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Chapter 8 Periodic Properties of the Elements

Chemistry: A Molecular Approach , 1 st Ed. Nivaldo Tro. Chapter 8 Periodic Properties of the Elements. Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA. 2007, Prentice Hall. Discovery.

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Chapter 8 Periodic Properties of the Elements

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  1. Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro Chapter 8Periodic Properties of the Elements Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA 2007, Prentice Hall

  2. Discovery • Elements are organized according to their physical and chemical properties in the Periodic Table • Historically, several people contributed to this effort in the late 19th century: • Dobereiner- “triads”: Ca,Sr,Ba; Li, Na, K (1817) • Newlands- similarity between every eighth element (1864) • Mendeleev (1870) - Arranged elements according to atomic mass. Similar elements were arranged together in a “group”

  3. Question Dobereiner’s “triads” (1817) Determine the Atomic mass of Sr by averaging the masses of Ca and Ba 87.62 amu

  4. Newlands • In 1863, he suggested that elements be arranged in “octaves” because he noticed (after arranging the elements in order of increasing atomic mass) that certain properties repeated every 8th element Law of Octaves

  5. Newlands “Law of Octaves” • Newlands (1864) arranged the elements as follows: • Did not contain the Noble gases (yet to be discovered) • Not valid for elements with Z > 20 • His table had 7 groups instead of 8

  6. Mendeleev • ordered elements by atomic mass • saw a repeating pattern of properties • Periodic Law – When the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically • put elements with similar properties in the same column • used pattern to predict properties of undiscovered elements • where atomic mass order did not fit other properties, he re-ordered by other properties • Te & I Tro, Chemistry: A Molecular Approach

  7. Mendeleev's Predictions Tro, Chemistry: A Molecular Approach

  8. The Periodic Table • Periodic Law - Both physical and chemical properties of the elements vary periodically with increasing atomic mass • Exceptions Te/I, Ar/K, Co/Ni • Placed Te (M = 127.6) ahead of I (M=126.9) because Te was similar to Se and S, and I was similar to Cl and Br • Moseley showed listing elements based on atomic no. resolved these issues

  9. What vs. Why • Mendeleev’s Periodic Law allows us to predict what the properties of an element will be based on its position on the table • it doesn’t explain why the pattern exists • Laws summarize behavior while theories explain them! • Quantum Mechanics is a theory that explains why the periodic trends in the properties exist Tro, Chemistry: A Molecular Approach

  10. Electron Spin • experiments by Stern and Gerlach showed a beam of silver atoms is split in two by a magnetic field • the experiment reveals that the electrons spin on their axis • as they spin, they generate a magnetic field • spinning charged particles generate a magnetic field • if there is an even number of electrons, about half the atoms will have a net magnetic field pointing “North” and the other half will have a net magnetic field pointing “South” Tro, Chemistry: A Molecular Approach

  11. Electron Spin Experiment Tro, Chemistry: A Molecular Approach

  12. Energy Shells and Subshells n = principal quantum no. (overall size) l = angular momentum quantum no. (shape of orbital) ml = magnetic quantum no. (orientation of orbital)

  13. Spin Quantum Number, ms • spin quantum number describes how the electron spins on its axis • clockwise or counterclockwise • spin up or spin down • spins must cancel in an orbital • paired • mscan have values of ±½ Orbital diagram for H Tro, Chemistry: A Molecular Approach

  14. Pauli Exclusion Principle • no two electrons in an atom may have the same set of 4 quantum numbers • therefore no orbital may have more than 2 electrons, and they must have with opposite spins • knowing the number orbitals in a sublevel allows us to determine the maximum number of electrons in the sublevel • s sublevel has 1 orbital, therefore it can hold 2 electrons • p sublevel has 3 orbitals, therefore it can hold 6 electrons • d sublevel has 5 orbitals, therefore it can hold 10 electrons • f sublevel has 7 orbitals, therefore it can hold 14 electrons Tro, Chemistry: A Molecular Approach

  15. Allowed Quantum Numbers Tro, Chemistry: A Molecular Approach

  16. Quantum Numbers of Helium’s Electrons • helium has two electrons • both electrons are in the first energy level • both electrons are in the s orbital of the first energy level • since they are in the same orbital, they must have opposite spins Tro, Chemistry: A Molecular Approach

  17. Electron Configurations • the ground state of the electron is the lowest energy orbital it can occupy • the distribution of electrons into the various orbitals in an atom in its ground state is called its electron configuration • the number designates the principal energy level • the letter designates the sublevel and type of orbital • the superscript designates the number of electrons in that sublevel • He = 1s2 Tro, Chemistry: A Molecular Approach

  18. unoccupied orbital orbital with 1 electron orbital with 2 electrons Orbital Diagrams • we often represent an orbital as a square and the electrons in that orbital as arrows • the direction of the arrow represents the spin of the electron Tro, Chemistry: A Molecular Approach

