Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Objectives Describe some of the experiments that led to the current atomic theory. Compare the different models of the atom. Explain how the atomic theory has changed as scientists have discovered new information about the atom. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Beginning of the Atomic Theory What Is an Element? Around 440 BCE, a Greek philosopher named Democritus thought that you would eventually end up with a particle that could not be cut. He called this particle an atom. From Aristotle to Modern Science Aristotle, another Greek philosopher, disagreed with Democritus’s ideas. He believed that you would never end up with a particle that could not be cut. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Beginning of the Atomic Theory, continued From Aristotle to Modern Science Democritus was right, though: Matter is made of particles, which we call atoms. An atom is the smallest particle into which an element can be divided and still be the same substance. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Dalton’s Atomic Theory Based on Experiments Dalton’s Theory John Dalton published his atomic theory in His theory stated that all substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. Atoms of the same element are exactly alike, and atoms of different elements are different. Atoms join with other atoms to make new substances. Not Quite Correct The atomic theory was then changed to describe the atom more correctly. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Thomson’s Discovery of Electrons Negatively Charged Particles Thomson experimented with a cathode-ray tube like the one shown on the next slide. He discovered negatively charged particles that are now known as electrons. Like Plums in Pudding After learning that atoms contain electrons, Thomson proposed a new model of the atom. Thomson thought that electrons were mixed throughout an atom, like plums in a pudding. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Development of the Atomic Theory Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Rutherford’s Atomic “Shooting Gallery” Negatively Charged Particles In 1909, Ernest Rutherford aimed a beam of small, positively charged particles at a thin sheet of gold foil. The next slide shows his experiment. Surprising Results Rutherford expected the particles to pass right through the gold in a straight line. To Rutherford’s great surprise, some of the particles were deflected. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Development of the Atomic Theory Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Where Are the Electrons? Far from the Nucleus Rutherford proposed that in the center of the atom is a tiny, positively charged part called the nucleus. Bohr’s Electron Levels In 1913, Niels Bohr proposed that electrons move around the nucleus in certain paths, or energy levels. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Where Are the Electrons?, continued The Modern Atomic Theory According to the current theory, there are regions inside the atom where electrons are likely to found. These regions are called electron clouds. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Atom Objectives Describe the size of an atom. Name the parts of an atom. Describe the relationship between numbers of protons and neutrons and atomic number. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Objectives, continued State how isotopes differ. Calculate atomic masses. Describe the forces within an atom. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Atom How Small Is an Atom? Three One-Hundred-Millionths of a Centimeter! Scientists know that aluminum is made of average- sized atoms. An aluminum atom has a diameter of about cm. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu What Is an Atom Made Of? The Nucleus Protons are positively charged particles in the nucleus. Neutrons are the particles of the nucleus that have no electrical charge. Outside the Nucleus Electrons are the negatively charged particles in atoms. Electrons are found around the nucleus within electron clouds. All the structures of the atom can be seen on the next slide. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Atom Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu How Do Atoms of Different Elements Differ? Starting Simply The hydrogen atom has one proton and one electron. Now for Some Neutrons The helium atom has two protons, two neutrons, and two electrons. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu How Do Atoms of Different Elements Differ? Building Bigger Atoms For bigger atoms, simply add protons, neutrons, and electrons. Protons and Atomic Number The number of protons in the nucleus of an atom is the atomic number of that atom. All atoms of an element have the same atomic number. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Isotopes Isotopes are atoms that have the same number of protons but have different numbers of neutrons. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Isotopes, continued Properties of Isotopes An unstable atom is an atom with a nucleus that will change over time. This type of isotope is radioactive. Telling Isotopes Apart You can identify each isotope of an element by its mass number. The mass number is the sum of the protons and neutrons in an atom. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Isotopes, continued Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Isotopes, continued Naming Isotopes To identify a specific isotope of an element, write the name of the element followed by a hyphen and the mass number of the isotope. Calculating the Mass of an Element The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes of that element. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Forces in Atoms Four Basic Forces Four basic forces are at work everywhere, even within the atom. These forces are gravitational force, electromagnetic force, strong force, and weak force. These forces work together to give an atom its structure and properties. Chapter 11

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Atom Chapter 11