Trends - General Periodic Trends PERIODIC TRENDS • •...

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Unformatted text preview: General Periodic Trends PERIODIC TRENDS • • • Atomic and ionic size Ionization energy Electron affinity •• Size goes UP on going down Size goes UP on going down a group. See Figure 8.10. a group. See Figure 8.10. •• Because electrons are Because electrons are added further from the added further from the nucleus, there is less nucleus, there is less attraction. attraction. •• Size goes DOWN on going Size goes DOWN on going across a period. across a period. Higher Z*. Electrons held more tightly. Larger orbitals. Electrons held less tightly. Atomic Radii Atomic Size Atomic Size SIZE SIZE Atomic Size Atomic Size Trends in Atomic Size See Figure 8.10 Radius (pm) Size decreases across a period owing Size decreases to increase in Z*. Each added electron feels a greater and greater + charge. 250 K 1st transition series 3rd period 200 Na 2nd period Li 150 Kr 100 Ar Ne 50 He 0 0 5 10 15 20 25 Atomic Number Page 1 30 35 40 Ion Sizes Ion Sizes Sizes of Transition Elements Sizes of Transition Elements See Figure 8.11 See Figure 8.11 • 3d subshell is inside the 4s subshell . • 4s electrons feel a more or less constant Z*. • Sizes stay about the same and chemistries are similar! and chemistries Li,152 pm 3e and 3p Ion Sizes Ion Sizes F,64 pm 9e and 9p Does the size go up or Does-the size go up or down when gaining an down when gaining an electron to form an F -electron to form an , 136 pm anion? 10 e and 9 p anion? Ion Sizes Ion Sizes Does the size go Does the size go + up or down up or down Li +, 60 pm when losing an when 3 p 2e and losing an electron to form electron to form a cation? a cation? F - , 136 pm 10 e and 9 p Forming Forming an anion. 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. Page 2 Li +, 60 pm 2e and 3 p Forming Forming a cation. a cation. • CATIONS are SMALLER than the atoms from which they come. • The electron/proton attraction has gone UP and so size DECREASES. Trends in Ion Sizes Ion Sizes Ion Sizes F,64 pm 9e and 9p + Li,152 pm 3e and 3p Ionization Energy Ionization Energy Ionization Energy Ionization Energy IE = energy required to remove an electron from an atom in the gas phase. IE = energy required to remove an electron from an atom in the gas phase. Redox Reactions Redox Reactions Why do metals lose Why do metals lose electrons in their electrons in their reactions? reactions? See Screen 8.12 See Screen 8.12 Mg (g) + 738 kJ ---> Mg + (g) + eMg+ (g) + 1451 kJ ---> Mg 2+ (g) + e(g) Why does Mg form Mg 2+ Why does Mg form Mg 2+ ions and not Mg 3+? ions and not Mg 3+? Why do nonmetals take Why do nonmetals take on electrons? on electrons? See Screen 8.12 See Screen 8.12 Mg (g) + 738 kJ ---> Mg + (g) + e- Ionization Energy Ionization Energy Ionization Energy Ionization Energy See Screen 8.12 See Screen 8.12 See Screen 8.12 See Screen 8.12 Ionization Energy Ionization Energy See Screen 8.12 See Screen 8.12 IE = energy required to remove an electron from an atom in the gas phase. Mg (g) + 735 kJ ---> Mg + (g) + e- Mg (g) + 735 kJ ---> Mg + (g) + e- Mg+ (g) + 1451 kJ ---> Mg 2+ (g) + e(g) Mg+ (g) + 1451 kJ ---> Mg 2+ (g) + e(g) Mg (g) + 738 kJ ---> Mg + (g) + e- Mg2+ (g) + 7733 kJ ---> Mg 3+ (g) + e- Mg2+ (g) + 7733 kJ ---> Mg 3+ (g) + eMg+ (g) (g) Mg 2+ + 1451 kJ ---> (g) + eMg+ has 12 protons and only 11 electrons. Therefore, IE for Mg+ > Mg. Energy cost is very high to dip into a shell of lower n. This is why ox. no. = shell This Group no. Page 3 Trends in Ionization Energy 1st Ionization energy (kJ/mol) 2500 He Ne 2000 Ar 1500 Kr 1000 Trends in Ionization Energy Trends in Ionization Energy • IE increases across a period because Z* increases. • IE decreases down a group • Metals lose electrons more easily than nonmetals. • Because size increases. • Metals are good reducing agents. 500 0 1 H 3 Li Lithium Sodium 5 7 9 11 Na 13 15 17 19 K 21 23 25 27 29 31 33 35 Trends in Ionization Energy Trends in Ionization Energy • Nonmetals lose electrons with difficulty. • Reducing ability generally increases down the periodic table. • See reactions of Li, Na , K Atomic Number Periodic Trend in the Reactivity of Alkali Metals with Water 2nd IE / 1st IE Li Figure 8.5 Na K Potassium Page 4 Electron Affinity A few elements GAIN electrons few GAIN to form anions. to anions Electron affinity is the energy involved when an anion loses an electron. A -(g) ---> A(g) + e- E.A. = ΔE Electron Affinity of Oxygen O- ion [He] ↑↓ ΔE is ENDOthermic because O has an affinity for an O atom e-. ↑↓ ↑ ↑ - electron [He] ↑↓ ↑↓ ↑ ↑ Electron Affinity of Nitrogen N- ion [He] ↑↓ N- ΔE is zero for for due to electronelectron N atom repulsions . [He] ↑↓ Trends in Electron Affinity Br 350 350 S4 S4 Period Period S1 S1 S 200 200 Se 150 150 100 100 Ge P S3 S3 S2 S2 O Si 250 250 K 1 1 2 2 3 3 4 4 5 5 Group Group 50 50 6 6 7 7 Electron affinity (kJ/mol) Electron affinity (kJ/mol) 300 300 C ↑ ↑ EA = 0 kJ F Cl ↑ - electron EA = + 141 kJ H ↑↑ 0 0 Page 5 ↑ Trends in Electron Affinity • See Figure 8.13 and Appendix F • Affinity for electron increases across a period (EA becomes more positive). • Affinity decreases down a group (EA becomes less positive). Atom EA Atom EA F F +328 kJ +328 kJ Cl +349 kJ Cl +349 kJ Br +325 kJ Br +325 kJ II +295 kJ +295 kJ ...
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This note was uploaded on 10/19/2011 for the course CHM 2210 taught by Professor Reynolds during the Fall '01 term at University of Florida.

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