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# Chlorine (Cl)

## What is chlorine?

Last updated date: 19th Mar 2023
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Chlorine is a chemical element that was discovered in 1774 by Carl Wilhelm Scheele. Initially, he mistakenly thought it contained oxygen. Chlorine got its name in 1810 by Humphry Davy, who claimed that it was in fact an element. Chlorine is also denoted with the symbol Cl (Cl full form is chlorine). The chlorine atomic number is 17. Chlorine belongs to group 17 (or VII-A) elements of the periodic table and the symbol of chlorine is ‘Cl’.

Being the second lightest halogen, it is positioned between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is present in gaseous form at room temperature with a yellowish-green appearance. Chlorine formula (chemical formula of chlorine) is Cl2. The electronic configuration of chlorine is 1s22s22p63s23p5 (or $\left [Ne\right ]$ 3s2 3p5).

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### Atomic Structure of Chlorine

As depicted in the picture, the first and second electron shells of chlorine atoms are completely filled up with 2 and 8 electrons respectively. The outermost shell contains 7 electrons in total. As the outermost shell has one electron less from attaining the nearest noble gas configuration, its tendency is to form an anion; ‘Cl-’.

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Some of the physical and Chemical properties of Chlorine are listed below.

## Properties of Chlorine

### Physical Properties

 Colour Greenish-yellow Phase Gaseous at room temperature Odour A discomforting and suffocating smell Density 3.21*10-3 g.cm-3 at 20°C Solubility Is readily soluble in water. Its aqueous solution is known as chlorine water and it contains a mixture of chlorine, hydrochloric acid, and hypochlorous acid Boiling Point –34.050C Melting Point –1010C.

Chemical Properties

 Atomic Number 17 Atomic Mass 35.453 g.mol-1 Electronegativity According to Pauling 3.0 Van Der Waals Radius 0.127 nm Ionic Radius 0.184 (-2) nm ; 0.029 nm (+6) Isotopes 4 Electronic Shell $\left [Ne\right]$3s23p5 The Energy of the First Ionisation 1255.7 kJ.mol-1 Energy of Second Ionisation 2298 kJ.mol-1 Energy of Third Ionisation 3822 kJ.mol-1 Standard Potential - 1.36 V Discovered By Carl Wilhelm Scheele in 1774 Toxicity Highly Toxic Compounds Polyvinyl chloride (PVC), hydrochloric acid and Sodium chloride (common salt) Reactivity With Metals Dry chlorine is reactive towards most of the metals but only upon heating Combustion The reaction of chlorine with alkali metals occurs with combustion in the presence of tiny amounts of moisture Explosive Mixtures of chlorine and hydrogen in some specific proportions become explosive Oxidation Cl2O, ClO2, O2O6, Cl2O7 and Cl2O8 are some oxides of chlorine. Hypochlorites (salts of hypochlorous acid), chlorites and chlorates also contain chlorine in the oxidised state

### Chlorine Uses

Some use of chlorine gas is given below.

• Water Treatment

The chemistry of chlorine has helped to a large extent in keeping the drinking water and water in pools safe. Before chlorine-based disinfectants became popular, waterborne diseases like typhoid, dysentery, cholera and hepatitis A took thousands of lives every year. Chlorine-based disinfectants for pools and spas are really helpful in killing waterborne pathogens and keeping the water safe from causing illness. This way, it prevents diseases like diarrhoea, rashes on the swimmer’s ear or skin etc.

• Food

Chlorine plays an important role in the abundant production of crops by protecting them against pests. It is also useful in keeping the kitchen surfaces germ free and safe to prepare food on them. E. coli, salmonella and hosts of some other foodborne germs are disinfected easily with chlorine-based disinfectants.

• Household Disinfectant

Chlorine is widely used in manufacturing bleaches which is further very helpful in cleaning, disinfecting and brightening clothes, kitchen and washroom surfaces. Germs found on household surfaces causing stomach bugs (norovirus), seasonal flu and other diseases can be easily killed with diluted bleach solutions.

