Electronegativity The ability of an atom, when in a compound, to attract e - density to itself. Linus Pauling 1930s The Nature of the Chemical Bond

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1 Study Guide for Exam #2 MSU LBS 171 Profs. Robert LaDuca and Ryan Sweeder Molecule: chem. combo. of 2 or more atoms bonded together Element: matter that is made up of only one type of atom Compound: matter made up of 2 or more kinds of an atom Molecular formula: describes the composition of a specific molecular structure Example: glucose is C 6 H Ionic compounds: compounds made of ions Ions: atom that has a charge (gained or lost e ) Cation: positively charged ion; anion: negatively charged ion Ionic compounds naming (charge on cation must = charge on anion, since compounds are neutral): CaO calcium oxide MgS magnesium sulfide Na 2 O sodium oxide Cu 2 O copper (I) oxide CuO copper (II) oxide Compounds which are hydrated are those that crystallize with the addition of H 2 O 1 H 2 O monohydrate 2 H 2 O dihydrate 3 H 2 O trihydrate. Periodic Trends: Atomic radius decreases L to R and increases down the periodic table. o As you go across, the positive charge on the nucleus increases, pulling the electrons closer to it. o As you go down, you are increasing the number of energy levels on the atom, increasing its size. Ionization energy (energy required to remove e from atom in gas phase) first ionization energy increases L to R, and decreases down the periodic table. o It gets harder to pull electrons from the atom as you move across the table, since the positive nucleus has a stronger hold over the electrons in the atom. o As you go down, the atom grows, since more energy levels are added, and it becomes increasingly easier to pull an e away, since it is not being held in by the nucleus as tightly. Electron affinity increases L to R, and decreases down the periodic table. o Effective nuclear charge increases L to R, and this makes it harder to ionize the atom. However, this makes the atom more attractive to additional electrons. Ionization energy and electron affinity are (for the most part) proportionately related. Electronegativity The ability of an atom, when in a compound, to attract e density to itself Linus Pauling 1930s The Nature of the Chemical Bond

2 Traveling up and to the right on the periodic table increases the electronegativity (χ) with Cs at the lowest value (0.8) and F at the highest (4.0). Figure 1. Increasing Electronegativity in the Periodic Table The difference in χ determines the type of chemical bond For compounds containing NONMETALS ONLY o χ 0.4 covalent bond (complete sharing) o 0.5 χ polar covalent bond (unequal sharing) For compounds containing METALS and NONMETALS o χ 1.5 ionic bond (complete transfer of valence e) o 0.5 χ 1.5 polar covalent bond (unequal sharing) Lewis Dot Structure 1918

3 First level view of chemical bonding each valence e on an atom is a dot Covalent or polarcovalent bond indicated by a dash (shared pair or e ) Lewis structures work well for main group elements, but not transition metals Drawing Lewis Structures o Count valence electrons (VE) on individual atoms o Put least electronegative atom (but not H) in center o Connect all other atoms with single bonds o Place lone pairs of e on outer atoms to satisfy octet rule until no e remain o If remaining e, place them on the central atom (only 8+ if it has a dsubshell) o Calculate Formal Charge = (original # VE) (# bonds) 2 (# lone pairs) o If central atom has + charge, form double bond. Recalculate. REMEMBER Atoms would prefer incomplete or expanded octet to a charge Cannot form expanded octet on first or second row elements If charge cannot be minimized, place negative FC on the more electronegative atom See lecture notes for examples of the formation of Lewis Dot Structures Resonance Localization of bonding e throughout the molecule Need more than one Lewis Structure to accurately represent the bonding Combination of equivalent Lewis Structures N N O OR N = N = O number of bonds NOTE: bond orders are equal, actual orders found by number of links

