History of the Atom

So what is a bar of Gold made from?

Can you make smaller pieces of gold from the Gold bar?

How small can you make the pieces?

Is there a difference between Iron bars and Gold bars?

What is the difference?

Where does this difference come from ?

The word Atom comes from the Greek language, átomos means indivisable, it cannot be broken down into more simple components.

A Gold bar is made up of millions and millions of these atoms.

The Iron bar is also made up of millions and millions of Iron atoms.

Can they be the same atoms in the Gold and Iron?

No! The Gold bar is made up of atom of gold while the Iron bar is made up of atoms of Iron.

So what do you think a Silver bar is made up of?

Each type of atom is different.

In the 17th and 18th centuries chemists began to isolate these differrent elements.

They saw that each element had its own properties,

Oxygen (O) was required for fires to burn.

Copper (Cu) was a better conductor than other metals,

Sodium (Na) reacted easily with other substance and burned with a different colour (orange) to the other elements.

Then came along John Dalton, from Cumberland in England.


He started off as a chemist, and worked with gases for a while,

he then thought about the gases and was curious that the GL Gas Laws were not in complete harmony with reality, and published his own Law of Partial Pressures, Daltons Law.

On the back of this great success he publishes his own Table of Relative Atomic Weights, this further adds to the idea that they truly believed there were just different types of atoms at this time. A great deal of the information he put in this table was little more than arranging other peoples work. Yet it is the first ordered table Chemistry has seen and the year is 1803.

Five main points of Dalton's Atomic Theory

  • Elements are made of tiny particles called atoms.
  • All atoms of a given element are identical.
  • The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative weights.
  • Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same set ratio of types of atoms.
  • Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.

And so now everything is decided ....

.......... or so they thought.

The next thing that happened the Atom was not for almost 100 years, when J.J. Thompson who was experimenting with Cathode Rays (more about that later) that had been discovered or first created by German scientist Johann Hittorf in 1869.

J.J. Thompson carried out 3 experiments

1 Was negative charge seperable from the cathode beam.

This experiment shows that however we twist and deflect the cathode rays by magnetic forces, the negative electrification follows the same path as the rays, and that this negative electrification is indissolubly connected with the cathode rays"

J.J. Thompson


Mr Kerr

2 We know they are deflected in a magnetic field, can they be deflected in an electric field

Yes they did, and with what was decided a Negative charge.

3 Lets try to measure the Mass to Charge ratio.

He compared the deflection of his cathode rays to hydrogen ions, he found that they were not as forcefu as the hydrogen ions. In fact they were over a 1000 times greater than the mass to charge ratio of the Hydrogen, this means the particle is over 1000 time more charged or less than 1/1000th the mass?

This last experiment was carried out in 1897, J.J. Thompson had discovered negatively charged particles that were very small parts of the atom. The idea of these particles existing had be forwarded by an Irish man by the name of George Johnstone Stoney, he had called these particles Electrons, and so the name stuck.



The new idea was that these electrons were stuck in the atom randomly like raisins in a christmas cake.

Until Ernest Rutherford came along, (a long way he came too), he was born in New Zealand and educated there too. In 1895 in order to continue his studies he went to England to study in the well sponsored Cavendish Laboratory in the University of Cambridge.

Rutherford is known as the Father of Nuclear Physics because it was he that showed that very heavy elements change from one atom to another, in nuclear disintigration. He also demonstrated half-life and from this proved that the earth was far older than had previously been believed.

All of a sudden Atom could be interchanged?

While they were all different they were similiar ??

the element rutherfordium, Rf, Z=104

In 1909, an undergraduate, Ernest Marsden, was being trained by Geiger. To quote Rutherford (a lecture he gave much later):

"I had observed the scattering of alpha-particles, and Dr. Geiger in my laboratory had examined it in detail. He found, in thin pieces of heavy metal, that the scattering was usually small, of the order of one degree. One day Geiger came to me and said, "Don't you think that young Marsden, whom I am training in radioactive methods, ought to begin a small research?" " Why not let him see if any alpha-particles can be scattered through a large angle?" I may tell you in confidence that I did not believe that they would be, since we knew the alpha-particle was a very fast, massive particle with a great deal of energy, the chance of an alpha-particle's being scattered backward was very small. Then I remember two or three days later Geiger coming to me in great excitement and saying "We have been able to get some of the alpha-particles coming backward …" It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.

A 40 cm wide missile being fired at tissue paper and it bouncing back, something very strange going on here!!

The feeling at the time was that the atom had its mass shared evenly over space in what they called the plum pudding model. That the whole space taken up by the atom had the atoms mass spread evenly across the entire space. Those experiments carried out in the Cavendish Lab suggested things were not so simple. Some of the Alpha particles were reflected back along the path they came in ?? The majority passes straight trough with little or no deviation from the path it was sent along. 1 in every 8000 was reflected back at an angle that was unexpected.

In 1908 Ernst Rutherford did an experiment where he beamed alpha particles at a piece of very thin gold foil. Alpha particles were known to be very small atoms (actually the nucleus of the helium atom) and gold was made up of big atoms. Most of the alpha particles passed through the foil, but some bounced back. The shocking conclusion was that most of the atom was empty space.

It was as if all the mass must be concentrated at the center of the atom.

This led Rutherford to propose in 1911 the idea that the atom looked something like the solar system, with a small dense positively-charged nucleus with electrons swirling around it.

Rutherford predicted the existence of the neutron in 1920. Twelve years later, his assistant James Chadwick found it. Chadwick had been a student at Manchester University. After graduating in 1911, he stayed at the laboratory doing research for Rutherford.