Why is chlorine’s atomic weight 35.45?
Most atomic weights sit close to a whole number, because a nucleus is protons and neutrons and both weigh roughly one unit. Chlorine sits almost exactly halfway between 35 and 36, which looks like an anomaly and is in fact the clearest demonstration of what an atomic weight is.
Natural chlorine is about 76% chlorine-35 and 24% chlorine-37. Neither isotope weighs 35.45; the value is the weighted average of the two. Take 0.76 × 35 plus 0.24 × 37 and you land near 35.5.
This matters historically. In the early nineteenth century, Prout proposed that all atomic weights were whole multiples of hydrogen’s, which would have made every element a condensate of hydrogen. Chlorine was the standing refutation — its weight was measured accurately and stubbornly refused to be an integer. Only after Soddy’s work on isotopes in 1913 did it become clear that Prout was roughly right about nuclei and wrong about elements, and that chlorine’s fractional weight came from mixing.
IUPAC publishes chlorine’s weight as the interval [35.446, 35.457], because the isotope ratio varies measurably between sources — seawater, evaporite deposits and volcanic emissions differ enough to matter. Chlorine is one of fourteen elements treated this way.
Why does chlorine have a higher electron affinity than fluorine?
Electron affinity is the energy released when a neutral atom gains an electron. The trend across the halogens should follow electronegativity, which puts fluorine first at 3.98 against chlorine’s 3.16.
The measured values reverse it. Chlorine releases about 349 kJ/mol, the highest of any element. Fluorine releases about 328 kJ/mol.
The cause is fluorine’s small size. A fluorine atom’s 2p subshell is compact, and the electrons in it are already crowded. Adding one more forces it into a small volume where electron–electron repulsion is severe, and that repulsion cancels part of the energy the nucleus would otherwise release.
Chlorine’s 3p subshell is larger and more diffuse. The incoming electron has more room, repulsion is milder, and more of the attraction survives as net released energy.
This is worth separating carefully from electronegativity, because the two are easy to conflate. Electronegativity measures how strongly an atom pulls electron density in a bond it is already part of — fluorine wins comfortably, because it is small and its nucleus is close to the bonding pair. Electron affinity measures the energy of adding a whole electron to an isolated gas-phase atom. Different questions, different answers.
The same size effect appears one column over: oxygen’s first electron affinity is lower than sulfur’s, for exactly the same reason.
How do you read chlorine’s electron configuration?
Chlorine’s ground state is [Ne] 3s² 3p⁵: a neon core, two electrons in 3s, five in 3p. Electrons per shell: 2, 8, 7.
Five electrons across three 3p orbitals means two orbitals hold pairs and one holds a single electron — visible in the filling diagram below. That single unpaired electron is why chlorine forms a diatomic molecule: two atoms share their unpaired electrons in a covalent bond, and each achieves an argon configuration.
One electron short of a filled shell, with a high effective nuclear charge, is the recipe for an aggressive oxidising agent. Chlorine takes an electron from almost anything, and this single fact accounts for its toxicity, its disinfectant power and its industrial importance.
The ground state term symbol is ²P₃/₂. The spin multiplicity of 2 confirms the single unpaired electron.
Why does chlorine disinfect water?
Adding chlorine to water produces hypochlorous acid, HOCl, and it is HOCl rather than Cl₂ that does the work. It is small, uncharged and able to cross bacterial cell membranes, where it oxidises proteins and nucleic acids indiscriminately. Nothing evolves resistance to being oxidised.
Water chlorination is among the most consequential public health measures ever adopted. Cholera and typhoid, which killed at large scale in nineteenth-century cities, are waterborne, and routine chlorination made urban water supplies safe in a way filtration alone did not.
The trade-off is real. Chlorine reacts with organic matter in water to form trihalomethanes and other disinfection by-products, some of which are regulated as probable carcinogens. Water treatment is a continuous balance between inadequate disinfection and excess by-product formation, and the balance is overwhelmingly worth striking — the diseases prevented are immediate and lethal, the by-product risks small and long-term.
Chlorine’s industrial reach goes far beyond water. Roughly a third of world production goes into PVC. It is essential to solvents, to titanium dioxide pigment manufacture, and to a large fraction of pharmaceutical syntheses.
Its first large-scale use was less benign. Chlorine gas was released at Ypres in 1915, the first major chemical weapons attack of the First World War. It is denser than air, so it settled into trenches, and it destroys lung tissue by the same oxidising action that makes it a disinfectant.
How is chlorine produced?
Chlorine is made by electrolysing brine — the chloralkali process — and it is one of the largest electrochemical operations in industry.
Passing current through concentrated sodium chloride solution produces three products at once: chlorine gas at the anode, hydrogen at the cathode, and sodium hydroxide left in solution. All three are commercially valuable, which is central to understanding the industry.
The coupling creates a permanent commercial awkwardness. Chlorine and sodium hydroxide are produced in fixed proportion, roughly one tonne of chlorine per 1.1 tonnes of caustic soda, but their markets are independent. When demand for PVC rises and demand for caustic soda does not, producers face a surplus of one and a shortage of the other, and prices move in opposite directions.
Three cell technologies have been used. Mercury cells gave very pure caustic soda but released mercury, and are being eliminated under the Minamata Convention. Diaphragm cells used asbestos. Membrane cells, which separate the compartments with an ion-selective polymer film, are now standard and use substantially less energy.
The energy demand is the dominant cost. Chloralkali plants consume on the order of 2,000 to 3,000 kilowatt-hours per tonne of chlorine, which is why they are sited near cheap electricity and why the industry is a significant participant in electricity demand-response markets.