3.2 the structure_of_the_atom2
- 1. Section 2 The Structure of the Atom Lesson Starter • Even though the two shapes look different, the characteristics of the various parts that compose them are the same. • The same is true with the atom. • Though atoms of different elements display different properties, isolated subatomic particles have the same properties. Chapter 3
- 2. 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. • Define atom. Chapter 3
- 3. Section 2 The Structure of the Atom The Structure of the Atom • An atom is the smallest particle of an element that retains the chemical properties of that element. • The nucleus is a very small region located at the center of an atom. • The nucleus is made up of at least one positively charged particle called a proton and usually one or more neutral particles called neutrons. Chapter 3
- 4. Section 2 The Structure of the Atom The Structure of the Atom, continued • Surrounding the nucleus is a region occupied by negatively charged particles called electrons. • Protons, neutrons, and electrons are often referred to as subatomic particles. Chapter 3
- 5. Properties of Subatomic Particles Section 2 The Structure of the Atom Chapter 3
- 6. Section 2 The Structure of the Atom Discovery of the Electron Cathode Rays and Electrons Chapter 3 • These particles were named electrons. • Experiments in the late 1800s showed that cathode rays were composed of negatively charged particles.
- 7. Section 2 The Structure of the Atom Discovery of the Electron, continued Charge and Mass of the Electron Chapter 3 • Joseph John Thomson’s cathode-ray tube experiments measured the charge-to-mass ratio of an electron. • Robert A. Millikan’s oil drop experiment measured the charge of an electron. • With this information, scientists were able to determine the mass of an electron.
- 8. Section 2 The Structure of the Atom Discovery of the Electron, continued Chapter 3
- 9. 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
- 10. Gold Foil Experiment Section 2 The Structure of the Atom Chapter 3
- 11. Gold Foil Experiment on the Atomic Level Section 2 The Structure of the Atom Chapter 3
- 12. 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. 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. Section 2 The Structure of the Atom Composition of the Atomic Nucleus, continued Forces in the Nucleus Chapter 3 • When two protons are extremely close to each other, there is a strong attraction between them. • A similar attraction exists when neutrons are very close to each other or when protons and neutrons are very close together. • The short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together are referred to as nuclear forces.
- 15. 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