History of the Atom & Atomic Theory

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1 Chapter 5 History of the Atom & Atomic Theory You re invited to a Thinking Inside the Box Conference Each group should nominate a: o Leader o Writer o Presenter You have 5 minutes to come up with observations on what is inside the box, WITHOUT opening it. What is/are the shape of the contents, weight, size, etc., and WHY do you think so Section 5. Atoms OBJECTIVES: Summarize Dalton s s atomic theory. Describe the size of an atom. When did the Idea of an Atom Come about? The Greek Philosophers tried to explain nature, and described an atom. Democritus, in 400 B.C., first suggested the existence of indivisible, indestructible atoms, called Atomos. Democritus hypothesized that if you divide matter into smaller and smaller pieces, eventually you will end up with tiny, indestructible pieces called Atoms. His ideas were untested, and rejected in lieu of the popular beliefs of Aristotle When did the Idea of an Atom Come about? Aristotle,, an earlier Greek philosopher, suggested that all substances are made of 4 elements: Fire - Hot Air - light Earth - cool, heavy Water - wet A blend of these 4 elements, in different proportions, would produce all substances What was the Next Important Discovery? John Dalton In the early 800 s, John Dalton, who was an English teacher, summarized the results of his experiments and those of others in... Dalton s s Atomic Theory Dalton proposed that all matter is made up of tiny particles, which are molecules or atoms. Molecules can be broken down into atoms by chemical processes. Atoms cannot be broken down by chemical or physical processes.

2 What was the Next Important Discovery? John Dalton Dalton combined the ideas of elements with that of atoms to try to explain 2 laws of chemistry: Law of Definite Composition: the % by mass of an element in a compound is always the same. i.e. the mass ratio of carbon to oxygen in Carbon dioxide (CO 2 ) is always the same carbon to 2 oxygen atoms. Law of Conservation of Mass: In chemical reactions, mass is conserved and is not created nor destroyed. i.e. Dalton proposed the creation of methane (CH 4 ) by substituting 4 hydrogen atoms for the 2 oxygen atoms in carbon dioxide. Dalton s s Atomic Theory. An element is composed of tiny, indestructible, indivisible particles called atoms. 2. All atoms of the same element are identical, and have the same properties. 3. Atoms of different elements combine to form compounds. 4. Compounds contain atoms in small whole number ratios. 5. Atoms can combine in more than one ratio to form different compounds, or simply, chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed. Dalton s s Atomic Theory How does Dalton s s Model of Atomic Theory look today, in light of new discoveries? Dalton s s first two suggestions are incorrect: Atoms are indestructible/indivisible particles: atoms are in fact divisible. All atoms of the same element are identical: subatomic particles were later discovered, the proton,, with a charge (p ) the electron,, with a - charge (e - ) and the neutron,, with neutral charge (n) Atoms of the same element can differ by the number of neutrons. Proposals 3, 4, and 5 are still accepted. Just How Small Is an Atom? Think of cutting a piece of lead into smaller and smaller pieces How far can it be cut? An atom is the smallest particle of an element that retains the properties of that element Atoms are very small; You would need to line up 00,000,000 copper atoms to measure cm. A penny that is made of pure copper would contain 2.4 x 0 22 copper atoms. Just How Small Is an Atom? Atoms are observable with proper instrument, a Scanning Tunneling Microscope (STM): Gold Atoms Section 5.2 Structure of the Nuclear Atom OBJECTIVES: Distinguish among protons, electrons, and neutrons in terms of relative mass and charge. Describe the structure of an atom, including the location of the protons, electrons, and neutrons with respect to the nucleus. 2

3 The Structure of the Atom Thomson s s Experiment Work done by J.J. Thomson, an English physicist in 897, proved that atoms had pieces called electrons. Made a piece of equipment called a cathode ray tube. Electrodes were hooked up to a high-voltage source, creating an anode (positively charged), and the cathode (negatively charged). A glowing beam flowed from the negative disk, to the positive disk, called the cathode ray. - Voltage source Vacuum tube Metal Disks Thomson s s Experiment Thomson s s Experiment - Voltage source Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end By adding an electric field he found that the moving pieces were attracted to the positive charge, and were therefore negative opposites attract. - Thomson s s Plum Pudding Model. The atom is breakable 2. Electrons are negative, so: Need a positive charge to balance the electrons Two negative charges repel each other In the Plum Pudding Model:. Electrons are suspended in a positively charged electric field 2. A lot of empty space in the atom to separate the electrons After Thompson Milliken (An American scientist in 9) determined the mass to charge ratio of an electron. The Electron s s charge is -; the mass is 9. x 0-28 g E. Goldstein discovered that a proton is a positively charged subatomic particle that is 840 times heavier than the electron The Proton s s charge is ; the mass is.67 x 0-24 g In 932, James Chadwick confirmed that the neutron has no charge but the same mass as a proton The Neutron s s charge is 0; the mass is.67 x 0-24 g 3

