Lithium Lithium Lithium (from ) is a chemical element with symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the lightest metal and the lightest solid element. Like all alkali metals, lithium is highly reactive and flammable, and is stored in mineral oil. When cut, it exhibits a metallic luster, but moist air corrodes it quickly to a dull silvery gray, then black tarnish. It never occurs freely in nature, but only in (usually ionic) compounds, such as pegmatitic minerals which were once the main source of lithium. Due to its solubility
Dense set close to it (see Diophantine approximation). Formally, a subset "A" of a topological space "X" is dense in X if for any point "x" in "X", any neighborhood of "x" contains at least one point from "A" (i.e., "A" has non-empty intersection with every non-empty open subset of "X"). Equivalently, "A" is dense in "X" if and only if the only closed subset of "X" containing "A" is "X" itself. This can also be expressed by saying that the closure of "A" is "X", or that the interior of the complement of "A" is empty. The density of a topological