The metal known as the lightest is lithium. With an atomic number of 3, lithium is the least dense metal and the lightest solid element at standard conditions. It is a soft, silvery-white metal that belongs to the alkali metal group and is highly reactive, especially in water. Lithium's lightweight characteristics make it particularly valuable in various industries, including rechargeable lithium-ion batteries, which power many electronic devices and electric vehicles. Additionally, lithium is used in mood-stabilizing medications for bipolar disorder, showcasing its versatility in both technology and health. Its unique properties continue to drive research and innovation.