# Activity: Atomic models Physicists believe that the atom has a positive nucleus surrounded by negative electrons. In this activity you will delve into a little history and philosophy of science to understand why the Thomson model was shown to be wrong by Rutherford's experiment. The historical development of atomic theory is an example of the Nature of Science.

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Doing this at home?

As hard as your teacher may try, they are unlikely to be able to zoom into any meaningful scale on the atom - so this is a great topic to learn at home!

The atom so far

The concept of matter being made up of particles was introduced in the unit on thermal physics ( Particle nature of matter activity ) to explain the following, in small groups discuss the explanations for each.

• the difference between solid liquid and gas
• where energy goes when work is done against friction
• the origin of air resistance
• buoyancy
• gas pressure
• Brownian motion

In this section atoms were treated like very small perfectly elastic balls, this model was also used to explain:

• the gas laws
• why a gas gets hot when compressed
• the relationship between internal energy of a gas and work done on it plus heat exchanged.

In the section on electricity the concept of charge was introduced to explain:

• the electric force
• current and resistance
• electrical energy and power

Electric current is the movement of charged particles called electrons. These can be seen in the Thomson's experiment. If you have one of these in class take a look at it (or watch the video below). Try holding a magnet near the line to see what happens. See if the direction of the force agrees with Flemings LHR (it does of course, this is really to see if you do it correctly).

(TOK)The blue line you see is actually light emitted from air atoms excited by the electrons. Are you really seeing electrons?

Using this apparatus it is possible to find the charge/mass ratio for an electron. This is done by balancing electric and magnetic forces so the electrons travel in a straight line. This isn't on the syllabus but it's good application of electric and magnetic fields. For the case shown determine

• The electric field strength
• The electric force on the electron and its direction
• The magnetic force on the electron and its direction
• Equate the forces and show that V = Bdv.

Electrons

The electron is the smallest known charge so to find the charge of an electron you just need to measure the charge of a lot of different objects and find the largest common factor. Milikan did this by balancing electric and gravitational fields to hold oil drops stationary. For this case determine

• The gravitational force
• The electric force
• If the drop is stationary show that q = mgd/V

The electron was found to have
charge = -1.6 x 10-19 C
mass = 9.1 x 10-31 kg

This is about 1/2000 of the mass of a hydrogen atom

• How does this support the explanation of electrons being the charge carrying particles in conduction?

Rutherford

Scattering experiments

To find out what is inside a box you could shoot a bullet at it. • For each of the examples given here, describe what would happen to the bullet.

Shooting a bullet at matter would not reveal anything about the structure of the atom for that you need a much smaller projectile. Some materials emit fast moving, very small (1/50 the mass of a gold atom), positive, particles called alpha particles. These can be fired at thin sheets of matter to see what happens. Before discussing the results let's make a prediction

(NOS) scientists often use evidence from experiments to define models that are used to make predictions.

Early models described the atom as a + ball with - charges embedded in it, often called the plum pudding model. A plum pudding.

Happy Christmas.

Note that the holly leaves and berries plus the white sauce is not part of the atomic model but do make the pudding look nicer. The sauce also makes it taste nicer but don't eat the holly.

• In a neutral atom what must be true about the + and - charge.
• Explain how a balloon becomes charged when rubbed using the plum pudding model.

If atoms are like plum puddings and alpha particles like bullets explain what would happen if an alpha particle would be fired at a thin gold foil only a few atoms thick. • Would there be any force between the alpha and the atoms?

Is this what you thought would happen?

What actually happens was quite unexpected.

1. Most alphas pass through.
2. Some are deflected
3. some bounce back

How can a bullet bounce of a pudding? It can't so the atomic model needs to be redesigned.

What if the atom had a heavy, positive, very small centre with the electrons around the outside. This implies that their is a lot of space around each atom so the scattering experiment would be more like this: • Predict how all the alpha particles shown would scatter.

This is more like the real case. One way the real case differs is that the nuclei are not fixed.

• Use Algodoo to show that the alpha can't be scattered by more than 90° if the nucleus isn't heavier than the alpha. To make the particles repel set attraction to a negative value.

The Rutherford model

If the nucleus is positive the electrons must be somewhere outside the nucleus, maybe they orbit the nucleus like planets orbiting the sun? Write equations for:

• The centripetal force.
• The electric force experienced by the electron.
• equate these forces to show that If you take a negative charge and spin it around on the end of a string it would create a changing electric and magnetic field. Any charged object nearby would be caused to move by the field.

• Is energy transferred in this process?
• Use this analogy to explain why the electron would lose energy if it orbited the nucleus.

You should have deduced that there is a problem with this model too. This will be resolved (for HL students) in a later section.

Summary

[NOS] This is a good example of the scientific method of hypothesis > prediction > experiment > new model.

Below is a prezi showing how scientific theories develop

Under the following headings, write a short account of how the scientific method was applied to your individual investigation (IA) or EE (if you did one in physics). Use the Prezi to help fill in the details.

• Exploration and discovery
• Testing ideas
• Community analysis and feedback
• Benefits and outcomes
Words: 1-500

Investigate the different models with this PhET simulation:

Assess yourself

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