Atomic Composition and Models

Dalton's Atomic Theory

Published in: 1803 

Summary of Dalton's theory: 

• Atoms are the smallest, indivisible building blocks of matter.

• All atoms of a given element have the same properties, including mass and size.

• Atoms combine in specific ratios to form compounds.

  • Ratios of atoms of elements in compounds determine their composition. 

• Chemical reactions rearrange atoms to form different compounds. 

What was missed: 

• Atoms are in fact not indivisible and are composed of subatomic particles. 

  • The existence of isotopes, which have the same chemical properties but different masses due to a difference in neutron count, invalidates the rule that "all atoms of an element have the same mass."

J.J. Thomson’s "Plum Pudding" Model

Published in: 1897 

Summary of J.J. Thomson's theory: 

• Validated everything previously discovered that was correct. 

• Discovered the existence of negatively-charged particles called electron from his cathode ray experiment. 

  • Theorized that atoms have a positively-charged medium (the pudding) with electrons (the plums) embedded into it. 

What was missed: 

• Atoms actually have a central nucleus that holds the concentrated positive charge of the atom. 

Rutherford’s "Nuclear" Model

Published in: 1911 

Summary of Rutherford's theory: 

• Validated everything previously discovered that was correct. 

• From his gold foil experiment, he discovered: 

  • Atoms have a small, dense, positively charged nucleus at the center.

  • Electrons surround the nucleus. 

  • Most of the atom is empty space. 

What was missed: 

• Did not explain why electrons don't fall into the nucleus from electrostatic attraction. 

Bohr’s "Planetary" Model

Published in: 1913 

Summary of Bohr's theory: 

• Validated everything previously discovered that was correct. 

• Based on spectroscopy results, Bohr theorized that: 

  • Electrons travel in circular orbits around the nucleus with their own fixed energy levels (quantized orbits). 

    • When enough energy is absorbed, electrons enter into an excited state and jump to higher energy levels. 

    • Excited electrons could return to its original energy level (ground state), releasing the energy as photons. 

What was missed: 

• Interpreted electrons as classical particles. 

• Did not accurately describe the probabilistic movement of electrons. 

Quantum Mechanical Model

Published in: 1926 

Summary of the theory: 

• De Broglie: Theorized that electrons display properties of both particles and waves, known as wave-particle duality. 

  • The wave must fit around the nucleus without overlapping on itself to form stable, fixed energy levels.

• Heisenberg: Established the uncertainty principle. He stated that there is a limit to how accurately we can measure the position and velocity of an electron simultaneously. 

• Schrödinger: Developed the wave function, which helps predict the probability of an electron's position. 

  • Visualized orbitals (s, p, d, f) as electron "clouds," where the density of the cloud represents the likelihood of finding an electron in that region.

This is currently the most accurate model of the atom.