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Chapter 8

Chapter 8. Modern Atomic Theory. Setting the Stage – Emission of Light. The beautiful color of fireworks comes from the particular elements used. The colors are characteristic of the elements and can be used to understand their properties.

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Chapter 8

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  1. Chapter 8 Modern Atomic Theory

  2. Setting the Stage – Emission of Light • The beautiful color of fireworks comes from the particular elements used. • The colors are characteristic of the elements and can be used to understand their properties. Malone and Dolter - Basic Concepts of Chemistry 9e

  3. Setting a Goal – Part AThe Emission Spectra of the Elements and Bohr’s Model • You will understand historically how elemental emission spectra were used to determine the structure electrons adopt within an atom. • For more details, see “Special Topics” Malone and Dolter - Basic Concepts of Chemistry 9e

  4. Objectives for Section 8-1 • 8-1a Describe the relationships among the wavelengths of light, energy, and color. • 8-1b Describe the relationship between emission spectra and the electronic structure of the hydrogen atom as explained by Bohr’s model. Malone and Dolter - Basic Concepts of Chemistry 9e

  5. Two Atom-Sized Jokes  • Two Hydrogen atoms sit down at a bar. One says to the other "I think I've lost an electron." The other says "Are you sure?", and he replies, "Yeah, I'm positive." • So a neutron walks into a bar, sits down and asks for a glass of soda. Finishing his drink, the neutron asks "How much?" The bartender says, "For you, no charge." Malone and Dolter - Basic Concepts of Chemistry 9e

  6. 8-1 The Emission Spectra of the Elements and Bohr’s Model The Nature of Light • Electromagnetic energy • Travels at 3.0  1010 cm/s • White light is a continuous spectrum of light (all energies represented) and can be separated. • Refer to Figure 8-1 in text. Malone and Dolter - Basic Concepts of Chemistry 9e

  7. The Visible Spectrum Malone and Dolter - Basic Concepts of Chemistry 9e

  8. Properties of Light • Light has intensity, which is termed amplitude. • Light is characterized by its wavelength, which is given the symbol  (lambda). • Wavelength is the distance between two adjacent peaks. Malone and Dolter - Basic Concepts of Chemistry 9e

  9. Wavelength and Energy • Wavelength and energy have an inverse relationship, as shown below. • h is Planck’s constant (6.626  10-34J·s) • c is the speed of light Malone and Dolter - Basic Concepts of Chemistry 9e

  10. Wavelength, frequency and velocity The velocity of propagated light waves in vacuum (c) = 2.998 x 108 m/s It is the product of the wavelength and wave frequency: c = ln Malone and Dolter - Basic Concepts of Chemistry 9e

  11. Period and Frequency The period (t) of the wave is the time taken for the wave to travel one complete wavelength. The frequency (n) is the reciprocal of the period (n = 1/t) Energy and frequency are directly proportional; E = hn (originally from Planck’s quantum theory) Malone and Dolter - Basic Concepts of Chemistry 9e

  12. Visible Light • Visible light comprises only a small fraction of the electromagnetic spectrum (400-800 nm). • Ultraviolet and infrared light cannot be seen with the unaided eye, but are important. Malone and Dolter - Basic Concepts of Chemistry 9e

  13. Energy Range of Light • Infrared light is of lower energy than visible light. • Ultraviolet light is of higher energy than visible light (some of the ultraviolet range is of high enough energy to damage living tissue). • X-rays and gamma rays are even higher energy forms of light (there are special hazards associated with this type of radiation). Malone and Dolter - Basic Concepts of Chemistry 9e

  14. Emission Spectra of Atoms • When atoms are excited (like in a flame) they emit discrete wavelengths of light, not a continuum. These are called atomic emission (line) spectra • Shown below is the visible light range at the top, the emission spectrum of Na in the middle and the emission spectrum of H at the bottom. Malone and Dolter - Basic Concepts of Chemistry 9e

  15. Measuring the Emission Spectrum • The H2 in the gas discharge tube is excited by electricity. • The light produced is separated by the prism. Malone and Dolter - Basic Concepts of Chemistry 9e

  16. Atomic Phenomena and Classical Physics Classical physics (Newtonian mechanics and Maxwell’s electromagnetism theory) could not explain: • Emission line spectra • The Rutherford (nuclear) model of the atom • Black body radiation • Photoelectric effect • Electron diffraction Malone and Dolter - Basic Concepts of Chemistry 9e

