Atomic models- early atomic theories.ppt feb 2024

Early Atomic Theories

Democritus's Atomic Theory

  • Matter cannot be endlessly divided; eventually, the smallest indivisible piece is reached.

  • Named this smallest piece "atomos," meaning "cannot be cut."

  • Atoms described as small, hard particles made of the same material but differing in shape and size.

  • His ideas were based on reasoning and logic rather than experimentation, leading to limited acceptance.

Dalton’s Atomic Theory

  • Rise of experimentation in scientific thought during the reign of Elizabeth I (1533-1603).

  • Emphasis on scientific methods and the importance of evidence.

  • In 1808, John Dalton proposed a theory explaining the Law of Definite Proportion, based on experimentation and observations.

Dalton Model of Atom

  • Proposed the atom as a solid, indivisible sphere.

  • His theory faced challenges as it did not explain how atoms acquire electrical charges.

Thomson’s Plum Pudding Model

  • In 1897, J.J. Thomson discovered that atoms consist of smaller particles: electrons and protons.

Discovery of the Electron

  • Used a cathode ray tube to demonstrate the presence of negatively charged particles.

  • Observed that a ray was emitted from the cathode and deflected by electric plates.

Observations and Conclusions from Cathode Ray Tube Experiments

  • The cathode ray was repelled by negative and attracted by positive plates, indicating it was negatively charged.

  • The negatively charged particles were referred to as "corpuscles" (later called electrons).

  • Thomson measured particle mass, finding it was about 2000 times smaller than the mass of hydrogen atom.

Plum Pudding Model Description

  • Electrons are embedded randomly in a positively charged sphere (the "plum pudding").

Rutherford’s Gold Foil Experiment

  • In 1911, Rutherford conducted experiments testing Thomson's model through the Gold Foil Scattering Experiment.

Gold Foil Experiment Details

  • Fired alpha particles at a thin sheet of gold foil and recorded the impacts.

  • He expected particles to pass straight through based on Thomson's model.

Results and Observations

  • Most particles passed through, with some deflecting at various angles and a few bouncing back.

  • This led to conclusions about the nucleus:

    • The nucleus is small, dense, and positively charged.

    • The phenomenon was likened to "howitzer shells bouncing off tissue paper."

Modifications to Atomic Models

  • Thomson's model could not explain Rutherford's observations.

  • Rutherford introduced the Nuclear Model where all positive particles (protons) are in the nucleus, with electrons orbiting around it.

  • The atom is mostly empty space.

Limitations of Rutherford's Model

  • The model asserts that electrons should spiral into the nucleus and collapse, which it does not explain.

Discovery of the Neutron by James Chadwick

  • In 1932, Chadwick found the neutron, a neutral particle in the nucleus, having nearly the same mass as a proton.

  • This discovery arose from observing that the mass of nuclei was not merely the sum of proton masses.

Bohr’s Model of the Atom

  • In 1913, Niels Bohr expanded on Rutherford’s model to clarify electron position and energy.

  • He studied hydrogen gas and observed a line spectrum of distinct colors, indicating discrete energy levels for electrons.

Bohr’s Proposals

  • Electrons exist only in specific orbits with associated quantities of energy.

  • Electrons do not lose energy in their allowed orbits.

  • Energy increases with distance from the nucleus.

Electron Transition and Energy Levels

  • Electrons can move between energy levels by absorbing or emitting specific energy quantities.

  • Electrons cannot exist between energy levels.

Limitations of Bohr’s Model

  • The model applied well to hydrogen but did not effectively explain the spectra of multi-electron atoms.

Quantum Mechanical Model

  • Proposed after fifteen years post-Bohr, the quantum mechanical model is statistical, predicting electron locations in an electron cloud.

  • Influential scientists included deBroglie, Schrödinger, and Heisenberg.

Characteristics of Quantum Mechanical Model

  • Electrons behave as both particles and waves; their position is uncertain if velocity is known.

  • The electron cloud signifies regions with varying probabilities of finding an electron; thus, it has a core nucleus with dense areas and areas of lesser density.