# 3-2 Discovering Atomic Structure

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3-2 Discovering Atomic Structure

Michael Faraday Suggested that the structure of atoms is related to electricity, and it turns out that atoms contain particles that have electrical charge.

Static Electricity Benjamin Franklin is one of the first to experiment with electricity. Concludes that electricity has two kinds of charges. Positive (+) Negative(-)

Cathode Rays and Electrons
Running electricity through a partially evacuated glass tube occurs in a cathode tube.

The negative end is called a cathode.
The positive charged end is called an anode When lined with a fluorescent material and passing electricity through it, you can see the radiation going through it.

It was found that putting a magnet near the particles deflects them, indicating a negative charge of particles J.J. Thomson experimented with cathode rays, with a magnet, and charged plates near the ray.

Discovered Magnetic and electric fields deflect the rays
Deflects rays in predictable ways The particles are Negative in charge Particles come from cathode Named the particles Electrons Meant Atoms were made smaller parts!!!! Particles had a charge of 1.76 x 108 coulombs/gram

Robert Millikan Oil Drop Experiment
Gave droplets of oil a negative charge using x-rays Droplets fell between two charges plates. Measured how the charge on the plates affected the rate of fall of oil drops. Found the charge to be 1.60 x coulombs From Thomson's findings he could calculate the mass of a single electron. 9.11 x grams

Problem: how could matter containing electrons be neutral and where was all the mass? His findings were 1/2000 the mass of an atom.

Radioactivity Radioactive elements are found in the late 1800’s. Henri Becquerel was the first to discover radioactivity Marie Curie and her husband Pierre find two radioactive elements after Becquerel

Ernest Rutherford See Figure 3-13 in book Discovers alpha particles
Deflects towards negative plate Discovers beta particles Deflect towards positive plate Discovers gamma particles Not affected by electric plates

The Nuclear Atom J.J. Thomson's problem again: if electrons are negative, why are atoms neutral? Must contain positive parts equal to the negative parts. Where are they? The Plum Pudding Model All particles evenly distributed. Rutherford changes this concept

The Gold Foil Experiment (figure 3-14)

Most particles passed through gold without a problem 1 in 8000 alpha particles deflected These were sent in ALL directions including straight back!

What does this mean? Most of the atoms positive charge, as well as the mass is in the middle, called the nucleus. Most pass through the empty space but occasionally one gets close enough to the positive nucleus to deflect