where their ratios are measured.

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All of the remaining thorium isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes.

Pa, and this process is often used to check the results of uranium–thorium dating.

Despite the anomalous electron configuration for gaseous thorium atoms, metallic thorium shows significant 5f involvement.

This was first realised in 1995, when it was pointed out that a hypothetical metallic state of thorium that had the [Rn]6d configuration with the 5f orbitals above the Fermi level should be hexagonal close packed like the group 4 elements titanium, zirconium, and hafnium, and not face-centred cubic as it actually is.

Thorium metal has a bulk modulus (a measure of resistance to compression of a material) of 54 GPa, about the same as tin's (58.2 GPa).

Aluminium's is 75.2 GPa; copper's 137.8 GPa; and mild steel's is 160–169 GPa.

Thorium is a moderately soft, paramagnetic, bright silvery radioactive actinide metal.

In the periodic table, it lies to the right of actinium, to the left of protactinium, and below cerium.

Thorium is an electropositive actinide whose chemistry is dominated by the 4 oxidation state; it is quite reactive and can ignite in air when finely divided. The most stable isotope, It is estimated to be over three times more abundant than uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare-earth metals.

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