  17. A Model for the Electrons in the Atom • A model is a description or analogy used to help visualize a phenomenon or entity. • Bohr proposed a model that violated classical physics, but started a new way of looking at atomic and molecular structure. • The central concept of this model is that the properties of atoms and electrons do not behave in a continuous fashion. Malone and Dolter - Basic Concepts of Chemistry 9e

  18. The Bohr Atom • Electrons revolve around the nucleus in certain stable orbits only. • Each stable orbit is quantized • the electron is a discrete distance from the nucleus • the orbit has a discrete energy • These orbits are referred to as energy levels. • The energy levels are characterized by an integral number called the principle quantum number, n. Malone and Dolter - Basic Concepts of Chemistry 9e

  19. The Bohr Atom (con’t.) • n has discrete values of 1, 2, 3,… • The quantum number n arose from Bohr’s idea of imposing quantization (from Planck’s Hypothesis, 1901) on the angular momentum of the electron. Otherwise Bohr’s description of the hydrogen atom uses classical physics. • Light is absorbed (giving an absorption line) or emitted (an emission line) only when an electron changes its stable orbit (excitation or relaxation) – see next slide Malone and Dolter - Basic Concepts of Chemistry 9e

  20. Balmer Emission Series of Hydrogen Explained by Bohr Theory Each energy level is associated with a particular Bohr orbit Malone and Dolter - Basic Concepts of Chemistry 9e

  21. Quantized Energy Levels • Quantized energy levels are like the steps of a staircase. • An electron can have one energy or another, but not an energy in between (can you stand between two steps?) Malone and Dolter - Basic Concepts of Chemistry 9e

  22. States of the Atom • Ground state • the lowest energy state • Excited state • all states that have higher energies • atoms and molecules have many excited states but only one ground state Malone and Dolter - Basic Concepts of Chemistry 9e

  23. Objectives for Section 8-2 • 8-2a Differentiate between a shell, a subshell, and an orbital. • 8-2b Identify an s, p, or d-type orbital from its shape. Malone and Dolter - Basic Concepts of Chemistry 9e

  24. 8-2Modern Atomic Theory: A Closer Look at Energy Levels • Bohr’s model of the hydrogen atom explained the emission spectra of hydrogen – a great success • However, even after several modifications, it was generally unsuccessful when applied to other atomic phenomena and failed totally to describe the helium atom Malone and Dolter - Basic Concepts of Chemistry 9e

  25. Modern Atomic Theory - Wave Mechanics • Modern atomic theory involves complex mathematical descriptions. • It takes into account that atomic-sized things behave more like waves than particles. The Schrödinger equation for a hydrogen atom Malone and Dolter - Basic Concepts of Chemistry 9e

  26. Wave Mechanics • We treat electrons as waves, since we cannot precisely determine the location of the electrons. The idea was originally put forward by de Broglie: all matter has some wave character • Schrödinger was the first person to apply de Broglie’s idea; he considered the electron in a hydrogen atom as a standing wave • Electrons are said to be described by orbitals, which are regions in space where there is a significant probability of finding the electron. Malone and Dolter - Basic Concepts of Chemistry 9e

  27. Extra Details Revealed • Wave mechanics reveals that the energy levels (sometimes called shells) have sublevels. • The sublevels are designated by letters:s, p, d, f • The sublevels increase in energy as s<p<d<f • Each sublevel has one or more orbitals. Malone and Dolter - Basic Concepts of Chemistry 9e

  28. Some Heroes of Modern Atomic Theory E. Rutherford N. Bohr W. Heisenberg L. de Broglie E. Schrödinger Malone and Dolter - Basic Concepts of Chemistry 9e

  29. Orbitals • The orbital is best thought of as an electron cloud. • The picture below is the density diagram for an s orbital. • It indicates the probability of an electron being located in a particular location near the nucleus. Malone and Dolter - Basic Concepts of Chemistry 9e

  30. Representing Orbitals • We often represent orbitals as spherical volumes of probability (volume in which the electron is found 90% of the time). • The orbitals of the s sublevel are shown below, for n = 1 and n = 2. 2s 1s Malone and Dolter - Basic Concepts of Chemistry 9e

  31. p-Orbitals • There are three orbitals in the p sublevel (starting at n = 2). • One is aligned along each of the Cartesian axes around the nucleus. • Orientation is indicated by subscript x, y or z. • This is the set of 2p orbitals: Malone and Dolter - Basic Concepts of Chemistry 9e

  32. d-Orbitals • There are five orbitals in the d-sublevel, starting at n = 3. • They are important to understanding the behavior of the transition metals and elements in period 3 and beyond. • This is the set of 3d orbitals: Malone and Dolter - Basic Concepts of Chemistry 9e

