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Chapter 8 BONDING: General Concepts
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Chemical Bonds Three basic types of bonds – Ionic Electrostatic attraction between ions – Covalent Sharing of electrons – Metallic Metal atoms bonded to several other atoms
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Ionic Bonding
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REVIEW Arrange the ions Se 2-, Br -, Rb +, and Sr 2+ in order of decreasing size.
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Which is the largest ion in each of the following groups? Li +, Na +, K +, Rb +, Cs + Ba 2+, Cs +, I -, Te 2-
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Energetics of Ionic Bonding It takes 495 kJ/mol to remove electrons from sodium.
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Energetics of Ionic Bonding We get 349 kJ/mol back by giving electrons to chlorine.
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Energetics of Ionic Bonding But these numbers don’t explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic!
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Energetics of Ionic Bonding There must be a third piece to the puzzle. What is as yet unaccounted for is the electrostatic attraction between the newly- formed sodium cation and chloride anion.
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Lattice Energy This third piece of the puzzle is the lattice energy: – The energy required to completely separate a mole of a solid ionic compound into its gaseous ions. The energy associated with electrostatic interactions is governed by Coulomb’s law: E el = Q1Q2dQ1Q2d
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Lattice Energy Lattice energy, then, increases with the charge on the ions. It also increases with decreasing size of ions.
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Energetics of Ionic Bonding By accounting for all three energies (ionization energy, electron affinity, and lattice energy), we can get a good idea of the energetics involved in such a process.
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Energetics of Ionic Bonding These phenomena also helps explain the “octet rule.” Metals, for instance, tend to stop losing electrons once they attain a noble gas configuration because energy would be expended that cannot be overcome by lattice energies.
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Use the following data to estimate the standard heat of formation for potassium chloride: K(s) + ½ Cl 2 (g) KCl(s) Lattice energy-690. kJ/mol Ionization energy for K419 kJ/mol Electron affinity of Cl-349 kJ/mol Bond energy of Cl 2 239 kJ/mol Enthalpy of sublimation for K64 kJ/mol Answer: -437 kJ/mol
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Covalent Bonding In covalent bonds atoms share electrons. There are several electrostatic interactions in these bonds: – Attractions between electrons and nuclei – Repulsions between electrons – Repulsions between nuclei
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Polar Covalent Bonds Though atoms often form compounds by sharing electrons, the electrons are not always shared equally. Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does. Therefore, the fluorine end of the molecule has more electron density than the hydrogen end.
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Electronegativity Electronegativity is the ability of atoms in a molecule to attract electrons to themselves. On the periodic chart, electronegativity increases as you go… – …from left to right across a row. – …from the bottom to the top of a column.
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Polar Covalent Bonds When two atoms share electrons unequally, a bond dipole results. The dipole moment, , produced by two equal but opposite charges separated by a distance, r, is calculated: = Q r It is measured in debyes (D).
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Polar Covalent Bonds The greater the difference in electronegativity, the more polar is the bond.
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Covalent Bond Strength Most simply, the strength of a bond is measured by determining how much energy is required to break the bond. This is the bond enthalpy. The bond enthalpy for a Cl-Cl bond, D(Cl-Cl), is measured to be 242 kJ/mol.
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Average Bond Enthalpies This table lists the average bond enthalpies for many different types of bonds. Average bond enthalpies are positive, because bond breaking is an endothermic process.
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Average Bond Enthalpies NOTE: These are average bond enthalpies, not absolute bond enthalpies; the C-H bonds in methane, CH 4, will be a bit different than the C-H bond in chloroform, CHCl 3.
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Enthalpies of Reaction Yet another way to estimate H for a reaction is to compare the bond enthalpies of bonds broken (energy required) to the bond enthalpies of the new bonds formed (energy released). In other words, H rxn = (bond enthalpies of bonds broken) - (bond enthalpies of bonds formed)
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Enthalpies of Reaction CH 4 (g) + Cl 2 (g) CH 3 Cl (g) + HCl (g) In this example, one C-H bond and one Cl-Cl bond are broken; one C-Cl and one H-Cl bond are formed.
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Enthalpies of Reaction So, H = [D(C-H) + D(Cl-Cl)] - [D(C-Cl) + D(H-Cl)] = [(413 kJ) + (242 kJ)] - [(328 kJ) + (431 kJ)] = (655 kJ) - (759 kJ) = -104 kJ
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Use the bond energies to calculate the enthalpy of the reaction of methane with chlorine and fluorine to form Freon-12. (answer: -1194 kJ) CH 4 (g) + 2Cl 2 (g) + 2F 2 (g) CF 2 Cl 2 (g) + 2HF(g) + 2HCl(g) C-H 413 kJ/mol Cl-Cl239 kJ/mol F-F154 kJ/mol C-F485 kJ/mol C-Cl339 kJ/mol H-F565 kJ/mol H-Cl427 kJ/mol
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Bond Enthalpy and Bond Length We can also measure an average bond length for different bond types. As the number of bonds between two atoms increases, the bond length decreases.