4 Electron Proton Neutron Properties of Subatomic Particles Particle Symbol e - p N 0 Relative Charge - 0 Relative Mass (mass of P = ) / x 0-24 But how are these subatomic particles arranged? Actual Mass (g) 9. x x 0-24 Rutherford and Radioactivity (908) There are three types of radiation:. Alpha Particles (α):( composed of positively charged Helium nuclei. 2. Beta Particles (β):( composed of negative charged electrons 3. Gamma rays (γ):( composed of high energy radiation Size: α > β > γ Ernest Rutherford -English physicist. (90) Believed in the plum pudding model of the atom. He designed an experiment to test the Plum Pudding Model. Rutherford used radioactivity, and shot the positively charged alpha particles at a gold foil which was a few atoms thick. Lead block Uranium Fluorescent Screen Gold Foil If the Plum Pudding model of the atom was correct, most α-particles should pass through un-deflected. What he expected... the positive charges were thought to be spread out evenly. Alone they were not enough to stop the alpha particles. 4

5 Since he thought the mass was evenly distributed in the atom: What he got Rutherford explained Since the atom is largely empty space, most of the α- particles passed through the foil. The Atomic Nucleus contains the atom s s protons, and it is located at the center of the atom. The α-particles that deflected and bounced backwards did so after nearing or striking the nucleus. Rutherford s s Model of the Atom (9) The atom is composed mainly of vacant space. all the positive charge and most of the mass is in a small area called the nucleus. This nucleus is dense, which would cause the backward deflection of the α-particles. Negatively charged electrons are distributed around a positively charged nucleus. An atom has a diameter of ~0-0 m, whereas the nucleus has ~ 0-5 m A New Look at Subatomic particles Rutherford predicted that because of the heaviness of the nucleus, the nucleus must contain neutral particles in addition to protons. Neutrons, n 0, were discovered ~30 years later by Chadwick. The neutron has no charge, but is as heavy as a proton. Rutherford s s Model, Simply put.. Protons & Neutrons in the Nucleus Charge Mass Proton Neutron 0 Electron - /836 5

6 Who was next? The Bohr Model in 95 It was proposed by Niels Bohr in 95. It is otherwise known as the Planetary Model. It is not completely correct, but it has many features that are approximately correct The Bohr Model Following Rutherford s s planetary model of the atom, it was realized that the attraction between the electrons and the protons should make the atom unstable Bohr proposed a model in which the electrons would stably occupy fixed orbits,, as long as these orbits had special quantized locations The Bohr Model In the Bohr model, the electron can change orbits, accompanied by the absorption or emission of a photon of a specific color of light. The Bohr Model In the Bohr model, the maximum number of electrons per energy level are determined as: Max e- e = 2n 2, where n is the energy level. Energy Level Maximum Number of Electrons What Came After Bohr? Quantum Theory o Modern quantum theories lead to stable locations of electrons, which are not exact planetary orbits, but are characterized by specific quantum numbers. o Each electron shell is characterized by a different principle quantum number, usually called n. o In quantum theory, the electron shells are not fixed orbits, but clouds of probability.. You can t t measure the exact location of the electron. o Quantum Theory Each electron orbital has a different shape, and no two electrons can be in the same orbital unless they have opposite spins. 6

7 Quantum Theory The quantum rules for the electron orbitals in an atom determine the row structure in the periodic table. The geometry of the electron orbitals determines the structure of an atom Quantum Theory The spin of the electron is another quantum property. In the planetary model, it is similar to the spin of the Earth on its axis. There are two choices for the orientation of the electron s spin axis: up or down. Lets Summarize Dalton s s Model Illustrated The remaining four Lets Summarize Review Questions. Aristotle suggested that matter existed through a combination of? Fire, Air, Earth and Water 2. What were the two characteristics of the Dalton Model of the atom that were later found to be untrue? Atoms are indestructible/indivisible particles All atoms of the same element are identical 3. What was most characteristic of Thomson s s Model of the Atom, that is most unlike what is accepted as true today? Plum Pudding Model, no nucleus, charges mixed with electrons 4. How did Rutherford s s experiments reveal that an atom consists of a dense, positively-charged nucleus, with a large electron cloud around it? Small ratio of alpha particles were deflected, and fewer bounced back after striking nucleus Review Questions 5. Why was the Bohr Model an improvement over the Rutherford planetary model? Bohr proposed a model in which the electrons would stably occupy fixed orbits 6. How did the Bohr Model explain the emission & absorption of light? Electrons can change orbits, accompanied by the absorption or emission of a photon of a specific color of light. 7. The Quantum Theory Model of the atom also describes Quantum levels, similar to Bohr. How did the new model differ? In quantum theory, the electron shells are not fixed orbits, but clouds of probability 7

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