k8-13 - 11/13/2008 Midterm 1 Chem 1A Possible: High score:...

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11/13/2008 1 Chem 1A Lewis structures Lecture 8: Midterm 1 Possible: 24 High score: 23 Mean:14 Arnold says: Way to go! Your exam will be scanned and sent to your eee page sometime next week. Hooking up From individual atoms to chemical bonds ? A chemical bond is formed when the energy of the aggregate is lower than that of the individual atoms Chemical bonding in a nutshell Atoms combine to achieve a more stable electron configuration. Maximum stability is achieved when atom is isoelectric with a noble gas. Only open shell electrons (valence electrons) are relevant in bond formation. Types of Chemical Bonds 1. covalent bond 2. ionic bond HH 3. metallic bond Na + Cl - Fe Fe Fe Fe Ionic vs. covalent bonds
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11/13/2008 2 Electronegativity Electronegativity is the ability of an atom to attract electrons to itself in a chemical bond Linus Pauling (1901 - 1994) Won the Nobel Prize for both Chemistry and Peace The higher the electronegativity, the greater the ability of the atom to attract electrons towards itself Electronegativity is a relative number Electronegativity trends Lighter atoms tend to be more electronegative Towards a closed shell: more electronegative FONClBrISCHP Electronegativity and bonding In a chemical bond, the atom with the highest electro- negativity carries the highest electron density in the bond Δ = I E N (atom 1) -EN (a tom2)I I E.N. (atom 1) E.N. (atom 2) I Δ < 2.0: covalent bond Δ > 2.0: ionic bond Bond electrons are mostly shared Bond electrons reside largely on one of the atoms Covalent Bond In a covalent bond, electrons are shared by nuclei repulsion Δ = 0 : electrons are equally shared by the atoms attraction Lewis Dot Symbols # dots = number of valence electrons = group number of representative elements
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11/13/2008 3 Lewis dot symbols: covalent bond H H + H H bonding pair or H H Cl Cl + Cl Cl Cl Cl or lone pairs Octet rule An atom other than hydrogen tends to form bonds until it is surrounded by eight valence electrons. Octet rules How to draw Lewis structures: the quick way 1. Add up the valence electrons of all the atoms 2. Use a pair of electrons to form a bond between each pair of bound atoms (skeleton structure) and subtract 3. Fill in electrons to satisfy the duet rule for H and the octet rule for C, N, O and F and subtract to get Δ N 4. If Δ N=0, done If Δ N < 0, add | Δ N|/2 extra bonds. If Δ N > 0, must violate octet rule with 3 rd (or higher) row elements Octet rule for water Water: H 2 O Total valence electrons: 1 + 1 + 6 = 8 1. Add up the valence electrons of all the atoms 2. Use a pair of electrons to form a bond between each pair of bound atoms HH O 3. Arrange remaining electrons to satisfy the duet rule for H and the octet rule for C, N, O and F O 2e - 2e - 8e - 8-4=4 Δ N=4-4=0 Multiple bonds:CO 2 Carbon dioxide: CO 2 1. Add up the valence electrons of all the atoms Total valence electrons: 4 + 6 + 6 = 16 2. Use a pair of electrons to form a bond between each pair of bound atoms OO C 3. Arrange remaining electrons to satisfy the duet rule for H and the octet rule for C, N, O and F C 16-4=12 Δ N = 12-(2x6+4)=-4, so 2 more bonds Multiple bonds:C 2 H 4
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k8-13 - 11/13/2008 Midterm 1 Chem 1A Possible: High score:...

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