Lithium is a soft, silvery-white metal with the atomic number 3, classifying it as the first member of the alkali metal family. This placement signifies that it possesses a single electron in its outermost shell, making it highly reactive. Lithium metal is never found in its pure state in nature because of this reactivity. The metal is so reactive, especially with water and the moisture in the air, that it must be stored immersed in liquids like mineral oil or paraffin oil.
The Underlying Chemical Process
The reaction between lithium and water is fundamentally a type of electron transfer. Lithium metal reacts with water molecules according to the balanced chemical equation: 2Li + 2H2O → 2LiOH + H2. The metal’s instability stems from its single valence electron, which is easily given up to achieve a stable outer shell. When the lithium atom loses its electron, it forms a positively charged lithium ion (Li+). The reaction yields lithium hydroxide (LiOH) and hydrogen gas (H2). Lithium hydroxide is a strong base that dissolves in the water, creating an alkaline solution and driving the reaction forward rapidly.
The Observable Effects of the Reaction
When a piece of lithium metal is dropped into water, the chemical reaction immediately begins. This reaction is intensely exothermic, meaning it releases a significant amount of heat energy into the surrounding environment. The heat produced is often sufficient to melt the lithium, which has a relatively low melting point of about 180 degrees Celsius. Because lithium is less dense than water, the metal floats on the surface. The rapid evolution of hydrogen gas causes the metal to fizzle and dart across the water in a phenomenon known as effervescence. This movement is accompanied by a characteristic hissing sound created by the escaping gas. The heat generated can ignite the flammable hydrogen gas. The ignition of the gas can cause a small flame, often crimson or red in color, to appear on the water’s surface.
Comparing Reactivity and Necessary Safety Measures
While lithium’s reaction with water is dramatic, it is actually the least vigorous of the common alkali metals, such as sodium and potassium. This difference in reactivity is explained by atomic structure: lithium’s single valence electron is closer to the nucleus due to its small atomic size. This proximity results in a stronger attraction, requiring more energy to remove the electron, which slows the overall reaction rate compared to its neighbors on the periodic table. Handling lithium requires strict safety protocols concerning moisture and fire. Water should never be used to extinguish a lithium fire because the metal reacts with the water, producing more flammable hydrogen gas and fueling the blaze. Instead, specialized fire suppressants, such as those classified as Class D for combustible metals, must be used to smother the fire and cut off the oxygen supply. For long-term storage, lithium must be kept in an inert environment, typically submerged in non-reactive substances like mineral oil, paraffin, or petroleum ether. This protective barrier prevents contact with atmospheric oxygen or water vapor, ensuring stability for use in applications like batteries and industrial processes.