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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
• Named “atomos” meaning indivisible
Aristotle (4BC)
• Claimed there were no such things as atoms.
• Believed everything was uniform
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) • 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) • 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
//micro.magnet.fsu.edu/electromag/java/rutherford/
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 - 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
• 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…
• 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