Evolution of the atomic theory
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Evolution of the Atomic Theory. Democritus (460 - 370BC). all things are composed of minute, invisible, indestructible particles of pure matter, which move forever in empty space stated that atoms are made of the same matter, yet they differ in shape, size, weight, sequence, and position

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Democritus 460 370bc
Democritus (460 - 370BC)

  • all things are composed of minute, invisible, indestructible particles of pure matter, which move forever in empty space

  • stated that atoms are made of the same matter, yet they differ in shape, size, weight, sequence, and position

  • Named “atomos” meaning indivisible


Aristotle 4bc
Aristotle (4BC)

  • Claimed there were no such things as atoms.

  • Believed everything was uniform


John dalton 1766 1844
John Dalton (1766 - 1844)

  • Theory:

  • Atoms are small, spherical objects.

  • Each different kind of element is represented by a different kind of atom.

  • Each different kind of atom has its own distinct mass.

  • Atoms are indivisible.


Sir joseph john thomson 1856 1940
Sir Joseph John Thomson (1856 - 1940)

  • Based on the evidence, Thomson came up with his "Plum Pudding" or what is more commonly known as the “Chocolate Chip Cookie Dough” model of the atom:

  • Atoms are composed of smaller particles.

  • These particles are the same for all different types of atoms.

  • These particles are negatively charged and are called electrons.

  • Electrons are embedded throughout the uniform sphere of positive charge to make up a neutral atom.


Ernest rutherford 1871 1937
Ernest Rutherford (1871 - 1937)

  • Based on the gold foil experiment, Rutherford made the following statements about the atom:

  • Atoms contain a nucleus, or core, that contains most of the mass of the atom.

  • The nucleus has a positive charge.

  • Most of the atom is empty space



Niels henrik david bohr 1885 1962
Niels Henrik David Bohr (1885 - 1962)

  • Electrons are only allowed to move in certain paths, or orbits.

  • Each of these orbits corresponds to a specific amount of energy, or energy level

  • Electrons can travel in one of the allowed orbits without losing energy.

  • The only way for an electron to change its energy level is for it to instantaneously change orbits.


Niels henrik david bohr 1885 19621
Niels Henrik David Bohr (1885 - 1962)

  • showed that an atom emits electromagnetic radiation only when an electron jumps from one quantum level to another

  • helped lead to the idea that electrons exist in shells and that the electrons in the outermost shell determine an atom's chemical properties


Electron cloud or quantum model
Electron Cloud or Quantum Model

  • it is impossible to know both the exact position and the exact momentum of an object at the same time


What we know so far
What we know so far…

  • We now know that electrons do not travel in distinct orbits

  • rather the model on the previous slide shows the most common probable location of an electron, much like the fan blades of a ceiling fan in rotation