Objectives

• To have an appreciation for the historical models the atom and how they have progressed specifically during the 20th and 21st century.
• Understand, reflect upon and describe the alpha-particle scattering experiment;
• Appreciate the alpha-particle scattering experiment as evidence of a small charged nucleus.

The History of the Atom

The structure of the atom has developed at a rapid pace over the past 200+ years. It would be well worth you looking into the history of the atom to determine how our knowledge on the atom has developed. People to research could include;

• John Dalton
• J. J. Thomson
• Ernest Rutherford
• Niels Bohr

After having research these scientists go to the following website, read a quick review and test yourself using the questions provided; absorblearning

The Plum Pudding Model

The Plum Pudding Model was first proposed by J. J. Thomson and was a model used for the atom.

Atoms were known to contain negative charges, known as electrons (although Thomson did make reference to them as being ‘Corpuscles’), atoms were also known to be neutrally charged overall. Thomson therefore created three plausible models to help represent the structure of the atom, the Plum Pudding Model, was the one that was most widely accepted.

This model stated that the negative electrons occupied a region of space that itself was a uniform positive charge (often considered as a kind of “soup” or “cloud” of positive charge).

The Alpha-Particle Scattering Experiment

The Alpha-particle scattering experiment was first conducted back in 1909, by two students of Ernest Rutherford; Hans Geiger and Ernest Marsden. Rutherford decided to continue Thomson’s studies of the atom  (he was a student of Thomson).

They fired a ‘beam’ of alpha particles at a gold leaf, which at the time they expected to travel straight through. They used a Zinc Sulphide screen over a microscope to detect where the alpha particle would end up.

The results showed that most of the alpha particle would travel straight through the gold leaf. Some were deflected at small angles and other at large angles, and some (although very few) were deflect back in the direction that they came from, this happened to fewer than 1 in 8000 alpha particles.

More information on the alpha-particle scattering experiment itself and what you need to know for your exams can be found here.

Three observations were announced:

1. Most of the alpha particles travelled straight through and were undeflected, this was expected for all the particles if the plum pudding model were correct.
2. Some of the alpha particles were deflected through large angles. This was not expected, but could potentially have been explained by the conservation of momentum.
3. A very few number of alpha particles were deflected back to the same side of the gold leaf as that of the radioactive source itself, this was not expected!

To explain these observations a new model of the atom was required. This model must include the following points about the nucleus

• It must be small – hence why so few were deflected.
• It must be massive – in the sense that it has lots of mass – it was known that the electrons had very little mass and the fact that all of the positive charges were concentrated into a small area.
• It is positively charged – because it repelled the alpha particles.
• The nucleus is in the centre of the atom, neutrons had not been discovered at that time – so there was no mention of them in this model.

As stated in the third bullet point above. The alpha particles, being positively charged, were repelled by the nucleus of the atom. This repulsion could be explained by Charles-Augustine de Coulomb who established, in 1785, that the magnitude of the electrostatic force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

With the alpha particle coming in close proximity of the nucleus, the electrostatic force is so strong that it can repel the alpha particle back in the general direction in which it came.

The Rutherford model was one that adopted the model of the Solar System, where the sun in the majority of all the mass and is concentrated at the centre with the rest of planets orbiting around it. This eventually led to the Bohr Model of the atom.

The Bohr Model

The Bohr model is the shortened name of the Rutherford-Bohr model and is more of a quantum interpretation of the Rutherford model of the atom.

It was known from the Rutherford model that electrons orbit, outside, the nucleus. The electrons can orbit the nucleus at approximately 1% the speed of light. If they are at a distance of 0.5 fm from th nucleus, this would mean they orbit the nucleus approximately $10^{17}$ times per second!

Bohr atomic model of a nitrogen atom.
Encyclopedia Britannica, Inc

Due to how fast they orbit, but also something known as the Heisenberg principle (which states, and can be shown mathematically, that the more precise the location is known the less precisely the momentum of that particle will be known), the electrons exist in ‘clouds’, around the nucleus. The idea of the electron cloud is that we can’t determine where or what direction the electronis moving in at any given moment, but it is somewhere in that ‘cloud’.

This model shows that the electrons in atoms have specific energy states around the nucleus (imagine planets and their orbits around the sun)

Bohr reffered to these energy states as energy levels (or energy shells), this helped describe the orbits as differing in energy. Bohr used the term energy levels (or shells) to describe these orbits of differing energy, this meant that electrons  had discrete amount of energy depending on which energy level they were in. They can have one energy level or another but nothing in between.

The lower the energy level (the closer to the nucleus) the electron orbits in, the less energy it has. In order to ‘jump’ up to the next energy level, the electron needs to gain some energy from a photon (electromagnetic wave). If the electron jumps down a level, it loses energy by emitting a photon. This emission and absorption of electromagetic radiation then accounts for conservation of energy. The lowest possible energy level for an electron is known as its ground state.