AP Chemistry Unit 3 - Elements

1. AP ChemistryUnit 3 - Elements Lesson 12 – Periodic Trends Book Section: 7.2-7.5, 8.3

2. 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.

3. 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)

4. 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.

5. 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

6. 1984 MC #43 • The elements in which of the following havemost 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

7. 1984 MC #43 • The elements in which of the following havemost 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

8. 1999 MC #50 • 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.

9. 1999 MC #50 • 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.

10. Sizes of Ions • Ionic size depends upon: • The nuclear charge • The number of electrons • The orbitals in which electrons reside

11. Sizes of Ions • Cations are smaller than their parent atoms. • The outermost electron is removed and repulsions between electrons are reduced.

12. Sizes of Ions • Anions are larger than their parent atoms. • Electrons are added and repulsions between electrons are increased.

13. Sizes of Ions • Ions increase in size as you go down a column. • This is due to increasing value of n.

14. Sizes of Ions • In an isoelectronic series, ions have the same number of electrons. • Ionic size decreases with an increasing nuclear charge.

15. 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…

16. 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.

17. 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)

18. 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.

19. Trends in First Ionization Energies • However, there are two apparent discontinuities in this trend.

20. Trends in First Ionization Energies • The first occurs between groups IIA and IIIA. • In this case a fulls-orbital is preferred, and it is harder to remove the electron from IIA than from IIIA.

21. Trends in First Ionization Energies • The second occurs between groups VA and VIA. • In this case, half-filled p orbitals are especially stable.

22. 1999 MC #37 • 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 • Na • Mg • Al • Si • P

23. 1999 MC #37 • 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 • Na • Mg • Al – 35% correct, hard • Si • P

24. Electron Affinity • Electron affinity is the energy change accompanying the addition of an electron to a gaseous atom: • Cl + e- Cl-

25. Trends in Electron Affinity • In general, electron affinity becomes larger as you go left to right across a row.

26. Trends in Electron Affinity • There are, again, however, two discontinuities in this trend.

27. Trends in Electron Affinity • The first is between IA and IIA. • IIA already has a full s-orbital, so adding another electron to the p-sublevel is not favorable.

28. Trends in Electron Affinity • The first is between IVA and VA. • IVA already has a half-full p-sublevel, so adding another electron is not favorable.

29. 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.

30. 1989 MC #1, 3 • O • La • Rb • Mg • 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?

31. 1989 MC #1, 3 • O • La • Rb • Mg • 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

32. 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