  19. Sublevel Splitting in Multielectron Atoms • the sublevels in each principal energy level (n = 2…) of Hydrogen all have the same energy (empty in lowest state) – called degenerate • for multielectron atoms, the energies of the sublevels are split • caused by electron-electron repulsion • the lower the value of the l quantum number, the less energy the sublevel has • s (l = 0) < p (l = 1) < d (l = 2) < f (l = 3) Tro, Chemistry: A Molecular Approach

  20. Penetration & Shielding Consider bringing an e- close to Li+ 1s2 Tro, Chemistry: A Molecular Approach

  21. Penetrating and Shielding 2p is shielded • the radial distribution function shows that the 2s orbital penetrates more deeply into the 1s orbital than does the 2p • the weaker penetration of the 2p sublevel means that electrons in the 2p sublevel experience more repulsive force, they are more shielded from the attractive force of the nucleus • the deeper penetration of the 2s electrons means electrons in the 2s sublevel experience a greater attractive force to the nucleus and are not shielded as effectively by 1s e- • the result is that the electrons in the 2s sublevel are lower in energy than the electrons in the 2p Tro, Chemistry: A Molecular Approach

  22. 6d 7s 5f 6p 5d 6s 4f 5p 4d 5s 4p 3d 4s 3p 3s 2p 2s 1s Energy • Notice the following: • because of penetration, sublevels within an energy level are not degenerate • penetration of the 4th and higher energy levels is so strong that their s sublevel is lower in energy than the d sublevel of the previous energy level • the energy difference between levels becomes smaller for higher energy levels

  23. Order of Subshell Fillingin Ground State Electron Configurations start by drawing a diagram putting each energy shell on a row and listing the subshells, (s, p, d, f), for that shell in order of energy, (left-to-right) 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 7s next, draw arrows through the diagonals, looping back to the next diagonal each time Tro, Chemistry: A Molecular Approach

  24. Tro, Chemistry: A Molecular Approach

  25. Filling the Orbitals with Electrons • energy shells fill from lowest energy to high • subshells fill from lowest energy to high • s → p → d → f • Aufbau Principle • orbitals that are in the same subshell have the same energy • no more than 2 electrons per orbital • Pauli Exclusion Principle • when filling orbitals that have the same energy, place one electron in each before completing pairs • Hund’s Rule Tro, Chemistry: A Molecular Approach

  26. Example 8.1 – Write the Ground State Electron Configuration and Orbital Diagram and of Magnesium. • Determine the atomic number of the element from the Periodic Table • This gives the number of protons and electrons in the atom Mg Z = 12, so Mg has 12 protons and 12 electrons Tro, Chemistry: A Molecular Approach

  27. 1s 2s 2p 3s 3p Example 8.1 – Write the Ground State Electron Configuration and Orbital Diagram and of Magnesium. • Draw 9 boxes to represent the first 3 energy levels sandp orbitals • since there are only 12 electrons, 9 should be plenty Tro, Chemistry: A Molecular Approach

  28. Example 8.1 – Write the Ground State Electron Configuration and Orbital Diagram and of Magnesium. • Add one electron to each box in a set, then pair the electrons before going to the next set until you use all the electrons • When pair, put in opposite arrows          1s 2s 2p 3s 3p Tro, Chemistry: A Molecular Approach

  29.  1s  2s  2p    3s 3p Example 8.1 – Write the Ground State Electron Configuration and Orbital Diagram and of Magnesium. • Use the diagram to write the electron configuration • Write the number of electrons in each set as a superscript next to the name of the orbital set 1s22s22p63s2 = [Ne]3s2 Tro, Chemistry: A Molecular Approach

  30. Valence Electrons • the electrons in all the subshells with the highest principal energy shell are called the valence electrons • electrons in lower energy shells are called core electrons • chemists have observed that one of the most important factors in the way an atom behaves, both chemically and physically, is the number of valence electrons Tro, Chemistry: A Molecular Approach

  31. Electron Configuration of Atoms in their Ground State • Kr = 36 electrons 1s22s22p63s23p64s23d104p6 • there are 28 core electrons and 8 valence electrons • Rb = 37 electrons 1s22s22p63s23p64s23d104p65s1 [Kr]5s1 • for the 5s1 electron in Rb the set of quantum numbers is n = 5, l = 0, ml = 0, ms = +½ • for an electron in the 2p sublevel, the set of quantum numbers is n = 2, l = 1, ml = -1 or (0,+1), and ms = - ½ or (+½) Tro, Chemistry: A Molecular Approach

  32. Electron Configurations Tro, Chemistry: A Molecular Approach

  33. Electron Configuration & the Periodic Table • the Group number corresponds to the number of valence electrons • the length of each “block” is the maximum number of electrons the sublevel can hold • the Period number corresponds to the principal energy level of the valence electrons Tro, Chemistry: A Molecular Approach

