Polarity in Covalent Bonds

Polar Bond polar means having polespolar means having poles uneven distribution of electronsuneven distribution of electrons one end of bond has more electrons than other endone end of bond has more electrons than other end large electronegativity difference between bonded atomslarge electronegativity difference between bonded atoms

Non-polar Bond non-nonon-polar = no poles even distribution of electrons electron cloud same on one end of bond as other end low or no electronegativity difference between bonded atoms

H 2 is symmetric: - both ends the same - electron cloud is football-shaped HCl is asymmetric: - both ends are different - electron cloud is lopsided - (Cl side more e - than H side)

Which bond(s) are polar? Which are nonpolar? polar: LiH & HF non-polar: H 2 red: e - rich blue: e - poor

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

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

Bond Polarity more electronegative atom pulls bonding e - more strongly & gets more than fair share of electron cloud –leads to separation of charge (delta notation) electron-rich side is partially negative (  -) electron-poor side is partially positive (  +)

BOND Polarity differencedepends on electronegativity difference between two atoms (A and B) in bond A B E A - E B - care only about SIZE of difference, not sign- care only about SIZE of difference, not sign - bigger the difference, the more polar the bond- 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 therefore bond will be non-polar

Electronegativity Difference ionic > 1.7 polar covalent >0.5 to 1.7 non-polar covalent 0.0 to 0.5 Type of Bond Difference in Electronegativities

calculate electronegativity difference for each of these bonds LiH : H 2 : HF: 2.1 – – – polar covalentnon- polar covalentpolar covalent

Electronegativity & Bond Type

Review Which of following bonds is most polar? A) O 2 B) HCl C) NH in NH 3 D) HBr 3.4 – 3.4 = – 3.2 = -1.1 = – 2.1 = 0.9 answer =(B) HCl

Review Which substance contains bond with greatest ionic character? A) KCl B) HCl C) Cl 2 D) CCl – 3.2 = -2.4 = – 3.2 = -1.1 = – 3.2 = – 3.2 = -0.6 = 0.6 answer = (A) KCl

Summary non-polar covalent bonds form between: –atoms having equal or low electronegativity values: polar covalent bonds form between: –atoms with greater electronegativity difference in values: >0.5 to 1.7

Predict the Polarity N 2 HF HCl O 2 Cl 2 HI HBr non-polarpolar polar non-polarnon-polarpolar polar

MOLECULE Polarity diatomic molecules: – molecular 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 look at kind & arrangement of bonds to determine overall polarity of molecule

Polarity of Molecules molecule may contain polar bonds, but 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 contain at least two mirror planes of symmetry diatomic elements CO 2 CH 4

Symmetric vs. Asymmetric CO 2 is nonpolar: electron cloud is symmetric H 2 O is polar: electron cloud lop-sided (asymmetrical)

Polarity of CO 2 ? two planes of symmetry therefore, molecule is NONPOLAR

Polarity of H 2 O? only one plane of symmetry not two, therefore water is polar!

structural formula can be used to predict Molecular Polarity! H H  C  H H   CF 4 is symmetric so is nonpolar

Ethane = C 2 H 6 Ethene = C 2 H 4 Ethyne = C 2 H 2 ALL hydrocarbon molecules are symmetrical - electron cloud is same on both ends - electron cloud is same on both ends - they are non-polar - they are non-polar