SW1: Scientific Models: The Atom

By the early 1900s, experiments had revealed that atoms consist of particles much smaller than the atom itself: one tiny, positively charged nucleus comprising almost all of the atom’s mass, plus a number of even tinier, negatively charged electrons, such that the total electric charge is zero. In this activity you will build and evaluate possible configurations of these particles to try to produce a stable model of the atom.

Part A: Static Model

The Law of Static Electricity states that OPPOSITE charges ATTRACT and LIKE charges REPEL.

1.Hydrogen is the simplestatom. It has one negatively charged electron and a positively chargednucleus. What would happen if you put the electron near the nucleus and “let go”?

2.How can Hydrogen exist as a stable atom if its electron and nucleus are attracted to each other? Can you thinkof a fix for this problem?

Part B: The Planetary Model

If electrons in the atom cannot be standing still, then they must be moving. Maybe the atom looks like a tiny solar system, with electrons orbiting around the nucleus,like planets around the Sun. As you consider this model, recall that objects that are moving will continue moving on a straight path unless pushed or pulled to the side.

1.What has to happen to a moving electron to change its direction of motion?

2.How might the positively chargednucleus of an atom bend the path of a moving electron?

3.A circular path, or orbit, is the simplest trajectory that an electron could follow. What would happen to the electron’s orbit if we gradually removed energy from the atom?

Part C:The Failure of the PlanetaryModel

Any charged object is surrounded by an electric field. It is this field of the nucleus that exerts an attractive force on an electron inside the atom. The electron, too, is surrounded by an electric field. Let’s use the PhET simulation ( to investigate what happens to that field when the electron accelerates (wiggles around).

1.Begin with the following settings: Manual, Full Field, Electric Field, Static Field.What happens to the electric field when you wiggle the electron in the transmitting antenna?

2.Change the settings to: Manual, Full Field, Electric Field, Radiated Field. What happens when you wiggle the electron in the transmitting antenna?

3.Change the settings to: Oscillate, Full Field, Electric Field, Radiated Field. Watch the electron in the receiving antenna. Where does it get the energy to move?

4.An electron orbiting around the nucleus is accelerating just like the electron you wiggled in the antenna.(Imagine looking at the atom from the side. As the electron orbits, it will appear to move up and down.) What would be emitted by the electron as it orbits around the nucleus?

5.Whenever a charged object accelerates (changes its speed or direction of motion), it emits electromagnetic (EM) waves. It takes energy to create these waves, and the waves carry this energy away. Why would this be a problem for the Planetary Model of the atom?

Summary:

1.Electrons can’t stand still because:

2.Electrons can’t move because:

There is no way to escape the catastrophic failure of any commonsense atom. This raises the question: if the electrons in an atom can’t stand still, and can’t move, what could they possibly be doing? The answer lies in Quantum Mechanics—a completely new set of laws that describe how nature behaves at a deeper level.