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Development of Atomic Theory. 8 TH Grade. 1. Democritus (430 BC ) proposes the idea that matter is formed of small pieces that could not be cut into smaller parts. He called the small pieces “ atomos ” – Greek for uncuttable . 2. John Dalton’s Atomic Theory (1803)

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1. Democritus (430 BC) proposes the idea that matter is formed of small pieces that could not be cut into smaller parts.

He called the small pieces “atomos” – Greek for uncuttable.


2.John Dalton’s Atomic Theory (1803)

Atoms are like smooth, hard balls that could not be broken into smaller pieces. Theory states:

  • Elements are collections of only one type of atom
  • Compounds are made of molecules which are combinations of atoms
  • Atoms of a given element are all the same, elements of different elements are different
  • Weight of atoms distinguished one from another

3. J.J. Thomson’s Model (1897)

Atoms contain negatively charged particles scattered throughout a ball of positive charge.

The positive charge was theorized because scientists knew that the overall atom was neutral.

Thomson named the negatively charged particle an electron.

Tomson’s Cathode Ray Tube Experiment


4.Ernest Rutherford’s Model (1919)

  • An atom’s positive charge is clustered in the center of the atom, called the nucleus, and the rest of the atom is mostly empty space in which the atom’s super tiny electrons move around at high speed.
  • Discovered the positively charged nucleus through the Gold Foil Experiment
  • He named the positively charged particles named protons.

5. Niels Bohr’s Model (1913)

Hypothesizes that electrons have only specific amount of energy, leading them to move at specific distances from the nucleus. These different distances were called orbitals.



As Rutherford’s students studied atomic disintegration, they kept seeing that the atomic number (number of protons in the nucleus, equivalent to the positive charge of the atom) was less than the atomic mass (average mass of the atom). For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus were adding to the mass.


1932 – James Chadwick proves the existence of the neutron. Updated Bohr’s model to include neutrons in the nucleus.


6. Modern Electron Cloud Model (development of model began in 1926)

  • Individual electrons do not move in fixed paths around the nucleus, but rather they move randomly and in every direction; but are predicted to likely be in certain regions around the nucleus based on their level of energy.
  • The cloud symbolizes where electrons are likely to be found.
  • Higher E electrons aremore likely to be furtheraway from the nucleus.

Hydrogen Atom