Polarity in Covalent Bonds Polar Bond polar means
Polarity in Covalent Bonds
Polar Bond • polar means having poles • uneven distribution of electrons • one end of bond has more electrons than other end • large electronegativity difference between bonded atoms
Non-polar Bond • non-polar = no poles on • 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: Li. H & 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 emore 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 • depends on electronegativity difference between two atoms (A and B) in bond A B 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 therefore bond will be non-polar
Electronegativity Difference in Electronegativities Type of Bond 0. 0 to 0. 5 non-polar covalent >0. 5 to 1. 7 polar covalent > 1. 7 ionic
calculate electronegativity difference for each of these bonds Li. H : 2. 1 – 1. 0 1. 1 polar covalent H 2 : 2. 1 – 2. 1 0 non- polar covalent HF: 2. 1 – 4 1. 9 polar covalent
Electronegativity & Bond Type
Review • Which of following bonds is most polar? A) O 2 3. 4 – 3. 4 = 0 2. 1 – 3. 2 = -1. 1 = 1. 1 B) HCl 3. 0 – 2. 1 = 0. 9 C) NH in NH 3 3. 0 – 2. 1 = 0. 9 D) HBr answer =(B) HCl
Review • Which substance contains bond with greatest ionic character? A) KCl 0. 8 – 3. 2 = -2. 4 = 2. 4 2. 1 – 3. 2 = -1. 1 = 1. 1 B) HCl 3. 2 – 3. 2 = 0 C) Cl 2 D) CCl 4 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. 5 • polar covalent bonds form between: – atoms with greater electronegativity difference in values: >0. 5 to 1. 7
Predict the Polarity • • N 2 non-polar HF polar HCl polar O 2 non-polar Cl 2 non-polar HI polar HBr 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
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 CH 4 CO 2
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 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 - they are non-polar
- Slides: 27