Historical Atomic Models and Their Limitations: Rutherford to Millikan

Ernest Rutherford

An atom is mostly empty space; small dense positive nucleus; electrons surround the nucleus in mostly empty space.

Limitations of Rutherford's Model

Had no neutrons; couldn't explain the atom stability; no nucleus structure.

Williams Crookes

Negative particles emitted; 'radiant matter'; independent of gas; fundamental property of matter.

Limitations of Crookes' Model

The stability of an atom is unexplained; incorrect interpretation of cathode rays; lacked subatomic particles.

Democritus

Proposed the atomic theory.

Limitations of Democritus' Theory

No experimental data; no subatomic particles; unable to explain bonding.

Antoine Henri Becquerel

Discovered radioactivity; atoms are indivisible.

Limitations of Becquerel's Work

No explanation of atomic structure; could not explain the reasoning of radioactivity; no classification of radiation types.

JJ Thompson

Plum Pudding Model: Neutral ---> Positive + Negative charges within; 'soup' of positive and negative particles; freedom of electron movement.

Limitations of Thompson's Model

Lacked a nucleus; unclear stability: didn't explain the relationship between positive and negative electrons; could not explain Rutherford's ideas; not supported with experimental evidence.

John Dalton

Atoms are smallest particles of matter; atoms of the same element are identical in mass, size, and chemical properties; atoms cannot be created, destroyed, or changed; compounds are formed by combining atoms.

Limitations of Dalton's Theory

Invisibility of atoms: subatomic particles; identical atoms in an element: isotopes; atoms of different elements are different in all respects: isobars; allotropes.

James Chadwick

Atoms of the same element could have different masses; neutron: no charge, and nearly the same mass as a proton; explained isotopes.

Limitations of Chadwick's Work

Theory of a neutron not being a fundamental particle, electron + proton; early models from Chadwick did not deal with neutron decay; measuring exact energy, velocity, behavior or neutrons was difficult.

Schrodinger and Heisenberg

Schrodinger: powerful model of an atom; assumes electrons move in waves; Heisenberg: uncertainty probability in the electron orbitals.

Limitations of Schrodinger and Heisenberg's Model

Cannot be used to solve for most atoms, only approximate; doesn't fully explain electron spin; is non-relativistic, and fails to account for scenarios like at light speed.

Niels Bohr

Explained hydrogen's spectral emission lines; introduced the concept of electron energy levels; first model to explain chemical bonding.

Limitations of Bohr's Model

Only successfully explained the spectrum of hydrogen.

Robert Millikan

Discovering the exact charge of an electron; supported others' atomic work (Thompson's plum model).

Limitations of Millikan's Work

Told us nothing about the nucleus; did not describe how the electrons are arranged; assumed electron charge was constant; excluded data that did not fit his results.