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Lecture 0802 Trends on the Periodic Table Li PERIODIC TRENDS Na K Effective Nuclear Charge Z* The 2s electron PENETRATES the region occupied by the 1s electron. 2s electron experiences a higher positive charge than expected. Effective Nuclear Charge, Z* Atom Li Be B C N O F Z* e- in Valence Orbitals est measured 1 +1.28 Increase in 2 ------Z* across a 3 +2.58 period 4 +3.22 5 +3.85 6 +4.49 7 +5.13 General Periodic Trends Atomic and ionic size Ionization energy Electron affinity Higher effective nuclear charge Electrons held more tightly Larger shells. Electrons held less tightly. Atomic Radius Is taken as the covalent radius for non-metallic elements and as the metallic radius for metals Atomic Radius Covalent radius is one-half the distance between the nuclei of two identical atoms that are singly bonded to one another. Chlorine Bond Length Atomic Radius Covalent radii for elements whose atoms do not bond to one another can be estimated by combining radii of those that do with the distances between unlike atoms in various molecules. Atomic Radius Metallic radius is one-half the closest internuclear distance in a metallic crystal. Prediction! Atomic Radius (pm) 200 150 100 50 0 1 3 5 7 9 11 13 Atomic Number 15 17 19 Atomic Size Size goes UP on going down a group. Because electrons are added further from the nucleus, there is less attraction. Size goes DOWN on going across a period. Atomic Size Size decreases across a period owing to increase in Z*. Each added electron feels a greater and greater + charge. Large Small Trends in Atomic Size Radius (pm) 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 30 35 40 Sizes of Transition Elements 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! General Periodic Trends Atomic and ionic size Ionization energy Electron affinity Higher effective nuclear charge Electrons held more tightly Larger shells. Electrons held less tightly. Ion Sizes Li,152 pm 3e and 3p Does + the size go up down when Li + , or 60 pm 2e and 3 pan electron to losing form a cation? Ion Sizes + Li,152 pm 3e and 3p CATIONS Li + , 78 pm 2e and 3 p Forming a cation. are SMALLER than the atoms from which they come. The proton/electron attraction has gone UP and so size DECREASES. Ion Sizes Does the size go up or down when gaining an electron to form an anion? Ion Sizes F, 71 pm 9e and 9p ANIONS F- , 133 pm 10 e and 9 p Forming an anion. are LARGER than the atoms from which they come. The proton/electron attraction has gone DOWN and so size INCREASES. Trends in Ion Sizes Trends in ion sizes are the same as atom sizes. Redox Reactions Why do metals lose electrons in their reactions? Why does Mg form Mg2+ ions and not Mg3+? Why do nonmetals take on electrons? Ionization Energy (General) Is the energy required to remove the outermost electron from an atom or a positive ion in the ground state. First Ionization Energy Energy required to remove the first electron from a neutral atom in the gaseous state. Ionization Energy Mg (g) + 738 kJ Mg+ (g) + e- Prediction! 1st Ionization Energy (kJ/mol) 2500 2000 1500 1000 500 0 1 3 5 7 9 11 13 Atomic Number 15 17 19 Trends in Ionization Energy 1st Ionization energy (kJ/mol) 2500 He Ne 2000 Ar 1500 Kr 1000 500 0 1 H 3 Li 5 7 9 11 Na 13 15 17 19 K 21 23 25 27 29 31 Atomic Number 33 35 Atomic Radii Trends in Ionization Energy IE increases across a period because Z* increases. Metals lose electrons more easily than nonmetals. Metals are good reducing agents. Nonmetals lose electrons with difficulty. Trends in Ionization Energy IE decreases down a group Because size increases. Reducing ability generally increases down the periodic table. Remember Li, Na, K Second Ionization Energy Energy needed to remove the outermost electron from a +1 ion. Energy needed to remove the second electron from a neutral atom. Ionization Energy Mg (g) + 738 kJ Mg+ (g) + e- Mg+ (g) + 1451 kJ Mg2+ (g) + eMg+ has 12 protons and only 11 electrons. Therefore, IE for Mg+ > Mg. 2nd Ionization Energy (kJ/mol) 8000 7000 6000 5000 4000 3000 2000 1000 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Atomic Number Ionization Energy Mg (g) + 735 kJ Mg+ (g) + eMg+ (g) + 1451 kJ Mg2+ (g) + e- Mg2+ (g) + 7733 kJ Mg3+ (g) + eEnergy cost is very high to dip into a shell of lower n. This is why ox. no. = Group no. General Periodic Trends Atomic and ionic size Ionization energy Electron affinity Higher effective nuclear charge Electrons held more tightly Larger shells. Electrons held less tightly. Electron Affinity A few elements GAIN electrons to form anions. E.A. is the energy released or absorbed when an electron is added to the valence level of a gas-phase atom. A(g) + e- A-(g) E.A. = ∆E Prediction! Trends in Electron Affinity Electron Affinity of Oxygen O atom [He] + electron O- ion [He] EA = - 141 kJ ∆E is EXOthermic because O has an affinity for an e-. Electron Affinity of Nitrogen N atom [He] + electron N- ion [He] EA = 0 kJ ∆E is zero for N- due to electronelectron repulsions. Trends in Electron Affinity See Figure 8.12 and Appendix F 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 General Periodic Trends Atomic and ionic size Ionization energy Electron affinity Higher effective nuclear charge Electrons held more tightly Larger shells. Electrons held less tightly.
