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UCM CHEM 1131 - Exam 3 Study Guide
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Chem 1131 1st EditionExam # 1 Study Guide Chapters 6,7,8Chapter 6: Representing MoleculesOctet RuleAccording to the octet rule, atoms will lose, gain, or share electrons in order to achieve a noble gas electron configuration.Only valence electrons contribute to bonding.F 1s22s22p5Lewis structureA representation of covalent bonding in which shared electron pairs are shown either as dashes or as pairs of dots between two atoms, and lone pairs are shown as pairs of dots on individual atoms. Only valence electrons are shown.The H and O atoms each achieve a stable noble gas configuration by sharing electrons, thus illustrating the octet rule. The Polarity and Electronegativity. Ionic and covalent bonds are simply the extremes in a spectrum of bonding, In a triple bond, atoms are held together by three electron pair.In a double bond, atoms are held together by two electron pair.* multiple bonds are shorter and stronger than single bonds.Electronegativity: the ability of an atom to draw shared electrons (the electron in a covalent bond) toward itself. ↑electronegativity ↑ tendency to attract electron densityAtoms of elements with widely different electronegativity = ionic compoundComparable electronegativity = polar covalent bonds (polar bond)Same electronegativity = pure covalent bondpurely covalent/nonpolar < 0.5 < polar covalent < 2.0 < ionic bond- the numbers are the difference between electronegativity of atoms.e.g. the difference between the electronegativities of F and Cl is 4.0-3.0 = 1.0, which means it’s polar. Percent Ionic Character → a purely ionic bond would have 100% (not known yet), and a purely covalent nonpolar bond has 0 percent ionic character. Dipole Moment and Partial ChargeDipole moment (μ ¿: a quantitative measure of the polarity of a bond. The shift of electron density in a polar bond is symbolized by placing a crossed arrow (a dipole arrow) above the Lewis structureDeltas (δ) denote a partial (positive or negative) charge.Formal ChargeFormal charge can be used to determine the most plausible Lewis Structure when more than one possibility exists for a compound.To determine the number of electrons associated with an atom in a Lewis structure, keep in mind : All the atom’s nonbonding electrons are associated with the atomHalf of the atom’s bonding electrons are associated with the atom.Formal charge = valence electrons – associated electronse.g.*a compound with the formal charge on most electronegative atom is plausible. * A Lewis structure in which all atoms have zero formal charge is the most preferred. -2 +1 0 -1 +1 -1 -3 +1 +1the middle one is most preferred since it has small formal charges and formal charge on most electronegative atom.Resonance StructuresResonance structure: one of two or more equally valid Lewis structures for a single species that cannot be represented accurately by a single Lewis structure.The double bond can be at either end of the molecule since neither one accounts for the known bond lengths in O3 Resonance structures differ only in the positions of their electrons – not in the position of their atoms. O XExcepts to the Octet RuleThe carbonate ion is another example of resonance.1. The central atom has fewer than eight electrons due to a shortage of electronse.g. Boron (five electrons) 3. The central atom as fewer than eight electrons due to an odd number of electronse.g. nitrogen (seven electrons)2. The central atom has more than eight electrons. (expanded octet)Atoms of second period elements cannot have more than eight valence electrons around them, but atoms in and beyond the third period, which have 3d orbitals that can be used in bonding, can form expanded octet.e.g. In some cases a species can be represented by a structure in which octet rule is obeyed, or by one in which the central atom has an expanded octet.e.g. Chapter 7: Molecular Geometry and Bonding TheoriesVSEPR – Molecular Geometry and PolarityMolecular shape can be predicted by using the valence-shell electron-pair repulsion (VSEPR) model. Molecular Geometry is the arrangement of bonded atoms. The basis of the VSEPR model is that electrons repel each other.Electrons are found in various domains around the central atom.ABX = A is a central atom surrounded by x B atoms and x can have integer value of 2 to 6.AB2AB3For CO2, we have two identical vectors pointing in opposite directions. Those too x components are equal in magnitude but opposite in sign. The sum of two vectors is zero so CO2 is nonpolar.For H2O, since the vector is not opposite in y direction, it is polar. Electron Domain GeometryElectron domain geometry is the arrangement of electron domains around the central atom. AB5 molecules contain two different bond angles between adjacent bonds. The tree bonds that are arranged in a trigonal plane are equatorial with 120 bond angle, and the two bonds that form an axis perpendicular to the trigonal plane are referred to as axial. Step 1: Draw the Lewis structure of the molecule or polyatomic ion.Step 2: Count the number of electron domains on the central atom.Step 3: Determine the electron-domain geometry by applying the VSEPR model.Step 4: Determine the molecular geometry by considering the positions of the substituent atoms that surround the central atom.Deviation from Ideal bond anglesSome electron domains are better than others at repelling neighboring domains. Lone pair-Lone pair> Lone pair–Bonding pair >Bonding pair-Bonding pairLone pairs (or lone electrons as in radicals) take up more space than bonded pairs of electrons. Thus, non-bonding electrons are more diffuse. Multiple bonds repel more strongly than single bonds.Valence Bond TheoryAccording to valence bond theory, atoms share electrons when atomic orbitals overlap.The nuclei of atoms are attracted to the shared pair of electrons, and it is this mutual attraction that holds the atoms together. (1) A bond forms when single occupied atomic orbitals on two atoms overlap.(2) The two electrons shared in the region of orbital overlap must be of opposite spin.(3) Formation of a bond results in a lower potential energy for the system.Problem: Need to know the actual bond angles/shape before proceeding to use the VBT(valence bond theory) to explain bondingThe H−H bond in H2 forms when the singly occupied1s orbitals of the two H atoms overlap:The F−F bond in F2 forms when the singly occupied2p orbitals


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UCM CHEM 1131 - Exam 3 Study Guide

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