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Chemists knew for decades that hydrofluoric acid must contain a new element related to chlorine. Every attempt to free it failed. In 1886 Henri Moissan succeeded, and the presentation speech at his Nobel ceremony called fluorine "that savage beast among the elements". This post explains why fluorine was so hard to isolate, how Moissan did it, works an electrolysis calculation, and ends with what you can take from his way of working.
Who was Henri Moissan?
His Nobel Prize facts page records that he was born in Paris on 28 September 1852 and died there on 20 February 1907. He received the 1906 Nobel Prize in Chemistry "in recognition of the great services rendered by him in his investigation and isolation of the element fluorine, and for the adoption in the service of science of the electric furnace called after him".
According to his Nobel biography, he began in plant biology before moving to inorganic chemistry, took his doctorate in 1880, turned to fluorine chemistry in 1884 and became Professor of Toxicology at the School of Pharmacy in 1886. It describes him as "an excellent lecturer and a meticulous and patient experimentalist". He died suddenly, shortly after returning from the prize ceremony in Stockholm.
Why was fluorine so hard to isolate?
The 1906 presentation speech names two problems. First, the element's "enormous energy": fluorine decomposes water even in the cold, so any method involving water fails. Second, anhydrous hydrofluoric acid, the obvious starting material, "was a non-conductor of electricity", so it could not simply be electrolysed.
The Nobel biography gives Moissan's way round both. In 1885 he found that solutions of potassium fluoride in hydrogen fluoride, at certain strengths, stayed liquid and conducted electricity at temperatures below zero. A year later he electrolysed these solutions and isolated fluorine for the first time. The presentation speech adds that his apparatus could eventually give a continuous flow of the gas for hours on end, which let him study it thoroughly.
What did Moissan find out about fluorine?
The presentation speech summarises the results. Fluorine had all the qualities expected from its place at the head of the halogens. At ordinary temperature it combines with carbon and silicon to form gases. It combines with hydrogen even at about −230 °C, and more heat is released when hydrogen combines with fluorine than with oxygen.
These are the properties your syllabus asks you to explain from size and electronegativity. Our post on groups 15 to 17 of the p-block covers why fluorine breaks so many group trends.
Worked example: how much fluorine does electrolysis give?
At the anode, fluoride ions lose electrons: 2F⁻ → F₂ + 2e⁻. So one mole of F₂ needs two moles of electrons.
Sample numbers: a current of 2.00 A runs for 1.00 hour.
Charge: Q = I × t = 2.00 × 3600 = 7200 C.
Moles of electrons: 7200 / 96,500 = 0.0746 mol, using the approximate value of the Faraday constant, 1 F ≈ 96,500 C mol⁻¹, given in the NCERT Class 12 electrochemistry chapter.
Moles of F₂: 0.0746 / 2 = 0.0373 mol.
Mass: 0.0373 × 38.00 = 1.42 g. Volume at STP (22.7 L/mol): 0.0373 × 22.7 = 0.847 L.
Common mistakes students make here
"Electrolyse aqueous sodium fluoride to get fluorine." Any fluorine formed would attack the water, which is why Moissan kept water out entirely.
Forgetting the factor of 2. Each F₂ molecule needs two electrons. Dividing by 1 doubles your answer.
Confusing electronegativity with electron gain enthalpy. Fluorine is the most electronegative element, but chlorine has the more negative electron gain enthalpy. Our post on periodic trends and their exceptions explains why.
When Faraday's-law problems trip you up at the set-up stage, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
What else did Moissan build?
His second achievement was the electric arc furnace. The Nobel biography says it reached temperatures up to 3,500 °C and that he used it to prepare carbides, silicides and borides and to make tiny artificial stones in his attempt to synthesise diamond. The presentation speech adds that the furnace let him prepare metals such as tungsten, molybdenum and titanium as pure ingots.
What can a student learn from Moissan's way of working?
Moissan did not attack fluorine head-on. He first found out exactly why earlier attempts failed, then changed the conditions until each obstacle was gone: no water, a conducting solution, a low temperature. When a problem defeats you, list the reasons it is hard before you try again. Each reason points to a fix.
Today, work out how long a 1.00 A current must run to produce 0.0100 mol of F₂.