  34. Tro, Chemistry: A Molecular Approach

  35. s1 s2 p1 p2 p3 p4 p5 p6 s2 1 2 3 4 5 6 7 d1 d2d3 d4 d5 d6 d7 d8 d9 d10 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13 f14 f14d1 Tro, Chemistry: A Molecular Approach

  36. Electron Configuration fromthe Periodic Table 8A 1A 1 2 3 4 5 6 7 3A 4A 5A 6A 7A 2A Ne P 3s2 3p3 P = [Ne]3s23p3 P has 5 valence electrons Tro, Chemistry: A Molecular Approach

  37. 4s 3d 6s 4f Transition Elements • for the d block metals, the principal energy level of the d orbital being filled is one less than valence shell • one less than the Period number • sometimes s electron “promoted” to d sublevel Zn Z = 30, Period 4, Group 2B [Ar]4s23d10 • for the f block metals, the principal energy level is two less than valence shell • two less than the Period number they really belong to • sometimes d electron in configuration Eu Z = 63, Period 6 [Xe]6s24f 7

  38. Electron Configuration fromthe Periodic Table 8A 1A 1 2 3 4 5 6 7 3A 4A 5A 6A 7A 2A 3d10 Ar As 4s2 4p3 As = [Ar]4s23d104p3 As has 5 valence electrons Tro, Chemistry: A Molecular Approach

  39. Practice – Use the Periodic Table to write the short electron configuration and orbital diagram for each of the following • Na (at. no. 11) • Te (at. no. 52) • Tc (at. no. 43) Tro, Chemistry: A Molecular Approach

  40. Practice – Use the Periodic Table to write the short electron configuration and orbital diagram for each of the following • Na (at. no. 11) [Ne]3s1 • Te (at. no. 52) [Kr]5s24d105p4 • Tc (at. no. 43) [Kr]5s24d5 3s 5s 5p 4d 5s 4d Tro, Chemistry: A Molecular Approach

  41. Properties & Electron Configuration • elements in the same column have similar chemical and physical properties because they have the same number of valence electrons in the same kinds of orbitals Tro, Chemistry: A Molecular Approach

  42. Electron Configuration & Element Properties • the number of valence electrons largely determines the behavior of an element • chemical and some physical • since the number of valence electrons follows a Periodic pattern, the properties of the elements should also be periodic • quantum mechanical calculations show that 8 valence electrons should result in a very unreactive atom, an atom that is very stable – and the noble gases, that have 8 valence electrons are all very stable and unreactive • conversely, elements that have either one more or one less electron should be very reactive – and the halogens are the most reactive nonmetals and alkali metals the most reactive metals • as a group Tro, Chemistry: A Molecular Approach

  43. Electron Configuration &Ion Charge • we have seen that many metals and nonmetals form one ion, and that the charge on that ion is predictable based on its position on the Periodic Table • Group 1A = +1, Group 2A = +2, Group 7A = -1, Group 6A = -2, etc. • these atoms form ions that will result in an electron configuration that is the same as the nearest noble gas Tro, Chemistry: A Molecular Approach

  44. Tro, Chemistry: A Molecular Approach

  45. Electron Configuration of Anions in their Ground State • anions are formed when atoms gain enough electrons to have 8 valence electrons • filling the s and p sublevels of the valence shell • the sulfur atom has 6 valence electrons S atom = 1s22s22p63s23p4 • in order to have 8 valence electrons, it must gain 2 more S2- anion = 1s22s22p63s23p6 Tro, Chemistry: A Molecular Approach

  46. Electron Configuration of Cations in their Ground State • cations are formed when an atom loses all its valence electrons • resulting in a new lower energy level valence shell • however the process is always endothermic • the magnesium atom has 2 valence electrons Mg atom = 1s22s22p63s2 • when it forms a cation, it loses its valence electrons Mg2+ cation = 1s22s22p6 Tro, Chemistry: A Molecular Approach

  47. Tro, Chemistry: A Molecular Approach

  48. Electron Configuration of Atoms in their Ground State • Electrons in some elements are “promoted” to form more stable configurations, e.g. • Cu = 29 electrons Expected: 1s22s22p63s23p64s23d9 Actual: 1s22s22p63s23p64s13d10 • Cr = 24 electrons Expected: 1s22s22p63s23p64s23d4 Actual: 1s22s22p63s23p64s13d5 The usual reason given for this configuration is that the half-filled or completely filled d-orbitals are more stable as compared to those which are not. Tro, Chemistry: A Molecular Approach

  49. Trend in Atomic Radius – Main Group • Different methods for measuring the radius of an atom, and they give slightly different trends • van der Waals radius = nonbonding • covalent radius = bonding radius • atomic radius is an average radius of an atom based on measuring large numbers of elements and compounds • Atomic Radius Increases down group • valence shell farther from nucleus • effective nuclear charge fairly close • Atomic Radius Decreases across period (left to right) • adding electrons to same valence shell • effective nuclear charge increases • valence shell held closer Tro, Chemistry: A Molecular Approach

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