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 Atomic Size Atomic Size SIZE SIZE Higher Z*. Electrons held more tightly. Larger orbitals. Electrons held less tightly. •• 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. Atomic Radii Atomic Size Atomic Size Size decreases across a period owing decreases across to increase in Z*. Each added electron feels a greater and greater + charge. Trends in Atomic Size See Figure 8.10 Radius (pm) 250 K 200 3rd period 2nd period Li Na 1st transition series 150 100 Kr Ne Ar 50 He 0 0 5 10 15 20 25 30 35 40 Atomic Number Page 1 Sizes of Transition Elements Sizes of Transition Elements See Figure 8.11 See Figure 8.11 • 3d subshell is inside the 4s subshell. subshell. • 4s electrons feel a more or less constant Z*. • Sizes stay about the same and chemistries are similar! Li,152 pm 3e and 3p 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? cation? Li,152 pm 3e and 3p Ion Sizes Ion Sizes + Li +, 60 pm 2e and 3 p Forming Forming a cation. a cation. cation. • CATIONS are SMALLER than the atoms from which they come. • The electron/proton attraction has gone UP and so size DECREASES. Ion Sizes Ion Sizes F,64 pm 9e and 9p Ion Sizes Ion Sizes F,64 pm 9e and 9p F - , 136 pm 10 e and 9 p Trends in Ion Sizes Forming Forming an anion. an anion. Does the size go up or Doesthe 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? • 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 Redox Reactions Redox Reactions Why do metals lose Why do metals lose electrons in their electrons in their reactions? reactions? 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? 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. 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) + 738 kJ ---> Mg + (g) + e(g) Mg+ (g) + 1451 kJ ---> Mg 2+ (g) + e(g) (g) Mg (g) + 738 kJ ---> Mg + (g) + e(g) 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) + 738 kJ ---> Mg + (g) + e(g) Ionization Energy Ionization Energy See Screen 8.12 See Screen 8.12 Mg (g) + 735 kJ ---> Mg + (g) + e(g) Mg+ (g) + 1451 kJ ---> Mg 2+ (g) + e(g) (g) Mg2+ (g) + 7733 kJ ---> (g) Mg 3+ (g) + e(g) Ionization Energy Ionization Energy See Screen 8.12 See Screen 8.12 Mg (g) + 735 kJ ---> Mg + (g) + e(g) Mg+ (g) + 1451 kJ ---> Mg 2+ (g) + e(g) (g) Mg2+ (g) + 7733 kJ ---> Mg 3+ (g) + e(g) (g) + 1451 kJ ---> (g) + e(g) Mg+ has 12 protons and only 11 has electrons. Therefore, IE for Mg+ > Mg. Mg. Mg+ (g) (g) Mg 2+ Energy cost is very high to dip into a shell of lower n. This is why ox. no. = Group no. Page 3 Trends in Ionization Energy 1st Ionization energy (kJ/mol) 2500 Trends in Ionization Energy Trends in Ionization Energy • IE increases across a period because Z* increases. • Metals lose electrons more easily than nonmetals. • Metals are good reducing agents. Trends in Ionization Energy Trends in Ionization Energy • IE decreases down a group • Because size increases. • Reducing ability generally increases down the periodic table. • See reactions of Li, Na, K Li, Na, He Ne 2000 1500 Ar Kr 1000 500 0 H 1 Li 3 5 7 9 Na 11 13 15 17 19 K 21 23 25 27 29 31 33 35 • Nonmetals lose electrons with difficulty. Atomic Number Lithium Periodic Trend in the Reactivity of Alkali Metals with Water 2nd IE / 1st IE Li Electron Affinity A few elements GAIN electrons to form anions. anions. Electron affinity is the energy involved when an anion loses an electron. A -(g) ---> A(g) + e- E.A. = ∆E Figure 8.5 Na K Sodium Potassium Page 4 Electron Affinity of Oxygen ∆E is ENDOthermic ENDOthermic because O has an affinity for an O atom e-. O- ion [He] ↑↓ Electron Affinity of Nitrogen N- ion [He] ↑↓ 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). ↑↓ ↑ ↑ ↑↑ ↑ - electron [He] ↑↓ ↑↓ ↑ ↑ ∆E is zero for due to electronelectron N atom repulsions. repulsions. N- - electron [He] ↑↓ ↑ ↑ ↑ EA = 0 kJ EA = + 141 kJ 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 Trends in Electron Affinity F Cl Br O Si S Se 350 350 300 300 250 250 200 200 150 150 100 100 50 50 0 0 Electron affinity (kJ/mol) Electron affinity (kJ/mol) H S4 S4 Period Period S3 S3 S2 S2 S1 S1 C Ge P K 1 1 2 2 3 3 4 4 5 5 Group Group 6 6 7 7 Page 5 ...
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