Dalton’s Atomic Theory: Postulates and What Went Wrong

Who proposed the atomic theory?

The word "atom" is older than Dalton. The Greek philosopher Democritus suggested around 400 BC that matter was made of tiny, uncuttable particles. But he had no experiments to support the idea. Dalton gave the first atomic theory backed by measurements, published in A New System of Chemical Philosophy in 1808. That is why the question "who proposed the atomic theory?" has two answers: Democritus the idea, Dalton the science.

Dalton’s atomic theory postulates

The postulates are five statements:

1. All matter is made of tiny particles called atoms.
2. Atoms cannot be created, destroyed or divided in chemical reactions.
3. All atoms of one element are identical in mass and properties.
4. Atoms of different elements have different masses and properties.
5. Atoms of different elements combine in simple whole-number ratios to form compounds.

Dalton pictured atoms as solid, indivisible spheres, like tiny billiard balls. This is the Dalton atomic model.

What laws did Dalton’s theory explain?

The theory explained three laws that chemists had already measured:

• Law of conservation of mass (Lavoisier, 1789): mass is not lost or gained in a reaction. Atoms are only rearranged, so the total mass stays the same.
• Law of definite proportions (Proust, 1799): a compound always contains the same elements in the same mass ratio. Water is about 1 part hydrogen to 8 parts oxygen by mass wherever it comes from.
• Law of multiple proportions (Dalton): when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in small whole-number ratios.

Worked example: the law of multiple proportions

Hydrogen and oxygen make two compounds, water (H₂O) and hydrogen peroxide (H₂O₂).

1. In water, 2 g of hydrogen combines with about 16 g of oxygen.
2. In hydrogen peroxide, 2 g of hydrogen combines with about 32 g of oxygen.
3. The oxygen masses are in the ratio 16 : 32, which is 1 : 2, a simple whole-number ratio.

Dalton explained this by saying that peroxide has twice as many oxygen atoms per hydrogen atom, which is just what the formulas show.

What is wrong with Dalton’s atomic theory?

Three parts of the theory had to be changed:

Chlorine is a good example: chlorine-35 and chlorine-37 are both chlorine, but they have different masses, so the average on the periodic table is 35.45 (read what is an isotope). The idea that atoms combine in whole-number ratios also fails for some compounds, called non-stoichiometric compounds, but it works for nearly all school chemistry.

How did the atomic model change after Dalton?

The atomic model changed as new experiments showed new particles:

1. 1897, J. J. Thomson: discovered the electron in cathode rays and proposed the "plum pudding" model.
2. 1911, Ernest Rutherford: the gold foil experiment showed that atoms have a tiny, dense, positive nucleus.
3. 1913, Niels Bohr: placed electrons in fixed shells (energy levels) around the nucleus.
4. 1932, James Chadwick: discovered the neutron.

Each step kept the part of Dalton’s idea that worked, that matter is made of atoms, and fixed the part that did not. You can see how electrons fill shells in how do ions form.

Why does Dalton’s atomic theory still matter?

It is the foundation of chemical formulas and equations. Whenever you balance an equation, you are using Dalton’s idea that atoms are rearranged, not created or destroyed, in a reaction. It is also the reason a formula like CO₂ has meaning: one carbon atom for every two oxygen atoms.

Common mistakes

• Saying Dalton discovered the electron. That was Thomson, nearly a century later.
• Stating that all of Dalton’s theory is wrong. Most of it holds for chemical reactions.
• Mixing up Dalton’s solid sphere with Thomson’s plum pudding or Rutherford’s nuclear model.
• Forgetting the date: 1803 to 1808, not the 1900s.

Frequently asked questions

What are the main points of Dalton’s atomic theory?

Matter is made of atoms; atoms cannot be created, destroyed or divided in chemical reactions; atoms of an element are identical; atoms of different elements differ; and atoms combine in simple whole-number ratios to form compounds.

What was wrong with Dalton’s atomic theory?

Atoms can be divided into protons, neutrons and electrons. Atoms of the same element can have different masses (isotopes). And atoms can change in nuclear reactions. These three points were discovered after Dalton.

What did Dalton’s atomic model look like?

Dalton imagined each atom as a small, solid, indivisible sphere with its own mass, and different elements having different atoms. It had no internal parts, no charge and no nucleus.

What came after Dalton’s atomic model?

Thomson’s plum pudding model (1897) came next, followed by Rutherford’s nuclear model (1911), Bohr’s shell model (1913) and today’s quantum model, in which electrons occupy orbitals rather than fixed paths.

Try it yourself

Open the Atom Builder, add protons, neutrons and electrons to make carbon, and then change the neutrons to make carbon-14. Which of Dalton’s postulates does the change contradict?