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The Structure of Matter Unit 2 Atomic Structure Chapter 3 Section 1 EARLY MODELS OF THE ATOM Objectives • Define the term “atom” • Explain how the idea of the atom originated • List the postulates of Dalton’s atomic theory Greeks • 450 BC—Greek philosopher Democritus • Proposed that all matter is made of tiny, indivisible particles called “atomos” (atom in English) • Theory was that if you cut a material over and over again, eventually you would have a piece that cannot be cut Atom • An atom is the smallest particle of an element that retains the chemical identity of that element • Democritus’ ideas were rejected at the time (what keeps atoms together?), but revisited in the 17th and 18th centuries…talk about waiting to be justified! Laws of Science • Late 1700s—Antoine Lavioser proposed the law of conservation of matter • No mass is gained or lost in the rxn • 1799—Joseph Louis Proust proposed the law of constant composition • A given compound always contains the same elements in the same proportions by mass John Dalton • 1803—Proposed the atomic theory of matter • Each element is composed of extremely small particles called atoms • All atoms of a given element are identical, but they differ from those of any other element • Atoms are neither created nor destroyed in any chemical rxn • A given compound always has the same relative numbers and kinds of atoms Elements • There are over 100 elements, therefore there are over 100 kinds of atoms • Do we only have 100 substances in the world? • Atoms combine to make up every substance in the world around us Atoms • Can we be absolutely 100% sure atoms exist? • Atoms are too small to be seen with a microscope • 1981—Scanning tunneling microscope gave us our closest look at an atom, but shows virtually nothing about what an atom is like on the inside Two(?) Worlds • The macroscopic world—ordinarysized objects that we can see • The submicroscopic world—smaller than we can see • Microscopic world—Need special techniques to see, but can be relatively easy to work with Chemist’s Work • Chemists work on the macroscopic and microscopic worlds and use their findings to understand the submicroscopic world In Summation • What is an atom? • List the postulates of Dalton’s atomic theory of matter • Why did the ancient Greeks reject Democritus’ ideas about atoms? • Why is there such diversity in nature if there are only about 100 different kinds of atoms? Section 2 DISCOVERING ATOMIC STRUCTURE Faraday Again • Proposed that atoms and electricity are somehow connected • Led to the discovery that atoms contain particles with electric charges Benjamin Franklin • Wait, this isn’t history class! • Franklin was a scientist as well • Flew a kite with a key attached to it during a thunderstorm • …The idiot • Lightning struck the key Benjamin Franklin • Concluded that an object could have one of two kinds of electric charge: positive (+) or negative (-) • Like charges repel each other, opposite charges attract Cathode Rays and Electrons • Using a vacuum sealed tube, scientists were able to observe a type of radiation • Called a cathode ray tube, used to be common in TVs (you can tell the age of the book from this comment) Cathode Rays and Electrons • 1896—J. J. Thomson worked with the cathode ray tube through several experiments • Concluded that the particles of radiation are negative • Named them electrons (e—) Cathode Rays and Electrons • Thomson determined that the total electrical charge to its mass is 1.76 x 108 coulombs per gram • Robert Millikan was able to determine the mass of the electron by determining a single electron’s charge • Charge: 1.60 x 10-19 coulomb • Mass: 9.11 x 10-28 gram Radioactivity • 1896—Henri Becquerel • Accidentally discovered that uranium is radioactive • Radioactivity: the spontaneous emission of radiation from an element • Marie and Pierre Curie isolated two other radioactive elements: radium and polonium Radioactivity • Radioactive decay causes fundamental changes in the atom • Rutherford studied this radiation and realized that there are three types – α radiation, which has a charge of +2 – β radiation, which has a charge of -1 – γ radiation, which has no charge or mass The Nuclear Atom • Thomson had shown the existence of electrons, but atoms are neutral • Rutherford fired alpha rays at gold foil • Most particles went straight through • About 1/8000 were deflected, and in all directions! The Nuclear Atom • Rutherford eventually realized that the “plum pudding” model of the atom was incorrect • Proposed that most of an atom’s mass, including its positive charge, is at a very small core in the center • Called this core the nucleus • EXTREMELY tiny! Section 3 MODERN ATOMIC THEORY Objectives • Name and describe the three subatomic particles of the atom • Explain how to determine the number of protons, neutrons, and electrons in an atom • Explain how an ion differs from an atom • Explain how isotopes of the same element differ • Explain the relationship between atomic mass and the relative abundance of isotopes The Structure of the Atom • Remember that the central core of an atom is a nucleus • Contains two types of