Atom structure time line

Atom structure time line

1. Democritus: Proposed that matter was composed of tiny indivisible particles

   • Aristotle: r Aristotle declared that all matter was made of only four elements: fire, air, water and earth.
     • Dalton: He formulated the first atomic theory since the “death of chemistry” that occurred during the prior 2000 years. Dalton theorized that all matter is made of atoms. Atoms are too small to see, “uncuttable,” and indestructible. All atoms of a given element are exactly alike and atoms of different elements are different. Billiard ball 1803 Dalton Th
       • Thomson: Thomson provided the first hint that an atom is made of even smaller particles. He discovered the presence of a negative particle in the atom – the electron. He proposed a model of the atom that is sometimes called the “Plum Pudding” model
         • Rutherford: Ernest Rutherford performed an experiment using positively charged particles fired at gold foil. Through his experiment, he proved that atoms are not a “pudding” filled with a positively charged material. He theorized that atoms have a small, dense, positively charged center, which he called the “nucleus
           • Bohr: Niels Bohr proposed an improvement. He built on the concept that the mass of an atom is contained mostly in the nucleus. He also theorized that electrons move in definite orbits around the nucleus, much like planets circle the sun.
             • Modern Cloud Theory: According to today’s atomic theory, electrons do not orbit the nucleus in neat planet-like orbits but move at high speeds in an electron cloud around the nucleus. In the electron cloud, electrons whirl around the nucleus billions of times in one second. They are not moving around in random patterns; an electron’s location depends upon how much energy the electron has.