1. Pudding and Clouds EQ: How has atomic structure evolved over time?

2. Atoms Atoms are extremely small particles, which cannot be seen, even with microscopes. The word atom has been around for 2500 years. It comes from the Greek word for indivisible – atomus. All matter is made up of atoms. Atoms are the smallest units of matter (although atoms can be divided further with great effort)

3. Models Scientists have created MODELS to describe atoms. Models are similar to theories, but often include a picture or physical representation. Models are usually simplified imitations of something you want to explain. – A model airplane is a small representation of a larger aircraft.

4. Models (cont.) Models can be a plan, a physical device, a drawing, or even a mental image. Architects usually construct a model of a building, bridge, or other project they are about to undertake. In the same way, chemists create models to explain how matter is constructed and how it behaves.

5. Scientific Evidence When a model is supported by lots of scientific evidence, it is often accepted by the scientific community. Scientific evidence is a collection of observations that many people have made. Everyone agrees on the same collection of observations. As new evidence is gathered, models are refined and changed.

6. Dalton’s Model In the early part of the 19 th 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.

7. JJ Thomspon’s Model First, came evidence of a tiny atomic structure with a negative charge. JJ Thompson 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).

8. JJ Thompson’s Model (cont.) 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.

9. Rutherford’s Model A physicist named Earnest 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 alpha particles went right through the gold foil (about 98% of them).

10. Rutherford’s Model (cont.) A few of the particles went through the foil but were deflected at a wild angle. The most interesting finding to Rutherford was that a small percentage of the particles (about.01%) actually bounced off the gold foil right back in the direction 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.

11. Rutherford’s Model (cont.) 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 around the nucleus, leaving a lot of space between them for alpha particles to pass through.

12. Bohr Model The scientific evidence collected by Niels 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.

13. Bohr Model (cont.) 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.

14. Electron Cloud Model 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. – This cloud represents the place where an electron would probably be found at any given moment.

15. PARTS OF AN ATOM Nucleus – located in the very center of the atom (protons and neutrons) Protons – positively charged part of an atom (found in the nucleus) Neutrons – neutrally charged part of an atom (found in the nucleus)

16. PARTS OF AN ATOM (cont.) Electron – negatively charged part of an atom (located on shells) Shells – layers surrounding the nucleus of an atom that house electrons Core shells – the innermost layers of an atom Valence shell – the outermost layer of an atom Valence electron – the electrons located on the outermost layer of the atom

17. Summarize Write one sentence for each atomic model summarizing the important information an absent student would need to know in order to do well on the test. – Dalton – JJ Thomson – Rutherford – Bohr – Electron Cloud