The element on the Periodic Table of Elements with the highest resistance to corrosion is iridium. Among all the elements, it also has the highest density. It is used to establish standards for weights and measures because it is corrosion-resistant. However, unless it is heated to extremely high temperatures, it is difficult to machine, mold, or work because it is so dense and brittle.

Iridium is a white metal with a hint of yellow that belongs to the platinum family. The density of this material is 22.65 grams per cubic centimeter. In contrast, iron has a density of 7.874 g/cm3, while lead has a density of 11.34 g/cm3. 

Chemistry Explained states that iridium is unaffected by acids, bases, and the majority of other powerful compounds. Its ability to withstand such materials renders it valuable in the creation of objects exposed to them.

According to the Royal Society of Chemistry, Tennant found iridium by dissolving crude platinum in dilute aqua regia, which is a mixture of nitric and hydrochloric acids, and then treating the black residue that was left behind with alkalis and acids one after the other. The residue split into two new components following this treatment. He presented his results before the Royal Institution in London, designating the element iridium for one element and osmium for the other. The Latin word iris, which meaning rainbow, is where the term iridium originates. This metal's multicolored compounds are what give it its name even if the metal itself isn't rainbow colored.

Ninety percent platinum and ten percent iridium made up the standard meter bar because iridium has a high corrosion resistance. However, in 1960, this bar was superseded as the accepted definition of a meter. The orange-red krypton spectral line was used to redefine the meter. Nonetheless, the platinum and platinum-iridium alloy international prototype kilogram, which establishes the unit of measurement, is still in use globally.

Sources of this metal:

Iridium is now extracted for use in commerce as a byproduct of nickel or copper mining. Iridium-containing ore can be found in Australia, Brazil, the US, Burma, South Africa, and Russia. 

According to Chemistry Explained, pure iridium is so uncommon that it only exists in the crust at a rate of roughly 2 parts per billion. 

One of the densest and rarest natural elements on Earth is iridium. It is primarily found in the Earth's core as opposed to the crust because it is so dense," Amanda Simson, an assistant professor of chemical engineering at the University of New Haven, stated.

However, there is some iridium in the crust. Significant concentrations of iridium were discovered in 1980 by scientists Luis and Water Alvarez in a particular region of the Earth's crust that is dispersed over the surface. "They speculated that it was caused by a meteor and linked this to the extinction of dinosaurs 66 million years prior," added Simson.

Uses:

It is employed in electrical connections and equipment that must withstand high temperatures. Certain optical lenses also employ it to lessen glare. Compass bearings and fountain pen tips are made of osmiridium, a compound consisting of osmium and iridium. An iridium and platinum alloy is also used to create extremely durable jewelry.