Polarity in Covalent Bonds

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

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

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)

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

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

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 (+)

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

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

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

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

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

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

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

Predict the Polarity 3.0 – 3.0 = 0 non-polar N2 HF HCl HI HBr 4.0-2.2 = 1.8 polar 3.2 – 2.2 = 1.0 polar 3.4 - 3.4 = 0 non-polar 3.2 – 3.0 = 0 non-polar 2.7 = 2.2 = 0.5 polar 3.0 – 2.2 = 0.8 polar

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

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

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

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!

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

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

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

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

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

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

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