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Dalton's atomic theory is often taught as four bullet points to memorise. It makes more sense once you know that it grew out of weather records kept by a schoolteacher, and that two of the four points turned out to be wrong. This post covers who John Dalton was, what he proposed, what changed, and a worked example of the mistake that made him write water as HO.
Who was John Dalton?
The Science History Institute biography describes a Quaker from a modest family in Cumberland, England, born in 1766, who earned his living as a teacher from the age of 12. After ten years at a Quaker school in Kendal he moved to Manchester and joined the Manchester Literary and Philosophical Society, which gave him a laboratory and an audience.
He was a meteorologist first. He kept daily weather records from 1787 until his death in 1844, and his first book was Meteorological Observations (1793).
How did weather lead to atoms?
Dalton wanted to know why the gases in air stay mixed rather than settling into layers. He concluded that the air is a mechanical mixture, in which each gas exerts its own pressure independently and the total pressure is the sum. That is his law of partial pressures, which you still use in gas problems. Explaining it led him to argue that the particles of different gases differ in weight. From there, the biography says, he began calculating atomic weights from the percentage compositions of compounds.
What did Dalton propose in 1808?
He set out the theory in A New System of Chemical Philosophy, published in parts from 1808. NCERT Class 11 Chemistry, Unit 1 summarises the main points:
Matter consists of indivisible atoms.
All atoms of a given element have identical properties, including identical mass. Atoms of different elements differ in mass.
Compounds form when atoms of different elements combine in a fixed ratio.
Chemical reactions reorganise atoms; atoms are neither created nor destroyed.
NCERT also credits him with the law of multiple proportions (1803), and notes that his theory explained the laws of chemical combination but not the laws of gaseous volumes.
Worked example: why did Dalton write water as HO?
To turn mass data into atomic weights, Dalton needed formulas, and he had no way to measure them. The biography explains that he assumed the simplest combination came first: one atom of A with one atom of B. So water was HO.
Water is 88.8% oxygen and 11.2% hydrogen by mass (from 16.00/18.02).
If water is HO: one O atom weighs 88.8/11.2 ≈ 7.9 times one H atom. Oxygen's atomic weight comes out near 8.
If water is H₂O: two H atoms share the 11.2%, so one O atom weighs 2 × 7.9 ≈ 15.9 times one H atom. That is the modern value, close to 16.
The data were not the problem. One wrong formula doubled or halved atomic weights all through his tables. Sorting this out needed Avogadro's idea about gas volumes, covered in our post on Avogadro's hypothesis.
Multiple proportions, the law Dalton did get right. In CO, 12 g of carbon combines with 16 g of oxygen. In CO₂, 12 g of carbon combines with 32 g of oxygen. The oxygen masses for a fixed mass of carbon are 16 : 32 = 1 : 2, a small whole-number ratio, as the law predicts.
Common mistakes:
Comparing total masses instead of a fixed mass of one element. Multiple proportions only works when one element's mass is held constant.
Assuming the simplest formula. That was Dalton's own error. Use data, not convenience.
Which parts of Dalton's theory changed?
Atoms are not indivisible. NCERT Unit 2 explains that experiments towards the end of the nineteenth and beginning of the twentieth century showed atoms contain electrons, protons and neutrons, "a concept very different from that of Dalton".
Atoms of one element need not have identical mass. Frederick Soddy won the 1921 Nobel Prize in Chemistry partly for "his investigations into the origin and nature of isotopes": atoms of one element with different masses.
Elements can exist as molecules. Hydrogen and oxygen gas are H₂ and O₂. NCERT Unit 1 notes that Dalton believed atoms of the same kind could not combine, so such molecules could not exist.
What survived is the core: atoms of each element have a characteristic relative mass, and reactions rearrange atoms without creating or destroying them. That is the base of the mole concept, and the JEE (Main) 2026 syllabus still opens its chemistry with "Dalton's atomic theory" and the laws of chemical combination. Check the current bulletin for your exam.
What about Dalton and colour blindness?
The first paper Dalton read to the Manchester society was on his own colour blindness, now sometimes called Daltonism. He guessed that the fluid in his eyes was tinted blue and asked for his eyes to be examined after his death. They were clear. In 1995, a study in Science extracted DNA from his preserved eye tissue and found he was a deuteranope, lacking the retina's middle-wavelength pigment.
What can a student take from Dalton's way of working?
Keep records. Decades of daily weather notes gave him the questions that led to atoms.
Make theories that can be tested. His was specific enough that others could check and correct it.
Expect to be partly wrong. Two of his four points needed revision, and the theory still shaped all of chemistry.
If the link between atoms, moles and masses is shaky, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise one idea at a time. Dalton's law of partial pressures returns in our guide to ideal gas problems.
Try this now: nitrogen forms N₂O, NO and NO₂. For 28 g of nitrogen, find the mass of oxygen in each and check the whole-number ratio.
