Page III-7-1 / Chapter Seven Lecture Notes MAR. Ionic - complete transfer of. Covalent - electrons shared MAR. CH 221 Flashback:

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1 Page III-7-1 / hapter even Lecture otes hemical onding onding and Molecular tructure hapter 7 ocaine hemistry 222 Professor Michael Russell Get the 222 ompanion and omposition Lab otebook efore irst Lab! Two Extreme orms of hemical onds hemical onding Ionic - complete transfer of electrons from one atom to another, metals + nonmetals Problems and questions ow is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? an we predict the structure? ow is structure related to chemical and physical properties? ovalent - electrons shared between atoms, mostly nonmetals Most bonds are somewhere in between Also Metallic - for metals, studied later onding verview Ionic orces - oulomb's Law 221 lashback: Mostly just for nonmetals metals plus nonmetals metals and alloys ionic force, higher melting point, etc. igher Page III-7-1 / hapter even Lecture otes

2 ovalent onding ovalent bonds arise from the mutual attraction of 2 nuclei for the same electrons. A covalent bond is a balance of attractive and repulsive forces. Page III-7-2 / hapter even Lecture otes ovalent onding ovalent bonding will be the focus of the first two chapters We will re-visit Ionic bonding and Metallic bonding in a future chapter Important to know when a compound is ionic, covalent or metallic! Electron Distribution in Molecules G.. Lewis Electron distribution is depicted with Lewis electron dot structures Valence electrons are distributed as shared or D PAIR and unshared or LE PAIR. ond and Lone Pairs Valence electrons are distributed as shared or D PAIR and unshared or LE PAIR. l shared or bond pair lone pair (LP) This is called a LEWI ELETR DT structure 3 lone pairs + 1 bond pair = 4 pairs total ond ormation A bond can result from a "head-tohead" overlap of atomic orbitals on neighboring atoms, making a sigma (σ) bond. + l l Valence Electrons Electrons are divided between core and valence electrons 1s 2 2s 2 2p 1 ore = [e], valence = 2s 2 2p 1 2 core e -, 3 valence e - verlap of (1s) and l (3p) ote that each atom has a single, unpaired electron in their atomic orbital. Page III-7-2 / hapter even Lecture otes r [Ar] 3d 10 4s 2 4p 5 ore = [Ar] 3d 10, valence = 4s 2 4p 5 28 core e -, 7 valence e -

3 Page III-7-3 / hapter even Lecture otes 1A Valence Electrons 8A 2A 3A 4A 5A 6A 7A uilding Lewis tructures o. of valence electrons of a main group atom = Group number or Groups 1A - 4A, no. of bond pairs = group number. or Groups 5A - 7A, Ps = 8 - Grp. o. umber of valence electrons is equal to the Group number uilding Lewis tructures o. of valence electrons of an atom = Group number or Groups 1A - 4A (14), no. of bond pairs = group number or Groups 5A (15) - 7A (17), Ps = 8 - Grp. o. Except for (and sometimes atoms of 3rd and higher periods), Ps + LPs = 4 This observation is called the TET RULE uilding a Lewis Dot tructure Ammonia, 3 1. ount valence electrons = 1 and = 5 Total = (3 x 1) + 5 = 8 electrons or 4 pairs of electrons 2. Decide on the central atom; never. entral atom is atom of lowest affinity for electrons. Therefore, is central 3. orm a sigma bond (single bond) between the central atom and surrounding atoms. 4. Remaining electrons form LE PAIR to complete octet as needed. uilding a Lewis Dot tructure 3 D PAIR and 1 LE PAIR. ote that has a share in 4 pairs (8 electrons), while shares 1 pair. ee uilding Lewis tructures handout ulfite ion, 3 2- tep 1. entral atom = tep 2. ount valence electrons = 6 3 x = 3 x 6 = 18 egative charge = 2 TTAL = 26 e- or 13 pairs tep 3. orm sigma bonds 10 pairs of electrons are now left. Page III-7-3 / hapter even Lecture otes

4 Page III-7-4 / hapter even Lecture otes ulfite ion, 3 2- Remaining pairs become lone pairs, first on outside atoms and then on central atom. arbon Dioxide, 2 1. entral atom = 2. Valence electrons = or pairs 3. orm sigma bonds. This leaves 6 pairs. 4. Place lone pairs on outer atoms. Each atom is surrounded by an octet of electrons. arbon Dioxide, 2 4. Place lone pairs on outer atoms. 5. o that has an octet, we shall form DULE D between and. Double and even triple bonds are commonly observed for,, P,, and 2 3 The second bonding pair forms a pi (π) bond. 2 4 ulfur Dioxide, 2 1. entral atom = 2. Valence electrons = 18 or 9 pairs 3. orm pi (π) bond so that has an octet - but note that there are two ways of doing this. bring in left pair ulfur Dioxide, 2 This leads to the following structures. R bring in right pair bring in left pair R bring in right pair These equivalent structures are called REAE TRUTURE. The true electronic structure is a YRID of the two. Page III-7-4 / hapter even Lecture otes

