1. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Structure of the Atom Objectives Summarize the observed properties of cathode rays that led to the discovery of the electron. Summarize the experiment carried out by Rutherford and his co-workers that led to the discovery of the nucleus. List the properties of protons, neutrons, and electrons. Chapter 3

2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Structure of the Atom Discovery of the Electron Cathode Rays and Electrons Chapter 3 Experiments in the late 1800s showed that cathode rays were composed of negatively charged particles. These particles were named electrons.

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Structure of the Atom Discovery of the Electron, continued Joseph John Thomson’s cathode-ray tube experiments measured the charge-to-mass ratio of an electron. Chapter 3 Charge and Mass of the Electron With this information, scientists were able to determine the mass of an electron. Robert A. Millikan’s oil drop experiment measured the charge of an electron.

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Structure of the Atom Discovery of the Electron, continued Chapter 3

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Expanding the Modern Atomic Theory J.J. Thompson is the person who is credited for discovering the electron. Thompson created a tube that had a positively charged anode on one side and a negatively charged cathode on the other side. Thompson then applied a magnet to the middle of the tube and discovered that negatively charged particles were emanating towards the positive magnetic field. From this, Thompson concluded that negatively charged particles, called electrons, were present in atoms. J.J. Thompson Thompson then created the Plum Pudding model, which suggested that electrons and protons were randomly placed throughout the atom. The “Plum Pudding”

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Structure of the Atom Discovery of the Atomic Nucleus More detail of the atom’s structure was provided in 1911 by Ernest Rutherford and his associates Hans Geiger and Ernest Marsden. The results of their gold foil experiment led to the discovery of a very densely packed bundle of matter with a positive electric charge. Rutherford called this positive bundle of matter the nucleus. Chapter 3

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Gold Foil Experiment Section 2 The Structure of the Atom Chapter 3

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Rutherford's Experiment continued… When the data from this experiment was gathered the scientists were shocked. Alpha particles were known to be dense and at that time atoms where thought to be far less dense. One experiment likened having an alpha particle bounce back to firing an artillery shell into a roll of toilet paper and having it bounce back.

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The nucleus is very small and VERY dense Relative scale model of an atom and the solar system Do you perceive a gold ring to contain a larger fraction of solid matter than the solar system? On this scale, the nearest star would be a little over 10,000 miles away.

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Rutherford's Experiment In 1911 British scientist Ernest Rutherford set out to test Thomson’s proposal by firing a beam of charged particles at atoms. Alpha particles are heavy particles with twice the positive charge of a proton. Alpha particles are now known to be the nuclei of helium atoms, which contain two protons and two neutrons. Ernest Rutherford's experiment was to emit alpha particles towards a thin gold sheet. Rutherford would then determine where the deflections of the alpha particles would go, and therefore be able to theorize what kind of placement protons and electrons had. Ernest Rutherford

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Rutherford's Experiment continued… Rutherford observed that most of the alpha particles went strait through the foil. However a large proportion were deflected through small angles and some (though very few) deflected straight back. Rutherford then theorized that there was something called a nucleus, which contained a high density of positively charged particles. Rutherford was able to say there was a nucleus because alpha particles that deflected right back must have hit something more massive and with a strong positive charge. This led Rutherford to propose a very different model for the atom. Instead of supposing that the positive charge and mass were spread throughout the volume of the atom, he theorized that it was concentrated in the center of the atom. Rutherford called this concentrated region of electric charge the nucleus of the atom.

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Structure of the Atom Composition of the Atomic Nucleus Except for the nucleus of the simplest type of hydrogen atom, all atomic nuclei are made of protons and neutrons. A proton has a positive charge equal in magnitude to the negative charge of an electron. Atoms are electrically neutral because they contain equal numbers of protons and electrons. A neutron is electrically neutral. Chapter 3

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Structure of the Atom Composition of the Atomic Nucleus, continued The nuclei of atoms of different elements differ in their number of protons and therefore in the amount of positive charge they possess. Thus, the number of protons determines that atom’s identity. Chapter 3

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Structure of the Atom Composition of the Atomic Nucleus, continued When two protons are extremely close to each other, there is a strong attraction between them. Chapter 3 Forces in the Nucleus The short-range proton-neutron, proton-proton, and neutron-neutron force that holds nuclear particles together is referred to as strong nuclear force. A similar attraction exists when neutrons are very close to each other or when protons and neutrons are very close together.

15. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Structure of the Atom The Sizes of Atoms The radius of an atom is the distance from the center of the nucleus to the outer portion of its electron cloud. Because atomic radii are so small, they are expressed using a unit that is more convenient for the sizes of atoms. This unit is the picometer, pm. Chapter 3