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