1. Polarity in Covalent Bonds

2. Polar Chemical Bonds being polar refers to:
uneven distribution of electrons
one end of bond has more electrons than other end
due to large electronegativity difference between bonded atoms

3. Non-polar Chemical Bonds
being non-polar refers to:
even distribution of electrons
electron cloud same on one end of bond as other end
due to low or no electronegativity difference between bonded atoms

4. H2 is symmetric:
- same # e- at each end
- football-shaped electron cloud
HCl is asymmetric:
- different # e- at each end
- electron cloud is lopsided
(Cl side has more e- than H side)

5. How can predict if BOND is polar or not?
compare difference in electronegativity values of two atoms joined in bond

6. Electronegativity
ability of atom to attract electrons in bond
values found in Table S!

7. Bond Polarity more electronegative atom has greater pull on bonding e-
gets more than fair share of electron cloud (uneven distribution of e-)
leads to separation of charge (delta notation)
electron-rich side partially negative (-)
electron-poor side partially positive (+)

8. BOND Polarity A B   EA - EB
depends on electronegativity difference between two atoms
A B
elements A and B chemically bonded together
 
EA - EB
- care only about SIZE of difference, not sign
- bigger the difference, the more polar the bond

9. Bond Polarity
What can say about polarity of bond where atom A is same as atom B?
both atoms are same so
electronegativity difference = 0
expect bond to be non-polar

10. electronegativity difference
ionic
≥ 1.7
polar covalent
>0.4 to <1.7
non-polar covalent
0.0 to 0.4
bond type
electronegativity difference

11. calculate electronegativity difference for each of these bonds
2.2 – 1.0
1.2
2.2 – 2.2
2.2 – 4
1.8
LiH : H2 : HF:
FALLS IN polar covalent range but is IONIC!!
FALLS IN ionic range but is POLAR COVALENT!!
non- polar covalent

13. Review Which of following bonds is most polar? A) O2 B) HCl
C) NH in NH3
D) HBr
3.4 – 3.4 = 0
2.2 – 3.2 = -1.0 = 1.0
3.0 – 2.2 = 0.8
3.0 – 2.2 = 0.8
answer =(B) HCl

14. Review Which substance contains bond with greatest ionic character?
A) KCl
B) HCl
C) Cl2
D) CCl4
0.8 – 3.2 = -2.4 = 2.4
2.2 – 3.2 = -1.0 = 1.0
3.2 – 3.2 = 0
2.6 – 3.2 = -0.6 = 0.6
answer = (A) KCl

15. Summary non-polar covalent bonds form between:
atoms having equal or low electronegativity values: 0.0 – 0.4
polar covalent bonds form between:
atoms with greater electronegativity difference in values: >0.4 to <1.7

16. Predict the Polarity 3.0 – 3.0 = 0 non-polar N2 HF HClHI
HBr
= polar
3.2 – 2.2 = polar
= 0 non-polar
3.2 – 3.0 = 0 non-polar
2.7 = 2.2 = polar
3.0 – 2.2 = polar

17. MOLECULE Polarity diatomic molecules: only ONE BOND so
molecule polarity is same as bond polarity

18. MOLECULE Polarity (3+ elements)
depends on 2 factors
1. type bonds in molecule
2. arrangement of bonds or shape of molecule
Think SYMMETRY!!

19. for larger molecules:
look at kind & arrangement of bonds to determine overall polarity of molecule

20. Polarity of Molecules
molecule may contain polar bonds, and not be polar!
depends on geometry of molecule
if molecule is symmetric:
“pull” of one polar bond is offset by “pull” of another polar bond
tug-of-war that no one can win!

21. Symmetric Molecules CH4 CO2
contain at least two mirror planes of symmetry
diatomic elements
CH4
CO2

22. Symmetric vs. Asymmetric
CO2 is nonpolar: electron cloud is symmetric
H2O is polar: electron cloud lop-sided (asymmetrical)

23. Polarity of CO2?
two planes of symmetry therefore, molecule is NON-POLAR

24. Polarity of H2O?
only one plane of symmetry not two, therefore water is polar!

25. structural formula can be used to predict Molecular Polarity!H
H  C  H  
CF4 is symmetric so is non-polar

26. ethane = C2H6
ethene = C2H4
ethyne = C2H2
ALL hydrocarbon molecules are symmetrical
- electron cloud is same on both ends
- they are non-polar

27. Compare & Contrast BOND POLARITY MOLECULE POLARITY
All about the ELECTRONEGATIVITY difference between the TWO atoms in a bond
NON-POLAR BOND: electronegativity difference is 0.4 or less
POLAR BOND: electronegativity difference is > 0.4 & <1.7
All about the SYMMETRY of the molecule
SYMMETRICAL molecules are NON- POLAR
ASYMMETRICAL molecules are POLAR