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Chapter 4 The Structure of the Atom 4.1 Early Ideas About Matter • Greek philosophers – formed explanations based on life experiences Democritus – 400 BC • Belived matter was made of tiny particles called atomos – Cannot be created, destroyed, or divided • Aristotle disagreed John Dalton - 1803 • • Revived & revised Democritus’ ideas Came up with 4 part atomic theory 1. All elements are composed of tiny particles called atoms which cannot be broken into smaller parts 2. All atoms of the same element have identical properties. Atoms of different elements have different properties 3. Atoms combine in whole-number ratios to form compounds 4. Chemical reactions take place when atoms rearrange. Atoms of one element are NOT changed into atoms of a different element Conservation of mass • Result of separation, combination, or rearrangement of atoms • Atoms are not created or destroyed 4.2 Defining the Atom • The smallest particle of an element that retains the properties of the element • Atoms are very small – One Cu atom = 1.28 x 10-10 m – Solid copper penny = 2.9 x 1022 atoms – If 1 atom = size of an orange, an orange would be as big as the earth! • Scanning tunneling microscope (STM) allows individual atoms to be seen – Scans across the surface of atoms – silicon J.J. Thomson – 1890s • Worked with a cathode ray tube • Discovered cathode ray was attracted to + charged plate • What does this mean? • By measuring the effects of both magnetic and electric fields on a cathode ray, Thomson determined the mass of the charged particle is less than the hydrogen atom • What does this mean? • Thomson’s model – Matter is neutral so some positive charge needs to balance out the negative electrons – Plum pudding model Rutherford - 1911 – Shot alpha particles (+ charged) at gold foil. – Based on Thomson’s model he expected to pass through and hit screen on other side but not all did!!! • What does this mean? • Rutherford concluded: – Atom consists of mostly empty space • If an atom had a diameter of 2 football fields the nucleus would be the size of a nickel – Almost all + charge & mass are found in nucleus • Rutherford model James Chadwick - 1932 • What we knew 1. Electrons & Protons have opposite charges 2. Neutral atom has same number of electrons as protons 3. Mass of atom was too much to just be made of protons & electrons • Showed nucleus contained a particle with the same mass as a proton but no charge – neutron Completing the model of the atom • Spherically shaped • Small, dense, + charged nucleus – Contains 99.97% of atoms mass • Fast moving electrons traveling through empty space – Held to atom by attraction to nucleus • Atoms are neutral so # of protons = # of electrons Electron Cloud Model Particle Electron Proton Neutron Symbol Location Charge Mass 4.3 How Atoms Differ • The number of protons in an atom identifies it as a particular element • Number of protons = atomic number • Periodic table is organized in order of increasing atomic number • All atoms are electrically neutral therefore • # of protons = • Determine the number of protons and electrons in an atom of Molybdenum Mass Number • Mass number = protons + neutrons • Neutrons = Atomic Shorthand • Beryllium- - -atomic number 4 mass number 9 • The atomic number is written as a subscript. • The mass number is written as a superscript. Be Element Atomic Protons Number 35 10 Electrons Neutrons 19 20 Mass number 80 20 Isotopes • Atoms with the same number of protons but different number of neutrons – Oxygen – 16, Oxygen – 17, Oxygen – 18 – One isotope is more common than the others Ions • Atoms with an electrical charge • Ions form by gaining or losing ELECTRONS • Cation • Anion Element Atomic # Protons Electrons Neutrons O2Al3+ 19 14 6C Potassium - 41 18 Mass number What can change in an atom? • Protons – NEVER!!! • Neutrons – if changed an isotope is formed • Electrons – if changed an ion is formed Proton Changed • Whole new atom!!! • Oxygen loses a proton it becomes Nitrogen • Oxygen gains a proton it becomes Fluorine Neutron Changed • Different version of the same atom is formed (isotope) • Oxygen – 16 has 8 Neutrons • Oxygen – 17 has 9 Neutrons • Oxygen – 18 has 10 Neutrons Electrons Changed • Charged version of the same atom is formed (ion) • Oxygen gains 2 electrons – 10 electrons & 8 protons – Cation • Oxygen loses 2 electrons – 6 electrons & 8 protons – Anion Mass of Atoms • Mass of protons & neutrons = 1.67 x 10-24 g • This is too small to be practical so scientists use atomic mass units (amu) • Carbon – 12 = 12 amu – 1 amu = 1/12 mass of carbon – 12 atom • Proton = 1.007276 amu • Neutron = 1.008665 amu • Electron = 0.000549 amu • Atomic mass = weighted average mass of the isotopes of an element • Boron has two naturally occurring isotopes: Boron – 10 (19.8% abundance, 10.013 amu) and Boron – 11 (80.2 % abundance, 11.009 amu). What is the atomic mass of Boron? • Unknown element X has two isotopes. 6X has an atomic mass of 6.015 amu and a 7.59 % abundance. 7X has an atomic mass of 7.016 amu and a 92.41% abundance. What is the atomic mass of the unknown element? What is the identity of the unknown element? 4.4 Unstable Nuclei and Radioactive Decay • Unstable atoms emit radiation to gain stability – Stability is gained by losing energy • Nuclear reactions can change elements into new elements Radioactivity • The process in which some substance spontaneous emit radiation • Radiation – rays and particles emitted by the radioactivity • Nuclear reaction – reaction that involves a change in an atom’s nucleus • Radioactive decay – spontaneous process in which atoms lose energy by emitting radiation – Unstable elements do this until they are stable (often a different element) Types of Radiation • Three types of radiation based on their electric charge – Alpha (α) – Beta (β) – Gamma (γ) Alpha Radiation • Radiation deflected toward negative charge – So its charge is • Made up of alpha particles – 2 protons + 2 neutrons Beta radiation • Radiation deflected toward positive charge – So its charge is • Made up of beta particles – electron Gamma radiation • • • • High energy radiation with no mass No charge Usually accompany alpha & beta radiation Account for most of the energy lost during radioactive decays Nuclear Stability • Primary factor in determining stability is ratio of protons to neutrons • Atoms with too many or too few neutrons are unstable – Will radioactively decay until they become stable