Chapter 2 CHEMICAL BONDS IONIC BONDS 2 1
Chapter 2. CHEMICAL BONDS IONIC BONDS 2. 1 2. 2 2. 3 2. 4 The Ions That Elements Form Lewis Symbols The Energetics of Ionic Bond Formation Interactions Between Ions COVALENT BONDS 2. 5 2. 6 2. 7 2. 8 Lewis Structures of Polyatomic Species Resonance Formal Charge 2013 General Chemistry I
Ø Chemical bond is the link between atoms. - ionic bond i. e. Na+, Cl- - covalent bond i. e. NH 3 - metallic bond i. e. Cu IONIC BONDS (Sections 2. 1 -2. 4) Ø ionic model: the description of bonding in terms of ions ionic solid: an assembly of cations and anions stacked together in a regular pattern 2013 General Chemistry I 55
2. 1 The Ions That Elements Form Ø Cations: Remove outermost electrons in the order np, ns, (n-1)d Ø Anions: Add electrons until the next noble-gas configuration is reached 2013 General Chemistry I 56
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58 2. 2 Lewis Symbols - valence electrons – depicted as dots; a pair of dots for paired electrons - cations and anions 2. 3 The Energetics of Ionic Bond Formation Na(g) → Na+(g) + e- (g) 494 k. J·mol-1 Cl(g) + e- (g) → Cl-(g) -349 k. J·mol-1 Na+(g) + Cl- (g) → Na. Cl(s) -787 k. J·mol-1 Na(g) + Cl(g) → Na. Cl(s) -642 k. J·mol-1 2013 General Chemistry I
59 2. 4 Interactions Between Ions - In an ionic solid, each cation is attracted to all the anions to a greater or lesser extent. → a “global” characteristic of the entire crystal Ø Lattice energy: the difference in energy between the ions packed together in a solid and the ions widely separated as a gas - strong electrostatic interactions in ionic solids → high melting points and brittleness 2013 General Chemistry I
60 - Coulomb potential energy of the interactions of two individual ions e is the fundamental charge; z 1 and z 2 are the charge numbers of the two ions; r 12 is the distance between the centers of the ions; e 0 is the vacuum permittivity. - molar potential energy of a three-dimensional crystal Ø The factor A is the Madelung constant, dependent on how the ions are arranged about one another 2013 General Chemistry I
61 - In a one-dimensional crystal in which cations and anions alternate along a line, ; A = 2 ln 2 or 1. 386 - real potential energy of an ionic solid → attractive and repulsive interionic interactions Ø Born-Mayer equation with d* = 34. 5 pm 2013 General Chemistry I repulsive effect
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61 s 2013 General Chemistry I
63 COVALENT BONDS (Sections 2. 5 -2. 8) 2. 5 Lewis Structures Ø covalent bond - a pair of electrons shared between two atoms - octet rule: In covalent bond formation, atoms go as far as possible toward completing their octets by sharing electron pairs. - Valence of an atom is the number of bonds it can form. - A line (-) represents a shared pair of electrons. - lone pairs of electrons – electron pairs not involved in bonding - Lewis structure – atoms by chemical symbols, covalent bonds by lines, and lone pairs by pairs of dots 2013 General Chemistry I
64 2. 6 Lewis Structures of Polyatomic Species - Lewis structure does not portray the 3 D shape of a molecule or ion, but simply displays which atoms are bonded together. - - bond order: the number of bonds that link a specific pair of atoms. 2013 General Chemistry I
65 Ø Writing a Lewis structure - terminal atom: bonded to only one other atom central atom: bonded to at least two other atoms 2013 General Chemistry I
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2. 7 Resonance - - double-headed arrows (↔), indicating a blend of the contributing structures - delocalization: a shared electron pair is distributed over several pairs of atoms and cannot be identified with just one pair of atoms. 2013 General Chemistry I 67
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69 Ø Benzene, C 6 H 6 - No reactions typical of compounds with double bonds - All the carbon-carbon bonds with the same length - Only one 1, 2 -dichlorobenzen exists. 2013 General Chemistry I© 2010, 2008, 2005, 2002 by P. W. Atkins and L. L. Jones
70 2. 8 Formal Charge Ø Formal charge – the charge it would have if the bonding were perfectly covalent in the sense that the atom had exactly a halfshare in the bonding electrons V = the number of valence electrons in the free atom L = the number of electrons present on the bonded atom as lone pairs B = the number of bonding electrons on the atom - A Lewis structure in which the formal charges of the individual atoms are closest to zero typically represents the lowest energy arrangement of the atoms and electrons. 2013 General Chemistry I
71 - Formal charge exaggerates the covalent character of bonds by assuming that the electrons are shared equally. - Oxidation number exaggerates the ionic character of bonds. It represents the atoms as ions, and all the electrons in a bond are assigned to the atom with the lower ionization energy. formal charge 2013 General Chemistry I oxidation state