  33. Shell Designation • The shell is indicated by the principle quantum number n. • The subshell is indicated by the letter (s, p, d, f). • The number of electrons in the subshell is indicated by a right superscript. • For example, 4p3 (4th shell, p subshell, three electrons). Malone and Dolter - Basic Concepts of Chemistry 9e

  34. Electronic Configurations • The electrons in the atom are indicated by an electron configuration. • We use only as many subshells and shells as are needed for the number of electrons. • The number of available subshells depends on the shell that is being filled. • n = 1 only has an s subshell • n = 2 has s and p subshells • n = 3 has s, p, and d subshells • n = 4 has s, p, d, and f subshells Malone and Dolter - Basic Concepts of Chemistry 9e

  35. The n = 3 Shell Malone and Dolter - Basic Concepts of Chemistry 9e

  36. Dorm Room Analogy • We can think of the levels as dorm rooms. • A student will choose the dorm that is closest to the campus and a room on the lowest floor. Malone and Dolter - Basic Concepts of Chemistry 9e

  37. Dorm Room Analogy, con’t • The dorms correspond to the shells • The floors correspond to the subshells Malone and Dolter - Basic Concepts of Chemistry 9e

  38. Setting a Goal – Part BThe Periodic Table and Electron Configuration • You will have a detailed understanding of the structure of electrons in atoms, and how this affects that element’s properties. Malone and Dolter - Basic Concepts of Chemistry 9e

  39. Objective for Section 8-3 • Using the periodic table, write the outer electron configuration of a specific element. Malone and Dolter - Basic Concepts of Chemistry 9e

  40. 8-3Electron Configurations of the Elements The Aufbau Principle • The electrons are added to the atom beginning with the lowest energy level. • As the sublevels and levels are filled, electrons are added to the next highest energy level. • Each orbital can hold two electrons, so each sublevel can hold twice as many electrons as there are orbitals. Malone and Dolter - Basic Concepts of Chemistry 9e

  41. The Aufbau Principle • Electrons occupy the available orbitals in the subshells of lowest energy. • The electronic configuration of an element is the assignment of all of the electrons of the atom into shells and subshells. Malone and Dolter - Basic Concepts of Chemistry 9e

  42. Assignment of Electrons into Shells and Subshells Nomenclature or symbolism for electron configuration Malone and Dolter - Basic Concepts of Chemistry 9e

  43. Ground States of the First Few Elements • Using the Aufbau Principle, the electron configurations for the first 10 elements are:H 1s1He 1s2Li 1s22s1Be 1s22s2B 1s22s22p1 C 1s22s22p2 N 1s22s22p3 O 1s22s22p4 F 1s22s22p5 Ne 1s22s22p6 Malone and Dolter - Basic Concepts of Chemistry 9e

  44. Abbreviated Electron Configurations • We develop a shorthand for the electron configuration by noting that the core is really the same as the electron configuration for the noble gas that occurs earlier in the periodic table. • Example: for S (1s22s22p63s23p4), the core is 1s22s22p6 which is the same as the electron configuration for Ne. • So the abbreviated form is [Ne]3s23p4 Malone and Dolter - Basic Concepts of Chemistry 9e

  45. Electron Configuration Shorthand • We note that the valence shell electron configuration has the same pattern for elements in the same group. • All elements in the same column as oxygen or sulfur (group VIA) have the valence electron configuration[core]ns2np4. Malone and Dolter - Basic Concepts of Chemistry 9e

  46. Periodic Nature of the Filling • We find that at the end of a period, the pattern of electron filling repeats with higher values of n. • F [He]2s22p5 • Cl [Ne]3s23p5 • Br [Core]4s24p5 • The noble gas in parentheses indicates the inner electrons. Malone and Dolter - Basic Concepts of Chemistry 9e

  47. Electronic Configurations • The levels increase in energy. • Within the levels, the sublevels also increase (recall s<p<d<f). • Recall that the electrons go into the lowest orbital first (1s, then 2s, etc.). Malone and Dolter - Basic Concepts of Chemistry 9e

  48. Shell, Subshell and Orbitals Malone and Dolter - Basic Concepts of Chemistry 9e

  49. Core and Valence Shells • Chemically, we find that the electrons in the shell with the highest value of n are the ones involved in chemical reactions. • This shell is termed the valence shell. • Electrons in shells with lower n values are chemically unreactive because they are of such low energy. • These shells are grouped together as the core. Malone and Dolter - Basic Concepts of Chemistry 9e

  50. Order of Filling of Atomic Orbitals The diagram opposite is a useful pictorial description of atomic orbital energy order Malone and Dolter - Basic Concepts of Chemistry 9e

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