The Development of Atomic Models

1. The Development of Atomic Models • According to legend, Democritus was supposed to be mad because he laughed at everything, and so he was sent to Hippocrates to be cured. Hippocrates pointed out that he was not mad, but, instead, had a happy disposition. That is why Democritus is sometimes called the laughing philosopher. • Democritus was a pre-Socratic Greek philosopher (born around 460 BC). • Democritus was originator of the belief that all matter is made up of various imperishable, indivisible elements which he called "atomos", from which we get the English word atom. BB - Model Dalton’s Model More to come

2. Plum Pudding Model Thomson's model was compared (though not by Thomson) to a British treat called plum pudding, hence the name. It has also been called the chocolate chip cookie model, but only by those who have not read Thomson's original paper • Proposed by J. J. Thomson (1856 - 1940), the discoverer of the electron in 1897. • The plum pudding model was proposed in March, 1904 before the discovery of the atomic nucleus. • In this model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electron's negative charge, like plums surrounded by pudding. The electrons were thought to be positioned throughout the atom. • Electrons could move like letters in alphabet soup • Instead of a soup, the atom was also sometimes said to have had a cloud of positive charge.

3. Nuclear Model 1. Atom is mostly empty space 2. Dense center called the nucleus 3. Electrons were stuck surrounding the nucleus. • The Gold foil experiment or the Rutherford experiment was an experiment done by Ernest Rutherford (1871 - 1937) in 1909. This experiment discovered the nucleus. • Led to the downfall of the plum pudding model of the atom. • Alpha particles (positive particles--Helium Nuclei) were shot at gold foil. • Particles passed through the gold foil. A few shot back. • Conclusions:

5. Planetary Model • Introduced by Niels Bohr, a Danish physicist (1885 - 1962), in 1913. • Because of its simplicity, the Bohr model is still commonly taught to introduce students to quantum mechanics. • The Bohr model depicts the atom as a small, positively charged nucleus surrounded by waves of electrons in orbit — similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. "The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth." Niels Bohr

6. Quantum Mechanical Model • Erwin Schrödinger (August 12, 1887 – January 4, 1961) • An Austrian physicist, achieved fame for his contributions to quantum mechanics, especially the Schrödinger equation, for which he received the Nobel Prize in 1933. • This model is based on probability • Where are you going to find and electron 90% of the time. • Atom is viewed as a fuzzy cloud. • Schrödinger equations create electron clouds (orbitals) with specific shapes.

7. Main Points For “Atoms” Video • What is the key to understanding atomic structure? • The discovery of what particle is associated with the Crook’s Tube? • What did Rutherford expect to happen in the gold foil experiment? • What was Rutherford’s genius? • What conclusions did Rutherford draw from the Gold foil experiment? • How much smaller is the nucleus than the electron cloud? • What determines the shape of the electron cloud?