The Atomic Theory: A Timeline Through History

Atomic Theory: A Historical Overview

The Origins of Atomic Theory

• Aristotle, a philosopher from 332 BC, proposed that all matter is composed of four elements: Earth, air, fire, and water. This belief was widely accepted despite lacking scientific evidence.

• Democritus, another Greek philosopher around 400 BC, introduced the concept of indivisible particles called "atomos," which means atoms. However, his ideas were not taken seriously due to the absence of experimental data.

• The dominance of Aristotle's theory set back atomic theory by nearly two millennia as many continued to believe in his elemental model rather than Democritus' particle-based view.

John Dalton's Contributions

• In the early 1800s, John Dalton revived atomic theory with a scientifically backed approach. He established that compounds are formed in consistent ratios (e.g., water is always H2O).

• Dalton posited that all matter consists of atoms; these atoms cannot be created or destroyed and are identical for each element but different across elements (e.g., carbon vs. gold).

• His theory emphasized that atoms combine to form compounds in fixed ratios, laying foundational principles for modern chemistry.

Discoveries by J.J. Thompson

• In the late 1890s to early 1900s, J.J. Thompson discovered electrons using a cathode ray tube experiment where negatively charged particles were deflected by positive plates. This led him to identify electrons as negatively charged components within atoms.

• Thompson proposed the "chocolate chip cookie" model (or "plum pudding" model), suggesting that atoms consist mostly of positive matter with negative electrons scattered throughout them. This model implied an overall neutral charge for the atom.

Ernest Rutherford's Findings

• Around 1907, Ernest Rutherford conducted experiments that revealed protons and the nucleus within an atom by bombarding gold foil with positively charged particles and observing their behavior upon impact.

• Contrary to Thompson’s model, Rutherford found that some particles were deflected at significant angles indicating a dense positively charged center—later identified as the nucleus—while most passed through empty space surrounding it.

Atomic Models and the Discovery of Subatomic Particles

Rutherford's Nucleus Model

• The nucleus is composed of positively charged protons, with most of the atom being empty space. Rutherford's model did not include electrons.

Bohr's Solar System Model

• Danish scientist Niels Bohr proposed in 1913 that electrons exist in specific energy levels around the nucleus, resembling a solar system.

• Electrons move in defined orbits and can jump between these orbits but cannot exist in between them.

Atomic Emission Spectrum

• When hydrogen atoms are energized, their electrons move to higher energy levels and emit visible light as they return to lower levels.

• This emission results in specific wavelengths of light rather than a continuous spectrum, indicating quantized energy levels for electrons.

Limitations of Bohr's Model

• Although Bohr was on the right track regarding quantized energy, his model only accurately described hydrogen and failed for other elements.

Schrodinger's Cloud Model

• In 1926, Erwin Schrodinger developed the cloud model, which describes electron locations as probabilities rather than fixed orbits.

• He agreed with Bohr on quantized energy but emphasized that precise electron locations cannot be determined; instead, he introduced atomic orbitals where there is a high probability (90%) of finding an electron.

Orbital Shapes and Probability

• Different orbital shapes include:

• S (sphere)

• P (dumbbell)

• D (flower-like)

• F (more complex flower shape)

Challenges in Electron Location

• Schrodinger noted that attempting to measure an electron’s position alters its location due to its wave-like nature. Thus, we can only predict where it has been previously.

Chadwick and the Neutron Discovery