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Lewis Structures Lewis structures are representations of molecules showing all electrons, bonding and nonbonding.
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Writing Lewis Structures 1.Find the sum of valence electrons of all atoms in the polyatomic ion or molecule. – If it is an anion, add one electron for each negative charge. – If it is a cation, subtract one electron for each positive charge. PCl 3 5 + 3(7) = 26
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Writing Lewis Structures 2.The central atom is the least electronegative element that isn’t hydrogen. Connect the outer atoms to it by single bonds. Keep track of the electrons: 26 - 6 = 20
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Writing Lewis Structures 3.Fill the octets of the outer atoms. Keep track of the electrons: 26 - 6 = 20; 20 - 18 = 2
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Writing Lewis Structures 4.Fill the octet of the central atom. Keep track of the electrons: 26 - 6 = 20; 20 - 18 = 2; 2 - 2 = 0
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Draw the Lewis structure for water.
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Draw the Lewis structure for HCN. (you may encounter a problem…)
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Writing Lewis Structures 5.If you run out of electrons before the central atom has an octet… …form multiple bonds until it does.
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Draw the Lewis structure for CN -
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Draw the Lewis structure for NO +
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Draw the Lewis structure for ammonia
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Draw the Lewis structure for N 2
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Draw the Lewis structure for CO 2
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What if you can draw more than one Lewis structure? Then assign formal charges. – For each atom, count the electrons in lone pairs and half the electrons it shares with other atoms. – Subtract that from the number of valence electrons for that atom: the difference is its formal charge.
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Writing Lewis Structures The best Lewis structure… – …is the one with the fewest charges. – …puts a negative charge on the most electronegative atom.
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Which of the following is the best Lewis structure?
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Resonance This is the Lewis structure we would draw for ozone, O 3. - +
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Resonance But this is at odds with the true, observed structure of ozone, in which… – …both O-O bonds are the same length. – …both outer oxygens have a charge of -1/2.
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Resonance One Lewis structure cannot accurately depict a molecule like ozone. We use multiple structures, resonance structures, to describe the molecule.
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Resonance Just as green is a synthesis of blue and yellow… …ozone is a synthesis of these two resonance structures.
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Resonance In truth, the electrons that form the second C-O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygens and the carbon. They are not localized; they are delocalized.
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Resonance The organic compound benzene, C 6 H 6, has two resonance structures. It is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring.
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Draw the resonance structures for the nitrite ion.
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Exceptions to the Octet Rule There are three types of ions or molecules that do not follow the octet rule: – Ions or molecules with an odd number of electrons – Ions or molecules with less than an octet – Ions or molecules with more than eight valence electrons (an expanded octet)
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Odd Number of Electrons Though relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of electrons.
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Fewer Than Eight Electrons Consider BF 3 : – Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine. – This would not be an accurate picture of the distribution of electrons in BF 3.
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Fewer Than Eight Electrons Therefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons.
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Fewer Than Eight Electrons The lesson is: if filling the octet of the central atom results in a negative charge on the central atom and a positive charge on the more electronegative outer atom, don’t fill the octet of the central atom.
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More Than Eight Electrons The only way PCl 5 can exist is if phosphorus has 10 electrons around it. It is allowed to expand the octet of atoms on the 3rd row or below. – Presumably d orbitals in these atoms participate in bonding.
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More Than Eight Electrons Even though we can draw a Lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygens.
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More Than Eight Electrons This eliminates the charge on the phosphorus and the charge on one of the oxygens. The lesson is: when the central atom in on the 3rd row or below and expanding its octet eliminates some formal charges, do so.
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Draw the Lewis structure for sulfur hexafluoride
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Draw the Lewis structure for ClF 3
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Draw the Lewis structure for XeO 3
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Draw the Lewis structure for RnCl 2
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Draw the Lewis structure for ICl 4 -
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Draw the Lewis structure for the sulfate ion.
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Molecular Shapes The shape of a molecule plays an important role in its reactivity. By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.
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What Determines the Shape of a Molecule? Simply put, electron pairs, whether they be bonding or nonbonding, repel each other. By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule.
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Electron Domains We can refer to the electron pairs as electron domains. In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain. The central atom in this molecule, A, has four electron domains.
