Models of the Atom Timeline

Timeline of Atomic Models

Scientists’ ideas about atoms have changed as experiments revealed more about matter. Each model improved on earlier explanations, though today’s models may also be refined as new evidence emerges.

Democritus and Leucippus — About 2,500 Years Ago

• The Greek philosophers proposed that matter was made of tiny, indivisible particles.
• Democritus called them atomos, meaning “uncuttable”—the origin of the word atom.
• He imagined that atoms had different shapes and sizes. For example, iron atoms might have hooks, while salt atoms might have sharp edges.
• Their proposal was philosophical rather than experimental, so it could not be tested with laboratory evidence. Aristotle’s competing idea—that matter consisted of varying amounts of earth, water, air, fire, and ether—became more influential.

John Dalton — 1808

• Dalton used scientific experiments to support the idea that matter is made of tiny particles.
• He pictured atoms as small, solid balls that combined in different ways to form different substances.
• Like Democritus, Dalton considered atoms indivisible. His model was later revised when smaller parts of atoms were discovered.

J. J. Thomson — Late 19th Century

• Thomson discovered electrons, which are much smaller than atoms, showing that atoms are divisible.
• His plum pudding model pictured negatively charged electrons embedded throughout a positively charged substance, like blueberries in a muffin.
• The positive and negative charges balanced, making the atom electrically neutral.

Ernest Rutherford — Gold Foil Experiment

• Rutherford’s findings showed that an atom’s positive charge is concentrated in a tiny central nucleus, rather than spread throughout the atom.
• The atom is mostly empty space, with electrons outside the nucleus.
• This became known as the nuclear model, or Rutherford model.

Niels Bohr — 1913

• Bohr proposed that electrons move around the nucleus in circular orbits, like planets around the Sun.
• This model was useful and intuitive, but later experiments showed that electrons do not literally travel in fixed circular paths.

Erwin Schrödinger and the Quantum Mechanical Model — 1920s

• Schrödinger and other physicists developed a more accurate description of electron behavior.
• Rather than fixed orbits, electrons are described by orbitals: regions and shapes associated with where electrons are likely to be found. The video compares these patterns to a hyperactive fly buzzing around and tracing out a shape over time.
• Orbitals can have different forms, including spherical and two-lobed shapes.

Further Details About the Nucleus

• Rutherford discovered protons in 1919.
• James Chadwick discovered neutrons in 1932.
• The modern introductory picture combines protons and neutrons in the nucleus with electrons described by orbitals around it.

Practical Takeaway

The quantum mechanical model is the more accurate way to describe atoms, but it can be cumbersome for explaining basic topics such as chemical bonding. The video therefore sometimes uses a simplified, Bohr-like diagram with electrons in rings. These diagrams can be useful, but they should not be mistaken for a literal picture of how electrons move.

Speakers and Sources Featured

• Speaker: The video’s narrator, explaining the timeline and comparing the models.
• Historical thinkers and scientists discussed: Democritus, Leucippus, Aristotle, John Dalton, J. J. Thomson, Ernest Rutherford, Niels Bohr, Erwin Schrödinger, and James Chadwick.