Summary
Highlights
Around 440 BCE, Democritus proposed that all matter consists of indivisible particles called "atomos" separated by empty space, varying in size and shape depending on the substance. His ideas were largely disregarded in favor of Aristotle's four-element theory.
In 1808, John Dalton challenged Aristotelian theory with experimental evidence. He showed that common substances break down into elements in fixed proportions, concluding that compounds are combinations of atoms with specific sizes and masses that cannot be created or destroyed.
Nearly a century after Dalton, J.J. Thomson discovered the electron in 1897. He proposed the "chocolate chip cookie model" where atoms were uniformly positive spheres embedded with negatively charged electrons. Thomson won a Nobel Prize for his discovery.
Ernest Rutherford, a student of Thomson, further investigated atoms by firing alpha particles at gold foil. His experiment revealed that atoms are mostly empty space with a dense, positively charged nucleus at the center, leading to the nuclear model of the atom.
In 1913, Niels Bohr, another of Thomson's students, expanded on Rutherford's model. He proposed that electrons orbit the nucleus at fixed energy levels and distances. However, further experiments and Heisenberg's uncertainty principle led to the quantum model, where electrons exist as waves within a range of possible locations, a complex theory whose implications are still being explored.
Despite continuous evolution in understanding, the fundamental concept of atoms remains key. The phenomenon of electrons shifting energy levels, absorbing or releasing light of specific wavelengths, beautifully illustrates atomic theory in action, vindicating Democritus's ancient hypothesis.