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AP Chemistry Unit 3 - Elements Lesson 12 – Periodic Trends Book Section: 7.2-7.5, 8.3 Effective Nuclear Charge In a many-electron atom, electrons are both attracted to the nucleus and repelled by other electrons. The nuclear charge that an electron experiences depends on both factors. Effective Nuclear Charge The effective nuclear charge, Zeff, is found this way: Zeff = Z – S Where Z is the atomic number and S is a screening constant, usually close to the number of inner electrons. (Assume this, don’t count valence electrons) What Is the Size of an Atom? The bonding atomic radius is defined as one-half of the distance between bonded nuclei. This is how we measure atomic radius. Sizes of Atoms Atomic radius tends to… …decrease from left to right across a row due to increasing Zeff …increase from top to bottom of a column due to increasing value of n 1984 MC #43 A) B) C) D) E) The elements in which of the following have most nearly nearly the same atomic radius? Be, B, C, N Ne, Ar, Kr, Xe Mg, Ca, Sr, Ba C, P, S, I Cr, Mn, Fe, Co 1984 MC #43 A) B) C) D) E) The elements in which of the following have most nearly nearly the same atomic radius? Be, B, C, N Ne, Ar, Kr, Xe Mg, Ca, Sr, Ba C, P, S, I Cr, Mn, Fe, Co – 48% correct, medium 1999 MC #50 A) B) C) D) E) In the periodic table, as the atomic number increases from 11 to 17, what happens to the atomic radius? It remains constant. It increases only. It increases, then decreases. It decreases only. It decreases, then increases. 1999 MC #50 A) B) C) D) E) In the periodic table, as the atomic number increases from 11 to 17, what happens to the atomic radius? It remains constant. It increases only. It increases, then decreases. It decreases only. – 62% correct, easy It decreases, then increases. Sizes of Ions Ionic size depends upon: The nuclear charge The number of electrons The orbitals in which electrons reside Sizes of Ions Cations are smaller than their parent atoms. The outermost electron is removed and repulsions between electrons are reduced. Sizes of Ions Anions are larger than their parent atoms. Electrons are added and repulsions between electrons are increased. Sizes of Ions Ions increase in size as you go down a column. This is due to increasing value of n. Sizes of Ions In an isoelectronic series, ions have the same number of electrons. Ionic size decreases with an increasing nuclear charge. Ionization Energy The ionization energy (I) is the amount of energy required to remove an electron from the ground state of a gaseous atom or ion. The first ionization energy (I1) is the energy required to remove the first electron. The second ionization energy (I2) is the energy required to remove the second electron, etc… Ionization Energy It requires more energy to remove each successive electron. When all valence electrons have been removed, the ionization energy takes a quantum leap. Trends in First Ionization Energies As one goes down a column, less energy is required to remove the first electron. For atoms in the same group, Zeff is essentially the same, but the valence electrons are farther from the nucleus (Coulomb’s Law – physics) Trends in First Ionization Energies Generally, as one goes across a row, it gets harder to remove an electron. As you go from left to right, Zeff increases. Trends in First Ionization Energies However, there are two apparent discontinuities in this trend. Trends in First Ionization Energies The first occurs between groups IIA and IIIA. In this case a full s-orbital is preferred, and it is harder to remove the electron from IIA than from IIIA. Trends in First Ionization Energies The second occurs between groups VA and VIA. In this case, half-filled p orbitals are especially stable. 1999 MC #37 Ionization Energies for Element X (kJ mol-1) First Second Third Fourth Fifth 580 1,815 2,740 11,600 14,800 The ionization energies for element X are listed in the table above. On the basis of the data, element X is most likely to be A) Na B) Mg C) Al D) Si E) P 1999 MC #37 Ionization Energies for Element X (kJ mol-1) First Second Third Fourth Fifth 580 1,815 2,740 11,600 14,800 The ionization energies for element X are listed in the table above. On the basis of the data, element X is most likely to be A) Na B) Mg C) Al – 35% correct, hard D) Si E) P Electron Affinity Electron affinity is the energy change accompanying the addition of an electron to a gaseous atom: Cl + e- Cl- Trends in Electron Affinity In general, electron affinity becomes larger as you go left to right across a row. Trends in Electron Affinity There are, again, however, two discontinuities in this trend. Trends in Electron Affinity The first is between IA and IIA. IIA already has a full s-orbital, so adding another electron to the psublevel is not favorable. Trends in Electron Affinity The first is between IVA and VA. IVA already has a half-full psublevel, so adding another electron is not favorable. Electronegativity Electronegativity is the ability of atoms to attract electrons to themselves. On the periodic table, electronegativity increases as you go… From left to right across a row From the bottom to the top of a column. 1989 MC #1, 3 O B) La C) Rb D) Mg E) N 1) What is the most electronegative element? 3) Which of the elements above has the smallest ionic radius for its most commonly found ion? A) 1989 MC #1, 3 O B) La C) Rb D) Mg E) N 1) What is the most electronegative element? A, 81% correct, very easy 3) Which of the elements above has the smallest ionic radius for its most commonly found ion? D, 33% correct, hard A) HW: 7.16, 19, 22, 24, 26, 28, 36, 42, 44, 54, 8.36, 38 This Week: Friday – Radioactive Decay & Nuclear Equations 10/18 – Gravimetric Analysis of a Chloride Salt Due 10/20 – Elements Exam 10/21 – Problem Set 2 Due