By combining Lavoisier’s law of conservation of matter and Proust’s law of definite proportions, Dalton was able to establish his atomic theory and to suggest a model for matter as tiny solid particles. Explaining the atomic theory was the stepping stone to developing the modern model for the atom. Dalton’s work provided a general explanation for matter. But what exactly are these atoms that Dalton and, previously, the atomists had proposed? What were they made of? What give them shape and form? What distinguished on atom from another?
It was not until the discovery and further investigation of the electron in the late 1800s that the true nature of the atom could be explored. Once associated with the atom, electrons appeared to universal component of all matter. It was believed that the negatively charged electron floated in a sea of positive chargethis was the only conceivable organization to support a stable atom composed of negatively charged particles. This model of the atom was referred to as the plum pudding modelit was likened to the English plum pudding; negative electrons represented the plums floating in the pudding.
In 1909, Ernest Rutherford performed his famous gold foil experiment that discredited the plum pudding model. Instead, Rutherford’s experiment established that the atom is mostly empty space with an extremely dense, positively charged nucleus.
This section will explore Rutherford’s nuclear model of the atom through a class discussion, review, and analysis of his classic experiment and a laboratory exercise that assigns the students to develop a logical model of a hidden object.
This section of the unit will partially fulfill Content Standards on scientific inquiry of New Haven Public Schools Academic Performance Standard by:
1. Designing and conducting scientific investigation
2. Communicating and defending a scientific argument
3. Recognizing that the results of scientific inquirynew knowledge and methodsemerge from different type of investigations and public communication among scientist.
The time limit for this section of the unit is 2-3 45 minutes class periods. It will begin with the review and analysis of Rutherford’s gold foil experiments and an interpretation of his results from this experiment. This review will be followed by a laboratory exercise that challenges students to determine the approximate shape, size, and location of an object hidden inside a black box. Students must develop their own procedures and draw conclusions based on their observation
The teacher may find Niel Jespersen’s book Chemistry quite helpful at explaining the experiments performed by scientists like Sir William Crookes (cathode ray tube experiment), Robert Millikan (oil drop experiment) whose work contributed to Rutherford’s experiments. The book can be purchased from Barns & Nobles or Amazon.com.
Review of Rutherford Gold Foil Experiment
Rutherford’s experiment verified that an atom is mostly space and the bulk of the mass is concentrated in a very small volume of the atom called the nucleus. Prior to his experiment, it was generally accepted that the mass was evenly distributed through out the atom as is demonstrated by the plum pudding model. To test this model, Rutherford performed an alpha scattering experiment where he aimed very small alpha particles through a very thin piece of gold foil. If the previous plum pudding model accurately depicted the atom, Rutherford expected that most of the alpha particles would be deflected as is shown in the figure 1 (appendix). Instead, Rutherford observed that the vast majority of the particles passed through the foil without any deflection, as though the foil was not there, while a few particles were deflected at large angles (figure 2; appendix). From this, he concluded that most of the particles pass through empty space, while a few collided with the relatively dense nucleus.
Introduction to the Experiment
The teacher will provide a brief (15 minutes) introduction to the plum pudding model and explain this model to the students. The important idea to get across is that the mass of is evenly distributed throughout the atom. An overhead projection of the plum pudding model atom as shown in figure 1 is helpful at providing students a visual representation of this model. Allow students to ask questions for clarification.
Once students have been introduced to the model, the teacher must discuss the concept of deflection. Ask students the following questions
What would they expect to occur if light is shined at mirror? (Answer: it will be reflected).
What happens then if the mirror reflective surface was not completely intact, but was instead transparent at certain areas? (answer: some of the light will pass through)
To better introduce the idea of deflection, the teacher could perform a demonstration by shinning light through the two different types of mirrors and allowing the students to observe for him or herself.
Analysis of the Models
Once the plum pudding model is introduced, assign students to groups of 2 to complete worksheet 1 in the appendix. The worksheet is design to get student to think creatively and critically about the plum pudding model and Rutherford’s gold foil experiment.
Developing a Model of a Hidden Object
The exercise was revised from Exercise and Experiments in Modern Chemistry. Like Rutherford’s experiment students will perform an experiment to determine the approximate location, size, and shape of an object located in a box. The teacher will prepare boxes sealed with a stationary object and a marble that acts as a probe. Working in groups of 2 students will develop their own procedure, record their observations and construct a logical model. (For a complete lab handout please refer to page 141 of Exercise and Experiments in Modern Chemistry). The exercise is designed to model Rutherford’s experiment in that the box represents the atom, the hidden object represents the nucleus, and the marble represents the alpha particle.