The chemical element with the atomic number 3 is lithium (Li). Lithium is classified as an alkali metal and is characterized by its low density and high reactivity. It is the lightest metal and the least dense solid element, making it essential in various applications, most notably in rechargeable lithium-ion batteries used in electronics, electric vehicles, and portable devices. Additionally, lithium has applications in medicine, particularly for the treatment of bipolar disorder. Its unique properties contribute to its growing importance in modern technology and mental health therapies.