This chemical element is an alkali metal, one of the lightest metals known. It is used in nuclear reactions, in batteries, in the glass industry, and as a medicine for the treatment of manic-depressive psychosis. What element is it?

This element's unique properties make it a perfect fit for all the clues provided. As the third element on the periodic table, it is the least dense of all solid elements and the first of the alkali metals. Its most famous modern application is in rechargeable lithium-ion batteries, which power everything from smartphones to electric vehicles. Beyond electronics, it plays a vital role in industrial processes, such as strengthening glass and ceramics by lowering their melting point. In a completely different field, lithium salts are a cornerstone medication for managing bipolar disorder, acting as a powerful mood stabilizer.

Discovered in 1817, its name derives from the Greek word 'lithos,' meaning 'stone,' because it was first identified within a mineral, unlike other alkali metals which were found in plant matter. As an alkali metal, it is highly reactive and will even react with water. But perhaps its most fascinating characteristic is its cosmic origin. Scientists believe that lithium is one of only three elements, alongside hydrogen and helium, to have been synthesized in significant quantities during the Big Bang, making it one of the universe's oldest building blocks.