Introduction to Lithium
Lithium (Li), a soft, silvery-white alkali metal, occupies the third position in the periodic table. It is the lightest metal element and the lightest solid element under standard conditions. Its atomic number is 3, and its electron configuration is [He] 2s$^1$. This single valence electron in the outermost shell is responsible for its characteristic chemical behaviour.
Chemical Reactivity of Lithium
Lithium is highly reactive due to its strong tendency to lose its single valence electron to achieve a stable noble gas configuration. This electron loss results in the formation of a positive ion (Li$^+$). Among the alkali metals, lithium is the least reactive due to its smaller atomic size and higher ionisation energy compared to sodium or potassium, meaning it holds onto its valence electron more strongly. However, it is still significantly reactive when exposed to various substances. India’s recent discovery of significant lithium reserves in Jammu and Kashmir underscores the element’s growing strategic importance, particularly for its chemical reactivity in battery technology.
Reaction with Water
When lithium comes into contact with water, a vigorous exothermic reaction occurs, producing lithium hydroxide (LiOH) and hydrogen gas (H$_2$). The reaction can be represented as:
2Li(s) + 2H$_2$O(l) → 2LiOH(aq) + H$_2$(g)
This reaction is less violent than that of sodium or potassium with water but still releases a substantial amount of heat. If the piece of lithium is sufficiently large, the heat generated can ignite the hydrogen gas, causing it to burn with a characteristic reddish flame. Handling of alkali metals, including lithium, in laboratories across India requires strict adherence to safety protocols to prevent accidents.
Reaction with Air
Lithium reacts readily with components of air. It tarnishes quickly upon exposure to oxygen, forming lithium oxide (Li$_2$O):
4Li(s) + O$_2$(g) → 2Li$_2$O(s)
Uniquely among alkali metals, lithium also reacts with nitrogen gas (N$_2$) at room temperature, forming lithium nitride (Li$_3$N):
6Li(s) + N$_2$(g) → 2Li$_3$N(s)
Due to its high reactivity with both oxygen and nitrogen, elemental lithium is typically stored under mineral oil or in an inert atmosphere, such as argon, to prevent its degradation. This storage method is crucial in industries in India, especially for battery manufacturing, where purified lithium is a key component.
Safety and Properties
Toxicity
Elemental lithium itself is not typically encountered or ingested. However, lithium salts, such as lithium carbonate, are used therapeutically in medicine for the treatment of bipolar disorder. While beneficial in controlled doses, high concentrations of lithium in the body can be toxic, primarily affecting the kidneys and the central nervous system. Lithium toxicity can lead to symptoms like nausea, vomiting, tremors, and confusion. Close monitoring of blood lithium levels is essential during therapeutic use.
Radioactivity
Naturally occurring lithium is not radioactive. It exists primarily as two stable isotopes: Lithium-7 (approximately 92.5%) and Lithium-6 (approximately 7.5%). While Lithium-6 is used in nuclear applications, such as the production of tritium for nuclear fusion research, the naturally occurring element does not emit radiation.
Flammability
Elemental lithium is highly flammable. It can ignite spontaneously in air if heated or when reacting vigorously with water. Once ignited, lithium fires are difficult to extinguish using common extinguishing agents like water or carbon dioxide, as these can react with the burning metal. Specialised Class D fire extinguishers, designed for metal fires and containing agents like lithium chloride or graphite, are required to suppress lithium flames.
A Notable Chemical Reaction
One of the most frequently cited chemical reactions illustrating lithium’s reactivity is its reaction with water. When a small piece of lithium metal is dropped into water, it floats and fizzes vigorously. The lithium skims across the surface of the water as hydrogen gas is produced, and the solution becomes alkaline due to the formation of lithium hydroxide. The reaction releases considerable heat, and sometimes the hydrogen gas produced can ignite, burning with a small flame. This demonstration vividly showcases the element’s strong metallic character and its tendency to react with protonic solvents like water.