The History of the Modern Periodic Table

1. The History of the Modern Periodic Table See separate slide show for Periodic Table History

2. Periodic Law • When elements are arranged in order of increasing atomic #, elements with similar properties appear at regular intervals.

3. Chemical Reactivity Families  Similar valence e- within a group result in similar chemical properties • Alkali Metals • Alkaline Earth Metals • Transition Metals • Halogens • Noble Gases

4. Effective Nuclear charge atomic size or radius ionization energy electron affinity electronegativity metallic character Reactivity bonding characteristics crystal configurations acidic properties densities Melting/Boiling points Periodic Table Reveals Periodic Trends

5. Electron screening or shielding • Electrons are attracted to the nucleus • Electrons are repulsed by other electrons • Electrons would be bound more tightly if other electrons weren’t present. • The net nuclear charge felt by an electron is called the effective nuclear charge ( Zeff ).

6. Quantum Mechanical Model Zeffis lower than actual nuclear charge. Zeff increases toward nucleus ns > np > nd > nf This explains certain periodic changes observed.

7. Effective Nuclear Charge ( Zeff) • The effective nuclear charge acting on an electron equals the number of protons in the nucleus, Z, minus the average number of electrons, S that are between the nucleus and the electron in question. Zeff = Z  S Zeff = attractive forces  repulsive forces Zeff = # protons  # shielding electrons

8. For Example, Lithium vs. Carbon Li Zeff = 3  2 = 1 C Zeff = 6  2 = 4 When moving across a row: The greater the Zeff value, the smaller the atom’s radius. So, carbon has a much smaller atomic radius compared to lithium: Rcarbon=77 pm Rlithium = 152 pm

9. Trend #1 Atomic Radii Increases to Left and Down • Why larger going down? • Why smaller to the right? • Higher energy levels have larger orbitals • Shielding - core e- block the attraction between the nucleus and the valence e- • Increased nuclear charge without additional shielding pulls e- in tighter

10. Atomic radii

11. The Periodic Table & Radii

12. Periodic Trend is Due to Effective Nuclear Charge Atomic Radii vs. Zeff:

13. Trends in Ionic Radii • Using your knowledge of Zeff, how would the size of a cation compare to neutral atom? Anion?

14. Trends in Ionic Radii • The cation of an atom decreases in size. • The more positive an ion is, the smaller it is because Zeff increases • The anion of an atom increases in size. • The more negative an ion, the larger it is because Zeff decreases.

15. Cations  lose electrons, become smaller

16. Anions  gain electrons, become bigger

17. Ion Radii Increases moving across, but depends if cation OR anion Increases down +3 +4 -3 -2 -1

18. Ions and Ionic Radii

20. Trend in Ionization Energy • Ionization NRG is the NRG required to remove an electron from an atom

21. Successive Ionization NRG • Ionization energy increases for successive electrons from the same atom.

22. Why do you think there is such a big jump for Mg3+? *Notice the large jump in ionization energy when a core e is removed.

23. The smaller the atom, the higher the ionization energy due to Zeff • Bigger atoms have lower ionization NRG due to the fact that the electrons are further away from the nucleus and therefore easier to remove.

24. Ionization Energy Increases Decreases

25. Electron Affinity • The energy change associated with the addition of an electron • Tends to increase across a period • Tends to decrease as you go down a group • Abbreviation is Eea, it has units of kJ/mol. Values are generally negative because energy is released. • Value of Eea results from interplay of nucleus electron attraction, and electron–electron repulsion.

26. Electron Affinity

27. Trends in Electronegativity • tendency for an atom to attract electrons when it is chemically combined with another atom. • decreases as you move down a group • increases as you go across a period from left to right.

28. Trend #5 Metallic Character • The metallic character of atoms can be related to the desire to lose electrons. • The lower an atom’s ionizatoin energy, the greater its metallic character will be. • On the periodic table, the metallic character of the atoms increase down a family and decreases from left to right across a period.

29. Metallic Character Increases moving down and across to the left Rb Cs Ba Fr Ra Lower left corner -- elements most likely to lose their valence electrons

30. # 6 Melting/Boiling Points • Highest in the middle of a period (generally).

31. Some Important Properties of Alkali Metals • Easily lose valence electrons (Reducing Agents) • React with halogens to form salts • React violently with water • Large Hydration NRG • Positive ionic charge makes ions attractive to polar water molecules • Radius and Ionization NRG follow expected trends.