5 Page III-7-5 / hapter even Lecture otes Violations of the ctet Rule Usually occurs with and elements of higher periods. oron Trifluoride entral atom = Valence electrons = or electron pairs = Assemble dot structure 3 4 The atom has a share in only 6 electrons (or 3 pairs). atom in many molecules is electron deficient. Also common for Al and e ulfur Tetrafluoride, 4 entral atom = Valence electrons = or pairs. orm sigma bonds and distribute electron pairs. 5 pairs around the atom. A common occurrence outside the 2nd period. dd # of electrons - 2 Paramagnetic compounds & free radicals or 2, central atom = Valence electrons = or pairs. dd e - occupies its own "space" orm sigma bonds and distribute electron pairs. ormal Atom harges Atoms in molecules often bear a charge (+ or -). The predominant resonance structure of a molecule is the one with charges as close to 0 as possible. ormal charge = Group number - 1 / 2 (number bonding electrons) - (number lone pair electrons) um of all formal charges in a molecule must equal ionic charge ee Guide to ormal harges Page III-7-5 / hapter even Lecture otes ormal Atom harges ormal charge = Group number - 1 / 2 (number bonding electrons) - (number lone pair electrons) more stable structure

6 Page III-7-6 / hapter even Lecture otes arbon Dioxide, 2 ormal harge omparison with (1/2)(2) - 6 = (1/2)(4) - 4 = (1/2)(8) - 0 = 0 um of formal charges = = = 0 which equals ionic charge on molecule This is a good structure! 6 - (1/2)(6) - 2 = +1 atom formal charge is still 0. um of formal charges = = = 0 which equals ionic charge on molecule This is a valid resonance form, but not as good as previous slide oron Trifluoride, 3 What if we form a - double bond to satisfy the atom octet? +1-1 Molecular Geometry never makes double bonds! MLEULAR GEMETRY VEPR Valence hell Electron Pair Repulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs. Molecule adopts the shape that minimizes the electron pair repulsions. o. of e- Pairs Around entral Atom Page III-7-6 / hapter even Lecture otes 2 3 e 180 Example 120 planar trigonal tetrahedral Geometry linear VEPR

7 o. of e- Pairs Around entral Atom Example e Geometry linear planar trigonal tetrahedral Page III-7-7 / hapter even Lecture otes VEPR o. of e- Pairs Around entral Atom Example e Geometry linear planar trigonal tetrahedral VEPR Ammonia, 3 1. Draw electron dot structure 2. ount Ps and LPs = 4 3. The 4 electron pairs are at the corners of a tetrahedron. lone pair of electrons in tetrahedral position The ELETR PAIR GEMETRY is tetrahedral. Ammonia, 3 The electron pair geometry is tetrahedral. lone pair of electrons in tetrahedral position The MLEULAR GEMETRY - the positions of the atoms - is TRIGAL PYRAMID Water, 2 1. Draw electron dot structure 2. ount Ps and LPs = 4 3. The 4 electron pairs are at the corners of a tetrahedron. Water, 2 The electron pair geometry is TETRAEDRAL The electron pair geometry is TETRAEDRAL The molecular geometry is bent Page III-7-7 / hapter even Lecture otes

8 Page III-7-8 / hapter even Lecture otes Methanol, 3 1. Draw electron dot structure Define bond angles 1 and 2 Methanol, 3 Define bond angles 1 and 2 Angle 1 = 109 o Angle 2 = 109 o In both cases the atom is surrounded by 4 electron pairs Acetonitrile, 3 Define bond angles 1 and 2 Angle 1 = 109 o Angle 2 = 180 o 109 ne is surrounded by 4 electron "lumps" and the other by 2 "lumps" 180 TRUTURE WIT ETRAL ATM TAT D T EY TE TET RULE Violations of the ctet Rule ompounds with 3 Pairs Around the entral Atom Usually occurs with Group 3A elements and with those of 3rd period and higher. onsider boron trifluoride, 3 The atom is surrounded by only 3 electron pairs. ond angles are 120 o Geometry described as planar trigonal or trigonal planar Page III-7-8 / hapter even Lecture otes

