Resonance Hybrids, Exceptions to Octet Rule Lecture

TP Which molecule is more polar? χH 1. 2. 3. 2.1, χB 2.04, χN 3.04, χF 3.98 NH trigonal pyramid BF trigonal planar Polarity is the same • • • • • Resonance hybrids Exceptions to octet rule Electronegativity and bond character Bond polarity Molecular geometry Next: Molecular polarity; Begin ch9: Covalent bonding Lewis formula of NO3– (nitrate) Exceptions to the octet “rule” Alternative equivalent formulas are known as resonance forms. Octet deficient: BF Bonding is an equal mixture of each resonance form, known as a resonance hybrid. Single bonds more stable, based on formal charge. The shared electrons are said to be delocalized across the four atoms, represented as a resonance hybrid. The mixture does not vary with time. Each bond is 1/3 of the way between single and double. 1 Lecture 16 CH101 A2 (MWF 11:15 am) Fall 2019 Copyright © 2019 Dan Dill dan@bu.edu Lecture 16 CH101 A2 (MWF 11:15 am) Fall 2019 Exceptions to the octet “rule” Exceptions to the octet “rule” Odd number of electrons: NO Expanded octet period 3 onward : POCl Copyright © 2019 Dan Dill dan@bu.edu Expanded octet more stable, based on formal charge. Electronegativity Electronegativity, χ Greek letter chi, “hee” Formal charge assumes electrons between atoms are shared equally. Electronegativity is a relative measure the tendency of an atom to attract electrons shared with another atom in a covalent bond. Unless two connected atoms are identical, sharing always favors one atom. Electronegativity is a relative measure the tendency of an atom to attract electrons shared with another atom in a covalent bond. 2 Electronegativity, χ Greek letter chi, “hee” Bond polarity Electronegativity is a relative measure the tendency of an atom to attract electrons shared with another atom in a covalent bond. The greater the electronegativity difference of two covalently bonded atoms, the more unequal the sharing of the electrons forming the covalent bond. BrI: 2.96 2.66 HCl: 3.16 2.1 NaCl: 3.16 0.93 0.30, covalent 1.0, polar covalent 2.23, ionic Please keep in mind that these definitions are qualitative, since sharing is always unequal unless the bonded atoms are identical. TP Which bond is more polar? χH 1. N H 2. B F 3. Polarity is the same 2.1, χB 2.04, χN 3.04, χF 3.98 Predicting molecular geometries How atoms are arranged in three dimensions around their central atom is determined by the steric number SN the central atom. SN attached atoms lone pairs 3 SN = attached atoms + lone pairs SN = attached atoms + lone pairs What is SN of C in CO ? What is the SN of B in BF ? SN 2: Two attached atoms and zero lone pairs What is the SN of O in H O? SN 4: Two attached atoms and two lone pairs What is the SN of N in NO ? SN 3: Two attached atoms and one lone pair SN 3: Three attached atoms and zero lone pairs What is SN of S in SF ? SN 5: Four attached atoms and one lone pair What is the SN of S in SF ? SN 6: Six attached atoms and zero lone pairs Predicting molecular geometries: SN = 2 Predicting molecular geometries: SN = 3 What is SN of C in CO ? What is the SN of N in NO ? SN 2: Two attached atoms and zero lone pairs SN 3: Two attached atoms and one lone pair Shape is linear, bond angle is 180° Shape is bent, bond angle a bit less than 120° 4 Predicting molecular geometries: SN = 4 Predicting molecular geometries: SN = 5 What is the SN of O in H O? What is SN of S in SF ? SN 4: Two attached atoms and two lone pairs Shape is bent, bond angle a little less than 109.5° SN 5: Four attached atoms and one lone pair Shape is seesaw, bond angles are a little less than 180°, 120°, and 90° Predicting molecular geometries: SN = 6 What is the SN of S in SF ? SN 6: Six attached atoms and zero lone pairs Shape is octahedral, bond angles are 90° 5