Chapter 4.1 (Boyle), 4.3 (Dalton), and 4.5 (JJ Thompson, Lord Kelvin & Rutherford)      Std 1h

 

4.1 – The Elements (Boyle)

Chemicals used by man since < 1000 BC – making tools from metal and embalming fluids

Greeks in 400 BC – attempted to explain chemical changes by defining 4 elements:

      Fire

      Earth

      Water

      Air

Boyle (1627 – 1691) Ireland

      Everything we conclude in science must be based on experimental, reproducible evidence

      Elements defined as substances that could not be broken down into simpler substances

 

4.3 – John Dalton’s Atomic Theory  (late 1800’s) England

 

The smallest part of an element is an atom

All atoms of an element are identical

Atoms of different elements are each unique, no two different elements share the same types of atoms

Atoms combine to form different substances

Atoms cannot be broken down by chemical means

 

4.5 – The Structure of the Atom

 

J.J. Thompson (late 1890’s) England

Used a cathode ray tube in his experiments (to see how they work click

J.J. Thompson’s Experiments with cathode ray tubes )

Electron – defined as a negative charge (-), will be repelled by the (-) area of an electric field.

All atoms have electrons

Theorized that there must be positive (+) charges in the atom somewhere to balance these (-) charges

 

Lord Kelvin (real name W.W. Thompson), before 1910 – England

Proposed the Plum Pudding model of the atom.  This theory thought of the atom as a homogeneous mixture of (+) and (-) charges.

 

Ernst Rutherford – about 1911 – New Zeland

Student of J.J. Thompson

Performed and reported experiment that disproved the Plum Pudding model of the atom

Experiment shown on page 97 and also available in a wonderful model on the web (click on

Rutherford Experiment – excellent interactive tutorial – (WW Norton)   and scroll down to 3.2).

Basics of the experiment:

α-particles (alpha-particles, helium nuclei with a positive charge, lost their 2 electrons) – from radioactive source such as Radium.  Radium must be in lead box to protect the scientist/observer, with a small hole that can be opened to allow α-particles to excape.  See brief description of radioactive decay at radioactive decay .

fluorescent screen – allows you to see alpha particle beam and where the particles hit

very thin gold foil – to show deflection due to the presence of solid masses (nuclei)

 

Interpreting results of experiment – 100% of the beam doesn’t just pass through, there are deflections, both major and minor that indicates there are large solid masses in the foil, not just mush .   PLUM PUDDING CANNOT BE RIGHT !!!

 

      CONCLUSION:  atoms have dense centers that are positively (+) charged – A NUCLEAR ATOM

                                                                                                                                        

In 1919 he concluded the (+) charges we protons.  However, it took until 1932 for James Chadwick, was able to show that the nuclei of most atoms contained neutrons which have no charge.  Neutrons allowed us to account for the large mass of the nucleus of the atom.

 

Demonstrations of Rutherford’s experiment in class

 

               First we had a person run into a pencil to simulate expected results if the atoms were like “plum pudding”

     

First Demo

 

 

 

PARTICLE

MASS

RATIO

 

OBJECT HIT

α-particles (+)

(150 lb person)

Actual  1,800 : 1

Demo  1,800 : 1

Electron (-)

(1/2 pencil, 4 g)

 

 

  Next, we threw a ping pong ball at a 500 g  mass to simulate expected results if the atom had a very dense nucleus that weighed 50 times more than an α-particle.

 

Second Demo

 

 

 

PARTICLE

MASS

RATIO

 

OBJECT HIT

α-particles (+)

(ping pong ball, 2.5 g)

Actual   1 : 50

  Demo   1 : 200

gold nucleus (+)

(500 g mass)

 

Next, we threw a ping pong ball (2.5 g)  at 2 different weights – each simulating the nucleus of a calcium nucleus (25 g) and gold nucleus (126 g)

 

Third Demo

 

 

 

PARTICLE

MASS

RATIO

 

OBJECT HIT

α-particles (+)

(ping pong ball, 2.5 g)

Actual 1 : 10

Demo  1 : 10

Calcium nucleus (+)

(25 g mass)

α-particles (+)

(ping pong ball, 2.5 g)

Actual 1 : 50

Demo  1 : 50

Gold nucleus (+)

(126 g mass)

 

From this demonstration we can conclude that the more massive the nucleus of the atom, the more likely it will be that the nucleus will deflect an (alpha) α-particle.