9 Page III-7-9 / hapter even Lecture otes ompounds with 5 or 6 Pairs Around the entral Atom ompounds with 5 or 6 Pairs Around the entral Atom 90 Trigonal bipyramid P electron pairs 90 ompounds with 5 or 6 Pairs Around the entral Atom ctahedron 90 6 electron pairs ulfur Tetrafluoride, 4 umber of valence electrons = 34 entral atom = Dot structure Electron pair geometry = trigonal bipyramid (because there are 5 pairs around the ) ulfur Tetrafluoride, 4 ther Molecular Geometries Lone pair is in the equator because it requires more room Molecular geometry = seesaw Page III-7-9 / hapter even Lecture otes

10 Page III-7-10 / hapter even Lecture otes ond rder # of bonds between a pair of atoms Double bond (bo = 2) ingle bond (bo = 1) ond rder ractional bond orders occur in molecules with resonance structures. onsider 2 - ond rder Acrylonitrile Triple bond (bo = 3) ond order = 3 e - pairs in bonds 2 bonds The - bond order = 1.5 ond rder ond order is proportional to two important bond properties: (a) (b) bond length bond energy 110 pm 745 kj 414 kj 123 pm ond Length ond length, the distance between two nuclei, depends on the size of bonded atoms. ond length is inversely proportional to bond order - -l ond distances measured using Ahe software in Angstroms (1 Å = m) -I ond Length ond length depends on bond order As bond order increases, bond length decreases ond Energy is measured by the energy required to break a bond The GREATER the number of bonds (bond order) the LARGER the bond energy and the RTER the bond. ond distances measured using Ahe software in Angstroms (1 Å = m) Page III-7-10 / hapter even Lecture otes ond Energy

11 Page III-7-11 / hapter even Lecture otes ond Energy ond order = 1.5 bond energy =? Using ond Energies Estimate the energy of the reaction: - + l-l 2 -l et energy = rxn = energy required to break bonds - energy evolved when bonds formed ond order =2 498 kj/mol ond order = kj/mol - = 436 kj/mol l-l = 243 kj/mol -l = 431 kj/mol = bonds broken - bonds formed Using ond Energies Estimate the energy of the reaction - + l-l ----> 2 -l Molecular Polarity oiling point = = 436 kj/mol l-l = 243 kj/mol -l = 431 kj/mol um of - + l-l bond energies = 436 kj kj = +679 kj 2 mol -l bond energies = 862 kj et = = +679 kj kj = -183 kj exothermic! Why do ionic compounds dissolve in water? oiling point = -161 Why do water and methane differ so much in their boiling points? Molecular Polarity ond Polarity Water molecules are attracted to balloons that have a static electric charge +δ -δ l Page III-7-11 / hapter even Lecture otes l is PLAR because it has a positive end and a negative end (dipoles). l has a greater share in bonding electrons than does. l has slight negative charge (-δ) and has slight positive charge (+δ)

12 Page III-7-12 / hapter even Lecture otes +δ -δ ond Polarity l Dipole moment, µ, can measure dipole strength by placing molecules in electrical field. Polar molecules will align when the field is on. onpolar molecules will not. ond Polarity Due to polarity, the -l bond energy is GREATER than expected for a "pure" covalent bond. D "pure" bond real bond EERGY 339 kj/mol calc'd 432 kj/mol measured Difference = 92 kj. This difference is proportional to the difference in + Polar molecules + onpolar molecules ELETREGATIVITY, χ. ee Polarity Guide Electronegativity,χ Linus Pauling, χ is a measure of the ability of an atom in a molecule to attract electrons to itself. Electronegativities tend to increase up and to the right on the periodic table i 2 ond Polarity l 2.5 The only person to receive two unshared obel prizes (for Peace and hemistry) hemistry areas: bonding, electronegativity, protein structure A great regonian and a great cientist P 1.5 Electronegativity, χ Which bond is more polar (or DIPLAR)? - - χ χ is more polar than has maximum χ. Atom with lowest χ is the center atom in most molecules. -δ +δ Relative values of χ determine D PLARITY (and point of attack on a molecule). and polarity is "reversed" Page III-7-12 / hapter even Lecture otes +δ -δ

13 Page III-7-13 / hapter even Lecture otes Polar or onpolar? Molecular Polarity Molecules will be polar if a) bonds are polar AD b) the molecule is T "symmetric" ompare 2 and 2. Which one is polar? Polar bonds, nonpolar molecule All above are symmetric and T polar Polar bonds, polar molecule 4 through l4 Polarity Polar or onpolar? onsider A3 molecules: 3, l2, and 3. Methane (4) and carbon tetrachloride (l4) are symmetrical and T polar. All other compounds asymmetrical and polar. End of hapter 7 ee: hapter even tudy Guide hapter even oncept Guide Page III-7-13 / hapter even Lecture otes