How do we know atoms exist?

No single observation settled the matter. Atoms were first supported by indirect evidence from chemistry and the physics of gases, then by the first truly direct evidence from the study of Brownian motion in the early twentieth century. Today instruments can detect individual atoms and even image them. [1] [2]

By the early 19th century it was well established that the masses of reactants in specific chemical reactions always have a particular mass ratio. OpenStax calls this very strong indirect evidence for basic units, atoms and molecules, with those same mass ratios. John Dalton did much of this work, with contributions from Amedeo Avogadro, who developed the idea of a fixed number of atoms or molecules in a mole. [1]

Dalton found that elements that combine in several ways, such as tin and oxygen, do so only in simple proportions. If matter were infinitely divisible, any proportion should be possible, so simple ratios pointed to indivisible atoms. [3]

The kinetic theory of gases, developed in the 19th century, treats gases as atoms and molecules in random thermal motion. It explains gas laws, heat transfer and thermodynamics, and works so well that it was another strong indication of atoms. It was still indirect, and there were heated debates about its validity until direct evidence arrived. [1] [3]

In 1827 the botanist Robert Brown saw pollen grains in still water moving along complex paths. The motion comes from random collisions with fluid molecules; statistical fluctuations in the numbers hitting each side push the grain this way and that. The molecules themselves are not seen, but their effect on the visible grain is. [1]

In 1905 Albert Einstein published a theory showing how this motion could be used to determine the size of atoms and molecules, giving a way to go from something visible, the grain's movement over time, to something invisible, the mass of the fluid's particles. Jean-Baptiste Perrin carefully observed Brownian motion, confirmed Einstein's theory and obtained accurate atomic sizes, which allowed a precise value of Avogadro's number. He received the 1926 Nobel Prize. OpenStax notes that most scientists were already convinced, but this analysis was conclusive and was the first truly direct evidence. [1] [3]

Rutherford, Hans Geiger and Ernest Marsden fired alpha particles at thin gold foil. The plum pudding model predicted only slight scattering. Instead most particles passed through, but some scattered to large angles and a few came straight back, about 1 in 8,000. This showed that the nucleus is very small and holds nearly all the atom's mass, surrounded by mostly empty space. [4]

Modern tools add further evidence. Mass spectrometers accelerate ions and detect them individually while measuring their masses, and devices such as the scanning tunneling microscope observe individual atoms. OpenStax states that all of our understanding of the properties of matter is based on, and consistent with, the atom. [1] [2]