John Dalton

Who Got Atoms Out of the Weather · Chemist & Meteorologist · 1766–1844

The Quaker schoolmaster, barred from university as a Dissenter, whose fifty-seven years of daily weather readings led him to the atomic theory — and to the claim that compounds hold atoms in fixed whole-number ratios.

Who was Dalton?

John Dalton was born in 1766 to a Quaker weaver's family in Cumberland, began teaching at twelve, and never attended a university, because as a Dissenter he was barred from Oxford and Cambridge. He kept a daily meteorological journal for fifty-seven years, amassing some two hundred thousand observations, and it was the air that led him to atoms. Studying how gases mix, he concluded that each gas in a mixture exerts its pressure independently of the others, which makes sense only if a gas is a crowd of separate particles rather than a continuous fluid — and only if those particles differ in weight from gas to gas. From that came his atomic theory: each element consists of identical atoms of a characteristic weight; atoms are neither created nor destroyed in a chemical reaction but only rearranged; and compounds contain atoms combined in fixed, simple, whole-number ratios. That last claim is the load-bearing one, because it converts a chemical formula from a summary of proportions by mass into a statement about particles that can be counted. He published the first table of relative atomic weights, taking hydrogen as one. Many values were wrong, and for an instructive reason: he assumed the simplest possible formula for any compound, so he took water to be one hydrogen and one oxygen. He rejected Gay-Lussac's combining volumes and Avogadro's hypothesis, either of which would have corrected him. He also gave the first scientific account of colour blindness, from which he suffered.

Major achievements

• Proposed the atomic theory: elements as identical atoms of characteristic weight
• Established that compounds contain atoms in fixed, simple, whole-number ratios
• Stated that atoms are rearranged in reactions, never created or destroyed
• Published the first table of relative atomic weights
• Showed that each gas in a mixture exerts its pressure independently

Life in brief

• 1766 — Born in Eaglesfield: To a Quaker weaver's family; barred from university as a Dissenter.
• 1778 — Begins teaching: At twelve years old, in a Quaker school.
• 1787 — Starts the weather journal: Kept daily for fifty-seven years — about 200 000 observations.
• 1794 — Colour blindness: The first scientific account, based on his own vision.
• 1801 — Partial pressures: Each gas in a mixture exerts its pressure independently.
• 1803 — The atomic theory: Fixed whole-number ratios, and the first table of relative weights.
• 1808 — A New System of Chemical Philosophy: The theory set out in full.
• 1844 — Dies in Manchester: Forty thousand people filed past the coffin.

Explore the life of Dalton — five chapters

1. Teaching at Twelve — A Quaker education and a closed door
2. Fifty-Seven Years of Weather — Two hundred thousand observations, and where the atoms came from
3. Simple Whole Numbers — The claim that made a formula mean something
4. The Rule of Greatest Simplicity — One assumption, and half the table wrong
5. Forty Thousand People Filed Past — A plain Quaker with a state funeral

Read the five-chapter life of John Dalton →

Where Dalton appears in your course

John Dalton has a genuine claim on 7 lessons of the Pearson Edexcel International GCSE science course built into Incandio. Six of them:

• Reacting Masses and Percentage Yield — Chemistry: Step two of every calculation on this page uses the ratio in the balanced equation, and that ratio only means anything because of Dalton. He argued that a compound contains atoms combined in fixed, simple, whole-number ratios — which converts a formula from a summary of proportions by mass into a statement about particles that can be counted. He is also worth asking about being wrong: he assumed the simplest possible formula for any compound, so he took water to be one hydrogen and one oxygen, and the atomic weights that followed were wrong for exactly that reason.
• Covalent Bonding — Chemistry: This page assumes that a water molecule contains exactly two hydrogens and one oxygen — a fixed, small, whole number — and that assumption is Dalton's law of multiple proportions doing quiet work. Before it, a compound could in principle contain any proportion of its elements. He is also instructive about the limits of the idea: he had no way to determine how many atoms were in a molecule, so he assumed the simplest possibility, which made water HO and put his atomic weights out by a factor of two. The bond counting on this page is precisely what he lacked.
• Simple Molecular Substances — Chemistry: Dalton's route to the atomic theory ran through exactly the idea this page needs — that the particles of a gas are separate and largely independent. He found that each gas in a mixture exerts its own pressure as though the others were not there, which makes no sense for a continuous fluid and perfect sense for a crowd of separate particles that barely interact. That independence is the same fact as the weak intermolecular forces on this page, met from the other direction, and it is why he concluded that different gases must be made of particles of differing weights.
• Energy Diagrams and Bond Energies — Chemistry: A bond energy calculation counts bonds — one H–H, two H–Cl — and that is only possible because a compound contains a definite, small, whole number of atoms joined in a fixed way. That is Dalton's contribution: fixed whole-number ratios turned a chemical formula into something countable. He is worth asking here about what he could and could not determine, since he had no way of knowing how atoms were arranged within a molecule, which is the very thing a bond energy calculation depends on.
• Collision Theory — Chemistry: Collision theory takes for granted that matter is made of discrete particles in constant motion, which was Dalton's proposal and was contested for most of a century afterwards. His route to it was through gases: he found that each gas in a mixture exerts its pressure independently, which makes sense only if a gas is a crowd of separate particles that barely interact except by colliding. He is the right figure to ask what the evidence for particles actually was, since collision theory is unusable without them.
• Catalysts — Chemistry: The definition of a catalyst turns on a substance being chemically the same at the end as at the start, and that claim only has content if substances are made of atoms that are rearranged rather than created or destroyed. Dalton supplied exactly that: reactions rearrange atoms, and the atoms themselves persist. He is the right figure to ask what 'unchanged' actually means at the level of the particles, and how you would ever establish it — which is precisely the problem the practical on this page has to solve.

Begin

• Start a conversation with Dalton — a dramatised, historically grounded AI portrayal
• Read the Historical Brief
• Explore the life of Dalton — five chapters
• Find your exam topics · Meet all 208 figures