• Healthcare and Medicines

The chemistry of chlorine is quite important in pharmaceutical industries. In the manufacture of many medicines including those to control cholesterol, fix allergy symptoms and relieve arthritis pain use of chlorine is crucial. Several medical devices including blood bags, surgical stitches, contact lenses, safety glasses and some respiratory equipment also have the application of chlorine chemistry in their manufacturing process.

• Energy and Environment

Chlorine is used in the process of manufacturing various devices used in the extraction of renewable energy resources. Chlorine is used in the purification and extraction of silicon from sand grains which are then useful in the fabrication of solar cells and panels. The blades of wind turbines are made up of chlorine-based epoxy resins.

The chemistry of chlorine is important in the manufacture of advanced technology equipment such as high-speed processors which are used in computers, laptops and smartphones, etc. It is useful in manufacturing refrigerants that are used in air conditioners. High-performance magnets and hybrid car batteries are other such applications of chlorine.

• Defence and Law Enforcement

The manufacture of bullet-resistant vests, night vision goggles, parachutes, cockpit canopies as well as missile guidance techniques have applications of chlorine chemistry. This equipment is used in the defence and enforcement of the law.

• Building and Construction

Durable paints for walls and ceilings, energy-efficient vinyl windows, plastic foam insulation for home heating and air conditioning systems, etc. are some example applications of chlorine chemistry in building and construction-related components which help in improving the efficiency and durability of the construction.

• Transportation

The seat cushions, airbags, brake fluids, bumpers, shatter-resistant windows, cables and wires, navigation systems, steel hulls etc. used in automobiles, trains and planes have wide applications of chlorine chemistry. These components add comfort and safety for the passengers.

### Did You Know?

• Chlorine alone is toxic and harmful while its compound; ‘NaCl’ (common salt) is completely safe to intake (of course in very small amounts).

• Gaseous chlorine is about 2.5 times denser than the air mixture. Hence, in a container filled with air with chlorine, it will have a tendency to settle down at the bottom.

### Conclusion

Chlorine is a chemical element that was discovered in 1774 by Carl Wilhelm Scheele. Chlorine atomic number 17, belongs to group 17 (or VII-A) elements of the periodic table and the symbol of chlorine is ‘Cl’. It is in gaseous form at room temperature, yellowish-green in colour, toxic with a suffocating smell and soluble in water. Chlorine is used in domains of food, healthcare, water treatment etc. It has wide applications in manufacturing bleaches, drugs, pest killers, defence equipment, automobile components, silicon-based electronics, construction hardware etc.

## FAQs on Chlorine (Cl)

1. Why is chlorine added to drinking water?

Chlorine is really effective in destroying germs and bacteria that contaminate water making it unsafe to drink. Various waterborne diseases once killed thousands of American people. To control waterborne diseases, the use of chlorine treatment of water was first introduced in 1908 in Jersey City. After that, this practice spread rapidly and virtually, it eliminated major waterborne diseases like typhoid and cholera.

The practice of drinking water chlorine treatment proved so effective that it increased the life expectancy of Americans by around 50 per cent in the 20th century. This is the reason that the United States Centers for Disease Control and Prevention calls the chlorination of drinking water one of the most significant advances for public health in the United State’s history.

2. Is chlorine in drinking water safe?

The answer to this is ‘Yes’. But there are constraints on the permissible  amount of chlorine in drinking water from the safety point of view. The EPA (environmental protection agency) suggests that levels of chlorine in drinking water should not exceed 4 parts per million. This amount of chlorine is sufficient in controlling harmful pathogens while keeping them safe and consumable. Chlorine treated water also prevents the formation of algae at the reservoir and pipelines of water supply. It also kills bacteria and this way removes the bad odour and smells from the water making it more suitable to drink.