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Before 1807, potash and soda looked like elements. No chemical reagent could break them down. Humphry Davy broke them down with electricity, and in about a year he had isolated six metals that now sit in groups 1 and 2 of your periodic table. This post shows what he did, in his own words where we can, and then works through the electrolysis calculations that JEE and NEET still ask about the same metals.
Who was Humphry Davy?
The Royal Institution's biography gives the outline. Davy (1778 to 1829) was born in Penzance, Cornwall, and was apprenticed to an apothecary. In 1798 he moved to Bristol to work at Thomas Beddoes's Pneumatic Institution, where he discovered the physiological effects of nitrous oxide, laughing gas. In 1802 he became professor of chemistry at the Royal Institution in London, and there he used the new electric battery to isolate sodium and potassium.
The battery was the key. Volta's pile, described in our post on Alessandro Volta, gave chemists a source of continuous current. Davy treated it as a new kind of reagent.
How did Davy isolate potassium?
He described it in the Bakerian Lecture published in the Philosophical Transactions for 1808, read on 19 November 1807. He placed a small piece of pure potash, "which had been exposed for a few seconds to the atmosphere, so as to give conducting power to the surface", on a platinum disc connected to the negative side of a large battery, and touched the top with a platinum wire from the positive side.
The potash began to fuse. At the upper, positive surface there was violent effervescence. At the lower, negative surface "small globules having a high metallic lustre, and being precisely similar in visible characters to quicksilver, appeared, some of which burnt with explosion and bright flame, as soon as they were formed". The globules were the metal he was looking for. In the same lecture he named the new substances "Potasium and Sodium", the first spelled with a single s, for "the metals produced from potash and soda".
Which elements came next?
In a second 1808 paper, Electro-chemical researches on the decomposition of the earths, Davy turned to the alkaline earths. He wrote that the phenomena from his first trials supported the old idea that these earths were metallic in nature. In that paper, as the digitised 1808 volume shows, he proposed "to denominate the metals from the alkaline earths barium, strontium, calcium, and magnium". He admitted "magnium" was an awkward word, but magnesium had already been used for metallic manganese and "would consequently have been an equivocal term". The name magnesium won anyway.
Two years later he took on oxymuriatic acid, the green gas made from muriatic (hydrochloric) acid. In his Bakerian Lecture of 1810 he wrote that it was wrong to give the name oxymuriatic acid to a body "which is not known to contain oxygene, and which cannot contain muriatic acid", and proposed a name "founded upon one of its obvious and characteristic properties, its colour, and to call it Chlorine". He noted that its discoverer had called it dephlogisticated marine acid, a reference to Carl Wilhelm Scheele.
How much electricity does it take to make these metals?
Davy had no way to measure charge. Faraday, who toured Europe with Davy as his assistant from 1813 to 1815 according to the Royal Institution, later did. The NCERT Class 12 electrochemistry chapter gives Faraday's constant as 96487 C per mole of electrons and explains that molten NaCl gives sodium at the cathode and chlorine at the anode. Here are two worked examples in that style.
Example 1: sodium. How much charge deposits 1.00 g of sodium from molten NaCl?
Cathode: Na+ + e– gives Na, so one mole of electrons per mole of sodium.
Moles of Na = 1.00 / 22.99 = 0.0435 mol.
Charge = 0.0435 × 96487 = 4.20 × 10^3 C.
With a steady 2.00 A current, time = 4197 / 2.00 = 2.10 × 10^3 s, about 35 minutes.
Example 2: calcium. How much charge deposits 1.00 g of calcium from molten CaCl2?
Cathode: Ca2+ + 2e– gives Ca, so two moles of electrons per mole of calcium.
Moles of Ca = 1.00 / 40.08 = 0.0250 mol.
Charge = 0.0250 × 2 × 96487 = 4.81 × 10^3 C.
The same mass of calcium needs more charge than sodium even though calcium atoms are heavier, because each ion needs two electrons.
What mistakes do students make here?
Forgetting the charge on the ion. Group 2 ions need 2 F per mole. Write the electrode equation first, every time.
Using water instead of a melt. NCERT shows that in aqueous NaCl the cathode product is hydrogen, not sodium, because H+ reduction (E° = 0.00 V) beats Na+ reduction (E° = –2.71 V). Davy's metals need molten salts.
Mixing up electrodes. Metals appear at the cathode, the negative electrode in electrolysis, just as Davy's globules formed on the negative platinum disc.
For more practice, our guide to electrolysis and Faraday's laws has graded problems. If you want to check each step of your own working, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student take from Davy's way of working?
Point a new tool at old problems. The battery was new; potash was old. Ask what a newly learned method lets you solve that you could not before.
Record what you see, exactly. His description of the globules is so precise you can picture it two centuries later. Write observations in your practical file the same way.
Name things carefully. He chose names that would stay correct whatever theory came next. Precise words help in answers too.
Tonight, write the cathode equations for Na, K, Mg and Ca, then calculate the charge needed to deposit 1.00 g of each.
