Lithium (Li) is the lightest metal and the first element in the alkali metals group. As an alkali metal, it possesses a single electron in its outermost shell, which it readily loses in chemical reactions. When this metal is introduced to water (\(\text{H}_2\text{O}\)), a vigorous chemical process begins. This interaction is a single-replacement reaction, predictable based on Lithium’s specific placement.
The Primary Products of the Reaction
When lithium metal contacts water, two distinct chemical products form. The first product is lithium hydroxide (\(\text{LiOH}\)), a strong base that dissolves in the water to form an aqueous solution. The second product is hydrogen gas (\(\text{H}_2\)), which is released as bubbles.
The water molecule is split during the process. The lithium atom loses its single valence electron to become a positive ion (\(\text{Li}^+\)), which then bonds with the hydroxide ion (\(\text{OH}^-\)) from the water. Simultaneously, the liberated hydrogen atoms combine to form the diatomic hydrogen gas molecule (\(\text{H}_2\)).
The balanced chemical equation for this transformation is: \(2\text{Li}_{(s)} + 2\text{H}_2\text{O}_{(l)} \rightarrow 2\text{LiOH}_{(aq)} + \text{H}_2_{(g)}\). The reaction is driven by the strong tendency of lithium to achieve a stable electron configuration by donating its electron.
Characteristics of the Reaction
The reaction between lithium and water is classified as an exothermic process, meaning it releases thermal energy into the surrounding environment. The rate of heat release is relatively slow compared to other alkali metals. Since the metal is less dense than water, it floats on the surface, allowing the reaction to proceed freely at the water-metal interface.
The metal piece moves rapidly across the water’s surface as the escaping hydrogen gas provides propulsion. The release of hydrogen gas is seen as gentle fizzing or bubbling. Unlike heavier alkali metals, the heat generated is not sufficient to melt the lithium metal, which has a relatively high melting point of \(180.5^\circ\text{C}\).
The hydrogen gas produced is highly flammable, creating a safety consideration. If sufficient heat is generated, or if the experiment is performed in a confined space, the hydrogen can potentially ignite. The lithium metal eventually disappears as it is entirely converted into the soluble lithium hydroxide.
Lithium’s Place Among Alkali Metals
Lithium’s reactivity with water is the mildest among its counterparts like sodium (Na) and potassium (K). Reactivity within Group 1 metals increases systematically as one moves down the periodic table. This trend is directly related to the atomic structure of the elements.
Lithium has the smallest atomic radius of the alkali metals, meaning its single valence electron is held closest to the positively charged nucleus. This close proximity results in a higher ionization energy, which is the energy required to remove that outermost electron. Because it is harder for lithium to lose its electron than for sodium or potassium, the reaction proceeds less vigorously.
Sodium’s reaction is more dynamic, generating enough heat to melt the metal, which forms a distinct silvery ball that races across the surface. Potassium’s reaction is even more intense; it releases heat so quickly that the hydrogen gas ignites immediately, often with a lilac-colored flame. This periodic trend clarifies that while lithium reacts readily with water, it represents the least energetic end of the alkali metal-water interaction spectrum.