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Every time you write NH3 instead of NH4 or NH2, you are using the idea of valence: each element links to a fixed number of other atoms. Someone had to notice that rule first. Edward Frankland, an English chemist working on strange compounds of zinc and tin with carbon groups, put it into words in 1852. This post covers what he did, reads his key sentence, works through his formulas with a modern eye, and draws out what students can borrow from his habits.
Who was Edward Frankland?
According to the obituary in Nature (17 August 1899), Frankland was born at Churchtown near Lancaster on 18 January 1825 and died in Norway on 9 August 1899. He studied chemistry at Marburg and Giessen, became the first professor of chemistry at Owens College, Manchester in 1851, moved to London in 1857, and in 1865 succeeded August Wilhelm Hofmann at the College of Chemistry. He received the Royal Society's Copley Medal in 1894. The obituary adds that his later knighthood was for his work as a water analyst, not for his chemistry.
How did a failed hunt for "ethyl" lead to valence?
In the 1840s chemists believed organic compounds contained stable groups, or radicals, that might be isolated on their own. The obituary explains that Frankland set out to isolate the radical of common alcohol. In 1848 he obtained a substance he called ethyl, which the obituary says was "in strictness an error". The method he used, heating metals with alkyl iodides, gave him something more valuable: a new family of compounds containing metals bonded to carbon groups, such as zincmethyl and zincethyl.
His full account, On a New Series of Organic Bodies containing Metals, was received by the Royal Society on 10 May 1852 and read on 17 June 1852, and was printed in the Philosophical Transactions. Studying how these compounds combined with oxygen, iodine and sulphur, he noticed that the metals would only take on a set number of extra atoms.
What exactly did Frankland write?
Near the end of the paper he turns to inorganic formulas and writes that "even a superficial observer is struck with the general symmetry of their construction". Compounds of nitrogen, phosphorus, antimony and arsenic tend to contain "3 or 5 equivs. of other elements". His examples include NH3, PCl3 and AsH3 in the three-atom group and NO5 and PO5 in the five-atom group. Then comes the key sentence:
"no matter what the character of the uniting atoms may be, the combining power of the attracting element, if I may be allowed the term, is always satisfied by the same number of these atoms."
That "combining power" is what we now call valency. Nature's obituary says that this recognition "supplied the basis of the modern doctrine of valency", which Kekulé later built on to describe how atoms link into structures.
Worked example: reading Frankland's formulas today
His formulas look odd because chemists in 1852 used equivalent weights, not modern atomic masses. A report quoted in Perkin's 1896 lecture to the Chemical Society writes its formulas with "C=6 O=8". So one "O" in an 1852 formula stood for 8 parts of oxygen, half of today's atomic mass of 16.
Convert his nitrogen oxides:
NO5 means 14 parts of nitrogen with 5 × 8 = 40 parts of oxygen.
In modern atoms of oxygen, 40 ÷ 16 = 2.5 oxygen atoms per nitrogen atom.
Doubling to get whole numbers gives N2O5, dinitrogen pentoxide.
NO3 in the same way gives 24 ÷ 16 = 1.5 oxygen atoms per nitrogen, so N2O3.
NH3 needs no change, because hydrogen was 1 in both systems.
Now compare with NCERT's rule. The Class 11 Chemistry unit on chemical bonding says the group valence of an element is generally equal to the number of valence electrons, or 8 minus that number. Nitrogen, phosphorus, arsenic and antimony each have 5 valence electrons, giving 5 or 8 − 5 = 3. That is exactly Frankland's "3 or 5", found decades before anyone knew about electrons.
The same rule gives zinc, with 2 valence electrons, a valency of 2, which fits the modern formula of diethylzinc, Zn(C2H5)2.
What mistakes do students make with valence?
Treating valency as a charge. Valency counts bonds or combining partners; oxidation state and ionic charge are different ideas, even when the numbers match.
Expecting one value per element. Frankland himself saw two for nitrogen. Many elements show more than one.
Reading old formulas at face value. Before modern atomic masses, formulas like NO5 used equivalents, so convert before you judge them.
Forgetting the 8-minus rule. Chlorine has 7 valence electrons and a common valency of 1, not 7.
If the jump from valence electrons to bonds still feels unclear, read our post on Gilbert Lewis and electron pairs, then try a few formulas in the Fermi Problem Library, which has a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student learn from how Frankland worked?
Frankland went looking for one thing, a free ethyl radical, and did not get it. Instead he paid attention to what his new compounds would and would not do, and then stepped back to compare them with a long list of familiar inorganic formulas. The rule he found came from noticing a pattern across many examples, and he was careful to say he was not offering a hypothesis for its cause. When you revise, collect several formulas side by side and ask what they have in common before reaching for an explanation.
For a sense of where the idea went next, see our post on August Kekulé and the benzene ring.
Take PCl3 and PCl5, count phosphorus's valence electrons, and check that both formulas fit the 5 or 8-minus-5 rule.
