Introduction to Chlorine
Chlorine (Cl) is a chemical element with atomic number 17. It belongs to Group 17 of the periodic table, known as the halogens. In its elemental form, at room temperature, chlorine exists as a diatomic molecule, Cl₂, appearing as a greenish-yellow gas. It possesses a pungent, irritating odour.
Chemical Reactivity
Chlorine is a highly reactive element. This high reactivity stems from its electron configuration, possessing seven valence electrons. It readily gains one electron to achieve a stable octet configuration, thus becoming a negatively charged chloride ion (Cl⁻). Due to this strong tendency to gain electrons, chlorine acts as a powerful oxidizing agent.
Chlorine reacts readily with:
- Metals: It combines vigorously with many metals, often producing metal chlorides. For instance, finely divided sodium metal reacts explosively with chlorine gas.
- Non-metals: It reacts with various non-metals, such as hydrogen, phosphorus, and sulfur, forming their respective chlorides.
- Organic compounds: Chlorine can undergo substitution or addition reactions with organic compounds, playing a significant role in organic chemistry and industrial processes like the production of PVC.
Reaction with Water
When chlorine gas is dissolved in water, it undergoes a disproportionation reaction, meaning it is both oxidized and reduced. The reaction produces hydrochloric acid (HCl) and hypochlorous acid (HOCl):
Cl₂(g) + H₂O(l) ⇌ HCl(aq) + HOCl(aq)
Hypochlorous acid (HOCl) is a weak acid but a strong oxidizing agent and a potent disinfectant. This property makes chlorine highly valuable for purifying drinking water in many municipalities across India, ensuring water supplies are safe from harmful microorganisms.
Reaction with Air
Elemental chlorine typically does not react spontaneously with the main components of air, nitrogen (N₂) and oxygen (O₂), at ordinary temperatures. Nitrogen is highly unreactive due to its strong triple bond. While chlorine and oxygen can form oxides of chlorine, these reactions usually require specific conditions, such as high temperatures or electrical discharge, and are not direct reactions with atmospheric oxygen. Therefore, chlorine gas can be present in air without readily reacting with its primary constituents.
Toxicity, Radioactivity, and Flammability
Toxicity
Chlorine gas is highly toxic. Its strong oxidizing properties make it dangerous to living organisms. Upon inhalation, it reacts with moisture in the respiratory system (e.g., in the lungs) to form hydrochloric acid and hypochlorous acid. These acids cause severe irritation and damage to the mucous membranes, leading to symptoms such as coughing, breathing difficulties, and lung damage. In high concentrations, it can be fatal. Historically, chlorine was used as a chemical weapon during World War I.
Radioactivity
Naturally occurring chlorine primarily consists of two stable isotopes: Chlorine-35 ($^{35}$Cl) and Chlorine-37 ($^{37}$Cl). These isotopes are not radioactive. There are several known radioactive isotopes of chlorine, such as Chlorine-36 ($^{36}$Cl), but these are produced artificially or occur in extremely minute quantities in nature and do not contribute to the element’s common properties or uses.
Flammability
Chlorine gas is not flammable. It does not burn in the presence of a flame. In fact, due to its strong oxidizing nature, it can sometimes support the combustion of other substances that readily react with it, such as hydrogen or certain metals, but it does not combust itself.
Famous Chemical Reaction Example
One of the most fundamental and famous reactions involving chlorine is its combination with sodium metal to form common table salt, sodium chloride (NaCl). This reaction exemplifies chlorine’s strong tendency to accept electrons and sodium’s tendency to donate electrons, resulting in an ionic bond.
2Na(s) + Cl₂(g) → 2NaCl(s)
This reaction is highly exothermic and occurs vigorously. Sodium chloride is an essential compound for human life and a widely used ingredient in Indian cuisine and food preservation. It is also extracted from sources like the Sambhar Salt Lake in Rajasthan.