particles: protons and neutrons • Protons have an equal charge as an electron, only positive • Neutrons have no charge and are very slightly heavier than a proton The Structure of the Atom • Rutherford visualized the atom as a miniature solar system with electrons circling the nucleus • A more accurate model shows the likely positions of electrons The Structure of the Atom Particle Location Charge (C) Mass (g) Mass (amu) Proton Inside nucleus +1.602 x 10-19 1.673 x 10-24 1.0073 = 1 Neutron Inside nucleus 0 1.675 x 10-24 1.0087 = 1 Electron Outside nucleus -1.602 x 10-19 9.109 x 10-28 0.0006 = 0 The Structure of an Atom • For simplicity’s sake, an electron has a 1- charge and a proton has a 1+ charge • Mass is expressed in atomic mass units (amu) • Protons and neutrons are approximately 1 amu, and electrons are essentially 0 amu The Structure of an Atom • The diameter of atoms range from 0.100 to 0.500 nm • The nucleus of the atom is very much smaller! Atomic Numbers • Henry Moseley—Discovered that each element has a unique positive charge in the nucleus • The number of protons in the nucleus is equal to the atomic number • Neutral atoms have the same number of electrons Ions • When an atom gains or loses one or more electrons, it gains a net electrical charge • This charge causes the atom to become an ion • Charge of ion = # of protons - # of e— Isotopes • All atoms of an element always have the same protons in their nuclei • Not all have the same number of neutrons • Atoms with the same number of protons but a different number of neutrons are called isotopes Isotopes • Nature contains a mixture of isotopes, typically found in the same percentages • An atom’s mass number is the sum of the isotope’s protons and neutrons • Naming an isotope The Mass of an Atom • Because protons and neutrons have slightly different masses, the amu needed to be adjusted • Scientists assigned carbon-12 to be the standard at 12 amu The Mass of an Atom • Average mass of an element’s atoms is called the atomic mass • Listed on the periodic table Objectives in Review • What are the names and properties of the three subatomic particles? • How can you determine the number of protons, neutrons, and electrons in an atom or ion? • What is an isotope? What is atomic mass? Section 4 CHANGES IN THE NUCLEUS Objectives • Describe the changes that may occur in the nucleus of a radioactive element • Write a nuclear reaction to represent radioactive decay • Compare and contrast alpha, beta, and gamma radiation Nuclear Reactions • Neither physical nor chemical reactions cause changes in the nucleus • A nuclear reaction changes the composition of an atom’s nucleus Nuclear Stability • Most atoms are stable (not radioactive) • Instability is controlled by the number of protons and neutrons Nuclear Stability • If protons all have a positive charge, what is stopping the nucleus from repelling its own particles? • A strong nuclear force is an attractive force that overcomes the electric repulsion between protons • Only works when subatomic particles are extremely close Nuclear Stability • Neutrons add a net attractive force to the nucleus • Neutrons = glue Nuclear Stability • Look at the belt of stability on pg. 113 • More neutrons are needed for larger elements to be stable • ALL elements beyond bismuth (83) are radioactive Nuclear Stability • A nucleus can become unstable if it has too few neutrons, but also if it has too many • Excess neutrons often release beta radiation Types of Radioactive Decay • What are the three types of radiation? • Alpha (α), beta (β), gamma (γ) Types of Radioactive Decay • An alpha particle consists of two protons and two neutrons • Equivalent to a helium-4 nucleus • Not much power • Symbolized by: Types of Radioactive Decay • Radiation can be useful! • Read the article about smoke detectors on page 114 Types of Radioactive Decay • Beta radiation is a stream of highspeed electrons • Still a nuclear reaction; comes from a change in the nucleus • A neutron changes into a proton and an electron • The proton stays in the nucleus; the beta particle (electron) is fired out at high speed Types of Radioactive Decay • Much more penetrating than an alpha particle (100x) • Can penetrate 1-2 mm into solid material • Represented by: Types of Radioactive Decay • Gamma rays are a very energetic form of light • Outside of the visible spectrum • Does not consist of particles • Represented by: Types of Radioactive Decay • Accompanies the other two types of radiation • Much more penetrating; can only be stopped by substances such as concrete or lead Types of Radioactive Decay • When an atom emits one of these kinds of radiation, it is undergoing radioactive decay • Scientists use nuclear equations to keep track of a reaction’s components Practice! 1. Write a nuclear equation for the alpha decay of Protactinium-231 2. Write a nuclear equation for the beta decay of Francium-223 Other Nuclear Reactions • Radioactive decay is only one type of nuclear rxn • Fusion: the collision and joining of two nuclei • Fission: the splitting of a nucleus • More will be covered in Chapter 24 Objectives in Review • What changes accompany nuclear reactions? • What is radioactivity?