Atomic Structure

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Slide 7

Discovery of the Electron

In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Slide 8

Thomson’s Atomic Model

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

J.J. Thomson

Slide 9

Mass of the Electron

1909 – Robert Millikan determines the mass of the electron.

The oil drop apparatus

Mass of the electron is

9.109 x 10-31 kg

Slide 10

Conclusions from the Study of the Electron

Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.

Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons

Electrons have so little mass that atoms must contain other particles that account for most of the mass

Slide 11

Rutherford’s Gold Foil Experiment

Alpha particles are helium nuclei

Particles were fired at a thin sheet of gold foil

Particle hits on the detecting screen (film) are recorded

Slide 12

Try it Yourself!

In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

Slide 13

The Answers

Target #1

Target #2

Slide 14

Rutherford’s Findings

The nucleus is small

The nucleus is dense

The nucleus is positively charged

Most of the particles passed right through

A few particles were deflected

VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”

Conclusions:

Slide 15

Atomic Particles

Slide 16

The Atomic Scale

Most of the mass of the atom is in the nucleus (protons and neutrons)

Electrons are found outside of the nucleus (the electron cloud)