Uranium. It’s big. Clocking in (mostly) with 92 protons and 146 neutrons, it’s the heaviest “primordial” element. That makes it the most massive element on Earth to have been created before our planet existed. Not that you’ll ever find it as a pure element, only inside other compounds. When refined, it’s a silvery-white metal.
But uranium’s not just physically large. It’s also had a massive influence on our understanding of physics. More so, I’d argue, than any other element. Read more.