4 o Example: Ozone (O 3 ) O = O O three bonds, two links 3 o Bond lengths (also called bond order) = = Naming Molecular Compounds (nonmetals ONLY) Name of less electronegative atom stays the same Name of more electronegative atom ends in ide Indicate the number of each atom by a Greek prefix o CO 2 carbon dioxide (omit mono from first element) o N 2 O dinitrogen monoxide o P 4 O 10 tetraphosphorus decoxide Acids o Neutral molecular compounds that can liberate H + o If parent anion ends in ide, becomes hydro ic acid Cl hydrochloric acid CN (cyanide) hydrocyanic acid o If parent anion ends in ate, becomes ic acid o If parent anion ends in ite, becomes ous acid HClO 4 perchloric acid HNO 2 nitrous acid Acid a compound that donates H + in solution Base a compound that accepts H + in solution Amphoteric water (H 2 O) can be either acidic or basic Polyatomic Ions (LEARN THE SHEET! LEARN THE SHEET!) Cation Ion Name Anion Ion Name

5 ammonium OH hydroxide 2 SO 3 sulfite Anion Ion Name 2 SO 4 sulfate CN cyanide HSO 4 hydrogen sulfate CH 3 CO 2 acetate 2 CO 3 carbonate ClO hypochlorite 2 HCO 3 hydrogen carbonate ClO 2 chlorite ClO 3 chlorate NO 2 nitrite ClO 4 perchlorate NO 3 nitrate 3 PO 4 phosphate 2 CrO 4 chromate 2 HPO 4 hydrogen phosphate 2 Cr 2 O 7 dichromate H 2 PO 4 dihydrogen phosphate MnO 4 permanganate NH 4 + Molecular Geometry creates the 3D shapes molecules actually form VSEPR Valence Shell Electron Pair Repulsion Number of e groups determines basis shape of molecular compound If lone pair or electron dot is present, molecular shape is different # e groups basis shape free pair shape two free pairs three free pairs 2 linear 3 trigonal planar bent <120 4 tetrahedral trigonal pyramidal bent < trigonal bipyramidal "seesaw" "Tshaped" linear 6 octahedral square pyramidal square Molecular Polarity depends on two conditions One or more polar bonds (χ 0.5) Must be asymmetric o Draw 2D structure / 3D structure o Place polarity arrows (positive at atom with lower χ) on polar bonds o Tug on arrows. If central atom moves, molecule is polar

6 Deeper View of Chemical Bonding Linus Pauling 1930s bond forms due to valence orbital overlap If covalent bond is symmetrical, it is a σ bond o Single bonds are always σ bonds Pauling stated that atomic orbitals can mix (or hybridize) to form new atomic orbitals that overlap to create the correct geometries # e hybrid leftover groups name orbitals 2 sp two p's 3 sp 2 one p 4 sp 3 nothing 5 sp 3 d four d's 6 sp 3 d 2 three d's A double bond is formed by a hybrid bond and a porbital References o porbital bond is asymmetric o Asymmetric bond called a π bond Lecture notes from Prof. LaDuca Figure 1. Increasing Electronegativity in the Periodic Table < > Figure 2. 2methyl propane < Figure 3. Isomers of Butene: cis and trans <

7 Study Guide Questions for Exam #2 1. Name each of the following ionic compounds a) K 2 S b) CoSO 4 c) (NH 4 ) 3 PO 4 d) Ca(ClO) 2 2. Draw the Lewis Dot Structure for each of the following molecules or ions a) BrF 3 b) BrF 5 c) ClO 3 d) NH 3 e) SiF 6 2 f) O 3 g) N 2 O h) SO 2 3. Give the basis shape and actual geometry of the molecules and ions from Problem Draw all of the resonance structures for NO 3 5. Which molecules are polar? a) H 2 O b) CO 2 c) NH 3 d) NH 2 Cl e) CH 3 Cl f) CH 4 g) PCl 3 6. Compare the bond lengths of ClO 2, ClO 3, and ClO 4. Which ion has the longest bond length? Which has the shortest bonds? References Chemistry and Chemical Reactivity. Kotz. Study Questions.