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UMass Amherst CHEM 110 - Chemistry Chapter 8

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Chemistry Chapter 8Bonding and Molecular StructureKey Idea: Arrangement of nuclei and electrons of the bonded atoms results in a lower energythan for the separate atoms● Coulomb's Law:○ Coulomb's law says that the attractive force between ions of opposite charge isdirectly proportional to the product of the ion charges and inversely proportionalto the square of the distance between them○ F = (qA x qB) / r^2■ qA and qB = ion charges■ R = distance between ions.■ Negative number = attractive force, so disregard negative sign andbiggest number = bigger magnitude.● Lewis Structures:○ Key to bonding: electron sharing to achieve a full valence shell○ Octet rule: electrons are distributed in such a way that each element issurrounded by 8 electrons, an octet■ Exceptions:● For H_2_ molecule stability is reached with 2 electrons.● For expanded octets/valence: elements have more than 8electrons (often 10 or 12) in a Lewis structure.○ 3rd row and below can do this.● Incomplete Octet: elements have less than 8 electrons in a Lewisstructure.○ Molecules containing H, B and Be.■ Non-bonding valence electrons (usually in pairs) are called lone pairs○ Each dot represents a valence electron○ The element symbol represents the nucleus and core electrons.○ Hydrogen can only make one bond.○ For Oxygen containing acids, the Hydrogen of the acid is attached to the Oxygen.○ For organic molecules:■ Carbon forms 4 bonds (a double bond is 2 bonds) and is generally in the interior of themolecule.Hydrogen forms only 1 bond and is therefore a terminal atom (on the outside of themolecule).Oxygen usually forms 2 bonds.Nitrogen usually forms 3 bonds (may form 4 in a cation like NH4+).● Electronegativity (χ)○ The ability of an atom in a molecule to attract (pull) electrons to itself. The lesselectronegativity, the more bonds it forms and will be the central atom in acomplex molecule.● Formal Charge (FC)○ FC is a measure of the extent to which an atom has gained or lost an electron inthe process of forming a covalent bond○ We can assign FC to each atom in a molecule or ion○ The sum of the formal charges for all the atoms in a molecule or ion is equal tothe charge on the molecule or ion○ Not the same as oxidation number.○ FC = number of VE- [number of LPE + 1/2(number of BE)]○ FC = # VE - (dots + lines)■ VE = Valence electrons■ LPE = Lone pair electrons■ BE = Bonding electrons○ Best formal charge = Formal charge for as many of the atoms in a structureas possible are closest to zero. Charges should be closest to zero. Chargesshould be closest together.○ Best formal charge of a molecule, when adding the individual charges of theatoms together, the total charge should be the charge of the actual molecule.■ Ex: CO^2-. Total formal charge should be 2-● Using FC to Determine Lewis Structures○ Lewis structures where the formal charges on each atom are closest to 0 and thenumber of separate charges is minimized are the lowest energy.■ Lowest formal charge = lowest energy● Resonance○ Two or more valid Lewis structures for a species that differ only in arrangement ofelectrons are called resonance structures■ Examples: Ozone (O3)○ Both structures represent the same molecule/ion○ Have the same atom arrangement○ Same geometry○ Differ only in the arrangement of electrons○ Resonance hybrid: A blend between the resonance structures○ Experimental evidence: the two bonds are equal● Valence-Shell Electron-Pair Repulsion(VESPR)○ Based on Lewis structure principles that:■ Valence electrons pairs repel each other■ The geometry around the central atom will be such as to minimize thisrepulsion■ When drawing 3d shapes, any two consecutive bonds can be a wedgeand dash, can’t skip bonds.○ VSEPR nomenclature■ A = Central Atom■ X = # of atoms directly bonded to the central atom■ E = # of lone pairs on the central atom● VSEPR: AX3E○ Molecules with lone pairs:■ Lone-pair/bonding pair repulsion is greater than bonding-pair/bondingpair repulsion.● VSEPR: AX2E2○ Molecules with lone pairs:■ Lone-pair/lone pair repulsion is greater than Lone-pair/bonding pairrepulsion is greater than bonding-pair/bonding pair repulsion● VSEPR: AX4, AX3E and AX2E2○ Molecules with lone pairs:■ Lone-pair/lone pair repulsion is greater than Lone-pair/bonding pairrepulsion is greater than bonding-pair/bonding pair repulsion● Polar Covalent Bonds○ unequal sharing of bonding electrons are between the two atoms with differentelectronegativities (χ)○ If there is an odd number of lone pairs of electrons around the central atom, themolecule is polar.○ To determine the polarity of a covalent bond using numerical means, find thedifference between the electronegativity of the atoms; if the result is between 0.4and 1.7, then, generally, the bond is polar covalent.■ If the difference of the electronegativity between the two elements isgreater than 1.7 then the bond is ionic○ If the molecule is not symmetrical, then it is polar● Non-polar covalent bonds:○ Bonding electrons are shared equally between the two atoms.○ If there is an even number of lone pairs.○ Electronegativity values are identical of molecules○ If the difference between the electronegativity of the atoms is 0 and 0.4 then thebond is nonpolar covalent.○ If the molecule is symmetrical, the it is nonpolar● Dipole Moment (µ)○ Asymmetric electron distribution results in a dipole moment (µ)● Bond Order: Number of bonding electron pairs (number of bonds) between two bondedatoms○ bond order of each bind in resonance structure = number of bonds involved inresonance / number of resonance structures● Bond Length○ Bond length increases with increasing atomic size or atomic radius.○ For a series of bonds that differ only in bond order, bond length decreases withincreasing bond order○ More bonds = shorter bond length.● Bond energy: is the energy required to break a chemical bond in a gas-phase moleculeto individual atoms○ Always an endothermic (requires energy) process● Greater the bond order, the higher the bond strength/energy and shorter the bond● Enthalpy of Reaction (ΔrHo)○ ΔrHo = Σ (energies of bonds broken) - Σ (energies of bonds formed)■ Δr = change in reaction■ Ho = heat/enthalpy○ ΔrHo = Σ ΔHo(bonds broken) - Σ ΔHo (bonds formed)○ Bond broken = reactants○ Bonds formed = products○ If answer is positive, then


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