1. Atomic Model Theory

2. Early Greek Models Who was right? Aristotle was wrong. However, his theory persisted for 2000 years. Democritus (400 b.c.) Aristotle (350 b.c) • Thought matter could not be divided indefinitely • Led to the idea of atoms in a void • Modified an earlier theory that matter was made of four “elements”: earth, fire, water, air

3. John Dalton (1766 – 1844) • “Billiard Ball” Model • 1800 – proposed a modern atomic model based on experimentation, not on pure reason • Performed a number of experiments that eventually led to the acceptance of the idea of the atoms in 1803 • Postulates • All matter is made of atoms. • Atoms of an element are identical. • Each atom has different atoms. • Atoms of different elements combine in constant ratios to form compounds. • Atoms are rearranged in reactions. • His ideas account for the law of conservation of mass.

4. JJ Thomson (1856 – 1940) “Plum Pudding” or “Cookie Dough” Model • Discovered the electron in 1897 • Atoms were made from a positively charged substance with a negatively charged substance scattered about, like raisin in a pudding or chips in a chocolate chip cookie.

5. Ernest Rutherford (1871 – 1937) Gold Foil Experiment • Atoms mostly empty space and not filled with a positively charged material • Small positive charged center (nucleus) • Negatively charged particles around the nucleus

6. Neils Bohr (1885 – 1962) Planetary Model • Found that electrons move in definite orbits or energy levels around the nucleus in 1913 • Electrons orbit the nucleus in “shells”.

7. Electron Cloud Model Current Model • Describes a cloud-like region where the electron is likely to be found • Can not say with certainty, where the electron actually is at any point in time, yet can describe where it ought to be • Cloud represents a history where the electron has probably been and where it is likely to be going