Lithium (Li) is an alkali metal with atomic number 3, occurring naturally in two isotopes: 6Li and 7Li. The two make up all natural occurrence of lithium on Earth, although further isotopes have been synthesized. In ionic compounds, lithium loses an electron to become positively charged, forming the cation Li+. Lithium is the first alkali metal in the periodic table, and the first metal of any kind in the periodic table. At standard temperature and pressure, lithium is a soft, silver-white, highly reactive metal. With a density of 0.564 g⋅cm−3, lithium is the lightest metal and the least dense solid element.