Agenda

1. Agenda • Hand back Scantron Sheets • Portfolio • Demo • Pauling electron-negativity • Using Pauling electron-negativity to predict bond types • How do draw a bond dipole • Predicting molecule polarity using bond polarity

2. Learning Goals • To be able to predict the polarity of a molecule using: • Pauling electron-negativity values • Bond dipoles • Molecular shapes (VSEPR Theory)

3. Polar Molecules

4. Demonstration • PREDICT- What do you think will happen when I Place the rod near the water? • EXPLAIN – why do you think that will happen? • OBSERVE!!!!!!! • EXPLAIN – Did your prediction agree with your observations? How can we explain what we observed?

5. Polar Molecules • What is a polar molecule? • The electron density is greater on one half of the molecule than the other, resulting in a molecule with a positive and a negative end • Example – Water, that’s why it bent towards the rod

6. Electronegativity • In order to help predict the polarity of the molecule Pauling began to measure the atomic property of atoms which he called electronegativity • Electronegativity describes how strongly the atomic nucleus attracts the electrons participating in bonding

7. What is bonding? • Remember the idea of children fighting over a toy? • What happens if one child was slightly bigger than the other? • What if one child was MUCH bigger than the other?

8. Electronegativity • Each atom was assigned a value between 0 and 4. • 4 meaning the atom strongly attracted the electrons • 0 meaning it did not attract the electrons at all.

9. Elbow Partners • Do you think the electronegativity increases or decreases as you go across the periodic table (left to right)? Why? • Example: do you think that lithium is more electronegative than fluorine why? • Do you think that the electronegativity increases or decreases going down the periodic table? Why or Why not? • Example: do you think that lithium is more electronegative than Francium? Why or Why not?

10. Trends in the periodic table • Across a period: the electronegativities generally increase from left to right across a period • Down a group: the electronegativities generally decrease from top to bottom down a group

11. Trends in the periodic table • Most electronegative atom Fluorine – 4.0 • Least electronegative atom Francium – 0.7

12. Predicting bond types • The difference in electronegativity (ΔEN) and the percent ionic character can be used to predict the type of bond that occurs between 2 atoms. • YOU WILL NOT HAVE TO CALCULATE PERCENT IONIC CHARACTER. You will be given a reference chart

13. ΔEN • Using the concept of difference in electronegativity, three different bond types can be identified • Nonpolar covalent: equal sharing of electrons • ΔEN<0.5 • Polar covalent: un even sharing of electrons • 0.5≤ΔEN ≤ 1.7 • Ionic: Electron transfer • 1.7< ΔEN

14. Predicting Bond Types Polar Covalent Nonpolar Covalent Ionic 0.5 3.3 2.5 1.7 0 Electronegativity difference (∆EN ) 50% 75% 0 100% 25% Percent ionic character Refer to page 252

15. Practice Questions • Classify the bonds in the following compounds as either ionic, polar covalent or non-polar covalent • SH2 • NaCl • O2

16. Partial charges • For a polar covalent bond, the atom with the higher electronegativitypulls on the bond harder than the atom with lower electronegativity. • This results in a higher electron density around one atom, and a reduced electron density around the second atom

17. Partial charges HCl H2O

18. Partial charges • The result is that one end of the atom becomes partially positive, and the other end becomes partially negative. • Since the electron is not Transferredthere is not a full charge difference, but a partial charge difference • Show partial charges using δ+and δ-.

19. Partial charge versus Full Charge • Ionic- Full charge, electron is transferred • Polar covalent-partial charge, electrons are unequally Shared

20. Partial charges δ- δ- δ+ δ+ δ+ HCl H2O

21. Bond Dipole • What is a bond dipole? It is a Vector representation of the difference of electronegativity of two atoms. • Runs parallel to the bond, the length represents relative strengths compared to other bonds • Starts at the Positivecharge, goes to the Negativecharge

22. Bond Dipole • Example, H-Cl • ΔEN=3.0-2.1=0.9 • H―Cl • Example C-H • ΔEN=2.5-2.1=0.4 • H―C

23. Bond Dipole • Practice question, classify the bonds for CO2 • Group discussion time!!! • CO2 has polar covalent bonds, then why is CO2 a non-polar molecule? • (HINT: Draw the 3d structure)

24. Molecular polarity • The molecular polarity can be found by adding all of the bond dipole vectors together • For CO2 this results in a cancellation of vectors • O C O • The Two Vectors cancel, therefore there is NO NET POLARITY VALUE For the molecule

25. Molecular Polarity of water O H H Therefore the molecular polarity of water is not zero, which was proven experimentally during the demo

26. Steps for predicting polarityWalkthrough Using O3 • Determine the molecules Lewis Structure (NASL method) • Use VSEPR to predict the molecules 3D shape • Determine the ΔEN for each bond • Draw the bond dipole for each bond • Add all of the bond dipoles together, show molecular polarity vector

27. Practice Question • Predict the polarity of NO2-

28. Practice