History of the atom-Ernst Rutherford
Work done by a New Zealand scientist Ernst Rutherford brought us even closer to the present day model of the atom. He performed, his now famous, gold leaf experiment where he fired alpha particles (helium nuclei) at a gold leaf. Rutherford noticed that most of the high energy alpha particles passed through the gold lead as though it wasn't there. However, 1 in 8,000 alpha particles was deflected through a high angle and on the odd occasion one would come straight back.

Rutherford concluded that for such a large proportion of alpha particles to pass straight through the gold foil, most of the volume of the atom is empty space.

He suggested that the small amount of particles bounced back due to repulsion by the positive matter identified by earlier physicists. This positive matter Rutherford would call the nucleus.

Still other alpha particles that passed close to this positive mass were repelled to a lesser extent and deflected away from their linear path, as shown on the right.
Rutherford was soon able to predict the pattern of the scattering of alpha particles, as shown on the right. He knew that the closer the alpha particles came to the nucleus the more they were repelled.

Rutherford's calculation revealed that the distance of closest approach was
3 X 10-14 m. However knowing that the atom had a radius of 1 X 10-10 m he concluded that the nucleus is a small, solid, positive charged mass in the vastness of the atom.

To put it into perspective, if the MCG, Melbourne Cricket Ground, was the atom the nucleus would be the size of a flea in the middle of the ground.

From his studies Rutherford concluded that the atom had a structure that resembled the solar system, where the electrons revolve around the nucleus much like planets around the sun.

In 1911 Rutherford put forward his model of the atom that proposed:
- Most of the mass and all its positive charge must be located in a central region called the nucleus;
- Most of the volume of an atom is space;
- Electrons move in circular orbits around the nucleus;
- An electrostatic attraction exists between negative electrons and the positive nucleus.

 

In 1914 Rutherford concluded that the fundamental particle of positive charge was the nucleus of the hydrogen atom which he called the proton. Observation made through experimentation revealed that the charge of all nuclei was equal to the charge of a proton multiplied by a whole number.

However Rutherford's model could not explain why the helium, which was known to contain two protons and two electrons was four times heavier rather that just two times heavier. The problem was even more pronounced for heavier atoms such as lead or uranium. Neutrons were yet to be discovered.

Rutherford would later reject his original explanation and propose that the nucleus contained particles with significant mass but no charge.
Explain why Rutherford proposed the word nucleus?
What experimental evidence did Rutherford have to put forward his model of the atom?
Why did Rutherford reject his original explanation of the nucleus?
What particle was not yet discovered that would have assisted Rutherford in describing the nucleus?
When constructing a model of the atom a small spherical bearing of radius 0.1 mm is placed in the middle of the oval. Calculate the radius of the oval that would represent the outer edge of the atom.
Continue with Chadwick and Neils-Bohr