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Valence Shell Electron Pair Repulsion Theory (VSEPR) “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”
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Electron-Domain Geometries These are the electron-domain geometries for two through six electron domains around a central atom.
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Electron-Domain Geometries All one must do is count the number of electron domains in the Lewis structure. The geometry will be that which corresponds to the number of electron domains.
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Molecular Geometries The electron-domain geometry is often not the shape of the molecule, however. The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs.
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Molecular Geometries Within each electron domain, then, there might be more than one molecular geometry.
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Linear Electron Domain In the linear domain, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.
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Trigonal Planar Electron Domain There are two molecular geometries: – Trigonal planar, if all the electron domains are bonding, – Bent, if one of the domains is a nonbonding pair.
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Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.
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Multiple Bonds and Bond Angles Double and triple bonds place greater electron density on one side of the central atom than do single bonds. Therefore, they also affect bond angles.
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Tetrahedral Electron Domain There are three molecular geometries: – Tetrahedral, if all are bonding pairs, – Trigonal pyramidal if one is a nonbonding pair, – Bent if there are two nonbonding pairs.
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Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry: – Axial – Equatorial
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Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.
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Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain: – Trigonal bipyramidal – Seesaw – T-shaped – Linear
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Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries: – Octahedral – Square pyramidal – Square planar
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Predict the geometric structures of PCl 5, PCl 4 +, and PCl 6 -
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Predict the geometric structure of XeF 4
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Predict the geometric structure of I 3 -
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Larger Molecules In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.
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Larger Molecules This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.
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Polarity Just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.
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Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.
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Polarity
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Chapter 9 COVALENT BONDING: Orbitals
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Overlap and Bonding We think of covalent bonds forming through the sharing of electrons by adjacent atoms. In such an approach this can only occur when orbitals on the two atoms overlap.
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Overlap and Bonding Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion. However, if atoms get too close, the internuclear repulsion greatly raises the energy.
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Hybrid Orbitals But it’s hard to imagine tetrahedral, trigonal bipyramidal, and other geometries arising from the atomic orbitals we recognize.
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Hybrid Orbitals Consider beryllium: – In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals.
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Hybrid Orbitals But if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds.
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Hybrid Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. – These sp hybrid orbitals have two lobes like a p orbital. – One of the lobes is larger and more rounded as is the s orbital.
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Hybrid Orbitals These two degenerate orbitals would align themselves 180 from each other. This is consistent with the observed geometry of beryllium compounds: linear.
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Hybrid Orbitals With hybrid orbitals the orbital diagram for beryllium would look like this. The sp orbitals are higher in energy than the 1s orbital but lower than the 2p.
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Hybrid Orbitals Using a similar model for boron leads to…
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Hybrid Orbitals …three degenerate sp 2 orbitals.
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Hybrid Orbitals With carbon we get…
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Hybrid Orbitals …four degenerate sp 3 orbitals.
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Hybrid Orbitals For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.
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Hybrid Orbitals This leads to five degenerate sp 3 d orbitals… …or six degenerate sp 3 d 2 orbitals.
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Hybrid Orbitals Once you know the electron-domain geometry, you know the hybridization state of the atom.
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Describe the molecular structure of BF 4 - and predict the hybridization of B
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Describe the molecular structure of XeF 2 and predict the hybridization of Xe
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Valence Bond Theory Hybridization is a major player in this approach to bonding. There are two ways orbitals can overlap to form bonds between atoms.
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Sigma ( ) Bonds Sigma bonds are characterized by – Head-to-head overlap. – Cylindrical symmetry of electron density about the internuclear axis.
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Pi ( ) Bonds Pi bonds are characterized by – Side-to-side overlap. – Electron density above and below the internuclear axis.
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Single Bonds Single bonds are always bonds, because overlap is greater, resulting in a stronger bond and more energy lowering.
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Multiple Bonds In a multiple bond one of the bonds is a bond and the rest are bonds.
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Multiple Bonds In a molecule like formaldehyde (shown at left) an sp 2 orbital on carbon overlaps in fashion with the corresponding orbital on the oxygen. The unhybridized p orbitals overlap in fashion.
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Multiple Bonds In triple bonds, as in acetylene, two sp orbitals form a bond between the carbons, and two pairs of p orbitals overlap in fashion to form the two bonds.
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Delocalized Electrons: Resonance When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion.
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Delocalized Electrons: Resonance In reality, each of the four atoms in the nitrate ion has a p orbital. The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen.
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Delocalized Electrons: Resonance This means the electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion.
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Resonance The organic molecule benzene has six bonds and a p orbital on each carbon atom.
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Resonance In reality the electrons in benzene are not localized, but delocalized. The even distribution of the electrons in benzene makes the molecule unusually stable.
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