Atomic models

• Atomic models


  • Evolved over time as our understanding of atomic structure deepened through experimental observations and theoretical advances
  • Several important models of the atom had been proposed throughout history, each contributing to the understanding of atomic behaviour and properties
• Dalton's atomic theory


  • Proposed by the British chemist John Dalton in 1803
  • Main postulates: 1) All elements are composed of tiny indivisible particles called atoms
  • 2) Atoms of a particular element are identical. They have same mass and same volume
  • 3) During chemical reaction atoms combine or separate or re-arrange. They combine in simple ratios
  • 4) Atoms can neither be created nor destroyed
• Dalton was able to explain quantitative results that scientists of his time had obtained in the experiments and nicely explained the laws of chemical combinations
• Experiments in the 1850's and beginning of twentieth century clearly demonstrated that atom is divisible and consists of subatomic particles, electrons, protons and neutrons
• Rutherford's experiment


  1. Bombarded a very thin gold foil with alpha particles

  2. Most of the alpha particles passed straight through the foil

  3. Only a few were slightly deflected

  4. One in 1 million was deflected through an angle greater than 90 degrees from their straight paths
• Rutherford's conclusions from the experiment

  • Most of the space occupied by an atom must be empty
  • The deflection of a few alpha particles through angles greater than 90 degrees shows they are deflected by electrostatic repulsion between the positively charged alpha particles and the positively charged part of atom
  • Massive alpha particles are not deflected by electrons
• Rutherford's atomic model


  • An atom is a neutral particle
  • The mass of an atom is concentrated in a very small dense positively charged region called the nucleus
  • The electrons are revolving around the nucleus in circles called orbits
  • The centrifugal force due to the revolution of electrons balances the electrostatic force of attraction between the nucleus and the electrons
• Rutherford's model of an atom resembles our solar system
• Defects of Rutherford's atomic model Classical physics suggests that electron being charged particle will emit continuously while revolving around the nucleus, causing the orbit to become smaller until it falls into the nucleus

  • 2) If revolving electrons emit energy continuously it should form a continuous spectrum
• When was Bohr's Atomic model formulated

  In 1913, Neil Bohr proposed atomic model consistent with Rutherford's atomic model
• What Bohr explained missed by Rutherford's atomic model

  He explained the observed line spectrum of the H atom ⚛️
• Bohr's model of the atom

  Electron revolves around the nucleus in one of the circular orbits<|>Each orbit is also called an energy level<|>Energy of the electron in an orbit is proportional to its distance from the nucleus<|>Electron revolves only in those orbits for which the angular momentum is an integral multiple of Planck's constant
• Absorption and emission of light

  1. Electron jumps to a higher energy orbit when light is absorbed

  2. Electron falls into a lower energy orbit when light is emitted

  3. Energy of the emitted light is equal to the difference between the energies of the orbits
• Bohr model does not depict the three dimensional aspect of an atom
• Quantum Mechanical Model

  Incorporates the principles of quantum mechanics<|>Treats electrons as wave-particle entities<|>Instead of exact orbits, it defines probability regions called orbitals where electrons are likely to be found
• Heisenberg Uncertainty Principle


  It is impossible to simultaneously determine the exact location and future trajectory of an electron
• Plotting the electron orbit around the nucleus becomes an irresistible challenge
• Atomic model


  A tool for understanding the structure and behavior of atoms and their interactions in chemical reactions<|>Represents a conceptual imagination to explain experimental observations of atomic behavior<|>Simplified representation of complex reality
• Structure of an atom

  • Nucleus in the center containing protons and neutrons
  • Electrons in shells surrounding the nucleus
  • Mass of an atom is concentrated in the nucleus
• Nuclear force


  Strong attractive force that binds protons and neutrons together<|>Stronger than electrostatic or magnetic forces<|>Exists between neutrons and neutrons, protons and protons, and neutrons and protons