Homework – Investigation III – Lesson 1



Homework – Pudding and Clouds

1. Share the information you learned today about atoms with an adult or parent. Explain to this person what you learned in class today. Then ask that person to write a quick paragraph in your notebook explaining what you shared with them. Ask your adult or parent to sign and date the notebook for authenticity.

2. Review the following information. Make a labeled sketch to explain and illustrate the content of each paragraph.

In the early part of the 19th century, John Dalton revived the ancient notion that all matter is made up of indivisible particles called atoms. He pictured the atom as a solid sphere that could not be divided into anything smaller. Over the next two hundred years scientific evidence led to a more and more sophisticated model of the atom.

First, evidence came of a tiny atomic structure with a negative charge. J.J. Thomson named this structure a “corpuscle.” Later, these tiny structures came to be called electrons. Thomson pictured these electrons scattered throughout an atom as raisins might be in a pudding – hence the Plum Pudding Model. He also recognized that overall an atom is neutral (not negatively charged). He concluded that there must also be an equal positive charge in the atom to offset the negative charge of the electrons. In Thomson’s mind the “pudding” portion of the plum pudding consisted of positively charged stuff.

A physicist named Ernest Rutherford was the next one to identify more complex structures within the atom itself. His ingenious experiment used a very, very thin piece of gold foil, just a few atoms in thickness. He shot tiny alpha particles at the gold foil. Most of the particles went right through the gold foil (about 98% of them). A few of the particles went through the foil but were deflected at a wide angle. But the most interesting finding to Rutherford was that a small percentage of the particles (about 0.01%) actually bounced off the gold foil right back in the direction that they came from. Rutherford concluded that there was a small, very dense structure in the middle of the atom. He called this structure the nucleus. He also said that the nucleus had a positive charge on it and that the alpha particles were bouncing off the nucleus whenever they happened to strike it directly. Rutherford pictured the negatively charged electrons circling the nucleus, leaving a lot of space between them for alpha particles to pass through.

The scientific evidence collected by Neils Bohr led him to conclude that the particles called electrons could only be found around the nucleus at specific distances. These areas around the nucleus where electrons are found are also referred to as electron shells. These shells are sometimes represented as concentric spheres. According to Bohr, electrons could be located at specific distances from the nucleus, but not in the spaces between the shells. (The word shell implies that electrons occupy only the surface of each sphere).

The last model in our series is called the Electron Cloud Model. The work of many scientists led to this final model, which, you may notice, includes a second heavy particle in the nucleus (called a neutron). The other notable feature of this model is that the electron is shown in continuous motion and is depicted as a probability cloud. The “cloud” represents the place where an electron would probably be found at any given moment. It is thought that a German physicist named Erwin Schrödinger came up with the idea of the electron cloud.

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