Unit 9 The Relationship Between Electricity and the Atom Electrolysis (3.1) Cathode Ray Tubes (3.1) Mass-to-Charge Ratio for Electron (3.1) Positive Particles (3.1) Determination of Electron Charge (3.1)

The First Battery (3.1) Alessandro Volta (1800) invents a cell similar to today’s battery This was around the time a body of evidence was beginning to suggest that matter had electrical properties The battery proved to be useful in exploring these properties and in further understanding the atom

Experiments with Electricity (3.1) Humphry Davy (early 1800’s) used powerful batteries to break apart compounds and discover new elements Michael Faraday, a protegè of Davy, further developed a field called electrochemistry. Electrochemistry recognizes that charged particles must be present to conduct electricity

Cathode Ray Tubes (3.1) William Crookes (1875) developed a low pressure gas discharge tube as illustrated below. Apply voltage across here Screen coated with zinc sulfide – will fluoresce Metal electrodes Most of the air is removed. Application of a voltage across the electrodes leads to a fluorescence on the screen as seen in the right hand image (simulated beam). The beam is called a cathode ray, where the term cathode refers to one of the electrodes. The ray emanates from the cathode. Image from www.crtsite.com

The Nature of Cathode Rays (3.1) J.J. Thomson (1897) applied an electric field to the cathode ray in a Crooke’s tube with a result similar to that shown in the figure. The cathode ray was deflected away from the negative plate suggesting the cathode ray has properties of negatively charged particles – named electrons by Thomson. Based on the curvature of the deflection, Thomson could determine the ratio of the mass of the negative particle to its electric charge. Characteristics of the electrons were independent of the metal used as electrodes – suggests that the electron is common to atoms of all elements. Notice this is contrary to Dalton’s indivisible atom concept from nearly 100 years earlier.

Complementary Positive Particles(3.1) Eugen Goldstein (1886) performed experiments with a cathode ray tube which was perforated as in the image Found the usual electrons going away from the cathode toward the anode (right to left in the image) but also found positive particles moving left to right The positive particles had varying masses depending upon the metal used to make the electrodes in the cathode ray tube. Image from http://chemed.chem.purdue.edu/

Millikan’s Oil Drop Experiment (3.1) Oil droplets are sprayed into the upper chamber. As they leave the atomizer, they will acquire negative charges just like static electricity. Alternatively, X-rays or a similar source may be used to produce charges As the charged particles enter the chamber, they fall due to gravity The bottom electrical plate is negatively charged so it repels the falling charged particles. By adjusting the voltage, the charged particles may be made to “hang” in the air. The mass may be determined from the voltage required to suspend the charged particles. By looking at the smallest possible difference in charges, the mass of the electron may be determined. Image from http://library.thinkquest.org/