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PERIODIC TRENDS

PERIODIC TRENDS. PERIODICITY. Period Law - - physical and chemical properties of elements are a periodic function of their atomic numbers. General Periodic Trends. Atomic and ionic size Ionization energy Electron affinity, electronegativity. Effective Nuclear Charge Z*.

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PERIODIC TRENDS

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  1. PERIODIC TRENDS

  2. PERIODICITY Period Law- -physical and chemical properties of elements are a periodic function of their atomic numbers

  3. General Periodic Trends • Atomic and ionic size • Ionization energy • Electron affinity, electronegativity

  4. Effective Nuclear ChargeZ*

  5. Effective Nuclear Charge, Z* • Atom Z* Experienced by Electrons in Valence Orbitals • Li +1.28 • Be ------- • B +2.58 • C +3.22 • N +3.85 • O +4.49 • F +5.13 Increase in Z* across a period

  6. Higher effective nuclear charge Electrons held more tightly Larger orbitals. Electrons held less tightly. General Periodic Trends

  7. Lithium Periodic Trend in the Reactivity of Metals Sodium Potassium MOST

  8. 2. Reactivity for Metals • As you go down a group for metals the number of energy levels increase. • Because of this, reactivity increases because the atom is more willing to give away its electron (react).

  9. 3.Nonmetalic Trends: Gain electrons • Nonmetals on right side, form anions • Going right elements are more nonmetallic (better gainers of electrons) • Going UP elements become more nonmetallic (want to gain)

  10. 8. Reactivity nonmetals: Gain e • The reason Across = fill the energy level • Going UP a group, nonmetals have same valence but fewer total electrons • Flourine is the most reactive nonmetal.

  11. Atomic Radii Figure 8.9

  12. Atomic Size • Size increases, down a group. • Because electrons are added into additional energy levels, there is less attraction. • Size decreases across a period. • Because, increased effective nuclear charge.

  13. Atomic Size Size decreases across a period owing to increase in Z*. Each added electron feels a greater and greater + charge. Large Small

  14. Trends in Atomic SizeSee Figures 8.9 & 8.10

  15. + + Li , 78 pm 2e and 3 p Ion Sizes Forming a cation. • CATIONS are SMALLER than the atoms from which they come. • The electron/proton attraction has gone UP and so size DECREASES. Li,152 pm 3e and 3p

  16. - - F, 71 pm F , 133 pm 9e and 9p 10 e and 9 p Ion Sizes Forming an anion. • ANIONS are LARGER than the atoms from which they come. • The electron/proton attraction has gone DOWN and so size INCREASES. • Trends in ion sizes are the same as atom sizes.

  17. Trends in Ion Sizes Figure 8.13

  18. Ionization Energy IE = energy required to remove an electron from an atom in the gas phase. Mg (g) + 738 kJ ---> Mg+ (g) + e-

  19. Ionization Energy IE = energy required to remove an electron from an atom in the gas phase. Mg (g) + 738 kJ ---> Mg+ (g) + e- Mg+ (g) + 1451 kJ ---> Mg2+ (g) + e- Mg+ has 12 protons and only 11 electrons. Therefore, IE for Mg+ > Mg.

  20. Ionization Energy 1st IE: Mg (g) + 735 kJ ---> Mg+ (g) + e- 2nd IE: Mg+ (g) + 1451 kJ ---> Mg2+ (g) + e- 3rd IE: Mg2+ (g) + 7733 kJ ---> Mg3+ (g) + e- Energy cost is very high to dip into a shell of lower n (core electrons). This is why ox. no. = Group no.

  21. Trends in Ionization Energy

  22. Trends in Ionization Energy • IE decreases down a group • Because size increases. • IE increases across a period • Because effective nuclear • charge increases

  23. Electron Affinity A few elements GAIN electrons to form anions. Electron affinity is the energy involved when an atom gains an electron to form an anion. X(g) + e- ---> X-(g) E.A. = ∆E

  24. Trends in Electron Affinity

  25. Trends in Electron Affinity • Affinity for electron increases across a period (EA becomes more negative). • Affinity decreases down a group (EA becomes less negative). Atom EA F -328 kJ Cl -349 kJ Br -325 kJ I -295 kJ

  26.     -  [He] O ion + electron       [He] O atom Electron Affinity of Oxygen ∆E is EXOthermic because O has an affinity for an e-. EA = - 141 kJ

  27.     [He] N atom + electron   N- ion    [He] Electron Affinity of Nitrogen ∆E is zero for N- due to electron-electron repulsions. EA = 0 kJ

  28. Electronegativity • So how is this different from electron affinity? • Electron Affinity – is rating of how well an atom wants to gain an electron • Electronegativity – is rating of how well an atom keeps the electron once it is bonded to another atom

  29. Electronegativity

  30. Electron Configurations and the Periodic Trends

  31. “Your Best Friend” • Periodic table

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