Unit IV: Nature of Matter Lesson 1 Atomic Theories.

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Presentation transcript:

Unit IV: Nature of Matter Lesson 1 Atomic Theories

Objectives   You will be able to – Explain how the discovery of cathode rays contributed to the development of atomic models – Explain JJ Thomson’s experiment and the significance of it to the understanding of the atom.

Dalton’s Model   John Dalton took what was known about chemical reactions at his time and proposed the first atomic model. – Conservation of Mass – Law of Multiple Proportions – Law of Definite Composition

Billiard Ball Model   Dalton combined the observations into one theory which stated that all matter was composed of small indivisible particles that he called atoms.   Demitri Mendeleev used this theory when he constructed the first working periodic table.

Cathode Rays   Crookes worked in the areas of chemistry and physics. He had many accomplishments, one of which was the discovery of cathode rays.

Crookes Tube   A source of high potential difference was placed across the cathode of a glass tube that had gas at a very low pressure inside.   Noticed a glow coming from the negative terminal

Properties of Cathode Rays   A wide variety of cathodes (different metals) were tested and all produced same results.   Magnetic fields deflected the rays.   The rays produced some chemical reactions similar to those produced by light.

Properties of Cathode Rays   The rays traveled in straight lines, perpendicular to the surface of the cathode

Properties of Cathode Rays   Had trouble showing that electric fields caused deflection of the beam. This was eventually accomplished by J.J. Thomson.   The rays were believed to be streams of particles. Thomson named them electrons and changed the model of the atom.

Thomson’s Charge to Mass Ratio   It was noticed that the beam of electrons could be bent by a magnetic field. This means that Fnet = Fm, so :   mv2 = Bqvr   So q/m = v/Br

Derivation of Equation   Thomson did not have a way of measuring the velocity directly, but he knew that he could keep the beam traveling in a straight line if he balanced the electric and magnetic forces acting on it. Fe = Fm |E|q = Bqv so : v = |E|/B

Derivation of Equation   By substituting these results into the first equation he came to; q/m = v/Br = |E|/B 2 r   Thomson calculated the charge to mass ratio of the electron to be 1.76 x C/kg. This ratio is constant for all materials.

The Raisin Bun Model   Thomson was able to show that electrons had the following properties: 1. Emitted by a wide variety of cathodes 2. About 2000 times smaller than hydrogen   The new atomic model had the negative electrons (raisins) imbedded in a sea of positive charge (bun). Sometimes called the Plum Pudding Model.