Chapter 3.2 Atomic Structure Preview Objectives The Structure of the Atom Properties of Subatomic Particles Discovery of the Electron Discovery of the Atomic Nucleus Gold Foil Experiment Gold Foil Experiment on the Atomic Level • Composition of the Atomic Nucleus • The Sizes of Atoms • • • • • • • • 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. • 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. •Surrounding the nucleus is a region occupied by negatively charged particles called electrons. •Protons, neutrons, and electrons are often referred to as subatomic particles. Visual Concept What particle did Thomson discover? J.J. Thomson discovered that atoms are made of smaller negativelycharged particles called electrons. Thomson’s discovery was the result of doing experiments with “cathode ray tubes” Stream of electrons is attracted to positively charged plate here. “What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision?” quote by Thomson Attracted to the positive electrode Not visible but could make things “glow” Traveled in a straight line Could be bent by electric or magnetic fields A plate in it’s path acquired a negative charge Same regardless of material Thomson did not know how the electrons in an atom were arranged. He believed they were mixed throughout an atom. He proposed that the atom was a sphere of positively charged material. • Spread throughout the atom were the negatively charged electrons similar to plums in a pudding or chocolate chips in ice cream. Charge and Mass of the Electron Joseph John Thomson’s cathode-ray tube experiments measured the charge-tomass 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. Visual Concept Awarded the Nobel Prize in Chemistry for his discovery of alpha particles, positively charged particles emitted from radioactive elements Was a student of J.J. Thomson but disagreed with the “Plum Pudding Model” • Ernest Rutherford and his associates Hans Geiger and Ernest Marsden wanted to investigate the atom more. • 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. Most of the particles traveled straight through the gold foil What was the surprising behavior of a few of the particles? A few of the particles were deflected and some even bounced back • 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. •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. Forces in the Nucleus 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, protonproton, and neutron-neutron forces that hold the nuclear particles together are referred to as nuclear forces. • 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. The diameter of a pinhead is 100,000 times smaller than the diameter of a stadium. Likewise the diameter of the nucleus of an atom is 100,000 times smaller than the diameter of an atom • 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.