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Chapter 2. CHEMICAL BONDS EXCEPTIONS TO THE OCTET RULE 2. 9 Radicals and Biradicals 2. 10 Expanded Valence Shells 2. 11 The Unusual Structures of Some Group 13/III Compounds IONIC VERSUS COVALENT BONDS 2. 12 Correcting the Covalent Model: Electronegativity 2. 13 Correcting the Ionic Model: Polarizability THE STRENGTH AND LENGTHS OF COVALENT BONDS 2. 14 Bond Strength 2. 15 Variation in Bond Strength 2. 16 Bond Lengths 2013 General Chemistry I
72 2. 9 Radicals and Biradicals Ø Radicals: species with an unpaired electron, highly reactive Biradicals: molecules with two unpaired electrons 2. 10 Expanded Valence Shells - more than 8 electrons associated with an atom in a Lewis structure - hypervalent compound: a compound that contains an atom with more atoms attached to it than is permitted by the octet rule - variable covalence: the ability to form different numbers of covalent bonds 2013 General Chemistry I
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76 2. 11 The Unusual Structures of Some Group 13/III Compounds - incomplete octet: fewer than eight valence electrons - boron and aluminum - completing octets by a coordinate covalent bond, in which both electrons come from one of the atoms 2013 General Chemistry I
IONIC VERSUS COVALENT BONDS (Sections 2. 12 -2. 13) 2. 12 Correcting the Covalent Model: Electronegativity - partial charges: the charges on the atoms - polar covalent bond: a bond in which ionic contributions to the resonance result in partial charges lower energy structure - electric dipole: a partial positive charge next to an equal but opposite partial negative charge - electric dipole moment (m): size of an electric dipole Debye Cl-H bond: m = ~1. 1 D 2013 General Chemistry I 76
77 Ø Electronegativity (c) – Electron-pulling power of an atom when it is part of a molecule (by Linus Pauling) - Mulliken scale: c = ½(I + Eea) average of the ionization energy and electron affinity - Increases from left to right and from bottom to top - rough rules of thumb ionic polar covalent i. e. Na. Cl or KF : ionic C-O : polar covalent Ca-O : ionic 2013 General Chemistry I
78 2. 13 Correcting the Ionic Model: Polarizability - All ionic bonds have some covalent character. - highly polarizable atoms and ions: readily undergo a large distortion of their electron cloud i. e. large anions and atoms such as I-, Br-, and Cl- polarizing power: property of ions (and atoms) that cause large distortions of electron clouds - increases with decreasing size and increasing charge of a cation i. e. the small and/or highly charged cations Li+, Be 2+, Mg 2+, and Al 3+ 2013 General Chemistry I
THE STRENGTHS AND LENGTHS OF COVALENT BONDS (Sections 2. 14 -2. 16) 2. 14 Bond Strength Ø Dissociation energy (D): energy required to separate the bonded atoms - The bond breaking is homolytic, which means that each atom retains one of the electrons from the bond. - average dissociation energy for one type of bond found in different molecules i. e. C-H single bond: average strength of bonds in a selection of organic molecules, such as methane (CH 4), ethane (C 2 H 6), and ethene (C 2 H 4) 2013 General Chemistry I 79
80 2. 15 Variation in Bond Strength - Factors influencing bond strength 2013 General Chemistry I
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82 2. 16 Bond Length Ø Bond length: the distance between the centers of two atoms joined by a covalent bond - corresponding to the internuclear distance at the potential energy minimum for the two atoms - affecting the overall size and shape of a molecule evaluated by using spectroscopy or x-ray diffraction methods - Factors influencing bond length 2013 General Chemistry I
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82 Ø Covalent radius: contribution an atom to the length of a covalent bond - Approximately the sum of the covalent radii of the two atoms - Decreases from left to right (increasing Zeff) - Increases in going down a group (size of valence shells and better shielding by inner core electrons) 2013 General Chemistry I
INFRARED SPECTROSCOPY Ø Infrared radiation: electromagnetic radiation with longer wavelengths (lower frequencies) than red light ~ 1000 nm or ~ 3× 1014 Hz - Molecules by infrared radiation become vibrationally excited. - “stretching” mode: the atoms moving closer and away again. “bending” mode: bond angles periodically increase and decrease. Ø Vibrational frequencies - The stiffness of a bond measured by its force constant, k Force = -k × displacement by Hooke’s law - Vibrational frequency, n, of a bond between two atoms A and B of mass m. A and m. B 2013 General Chemistry I m = effective mass (or reduced mass) 90
91 Ø Normal modes of vibration A nonlinear molecule consisting of N atoms → 3 N-6 normal modes A linear molecule → 3 N-5 normal modes i. e. H 2 O, n = 3 → 3 normal modes CO 2, n = 3 → 4 normal modes Ø Characteristic frequencies of functional groups detectable in a spectrum - fingerprint region: a complex series of absorptions 2013 General Chemistry I
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