LEWIS STRUCTURES Dr Seemal Jelani 4/13/2015 1 Lewis “dot-line” representations of atoms and molecules • Electrons of an atom are of two types: • Core electrons and Valence electrons • Only the valence electrons are shown in Lewis dot-line structures Dr Seemal Jelani 4/13/2015 2 • The number of valence electrons is equal to the group number of the element for the representative elements • For atoms the first four dots are displayed around the four “sides” of the symbol for the atom. Dr Seemal Jelani 4/13/2015 3 • If there are more than four electrons, the dots are paired with those already present until an octet is achieved • Ionic compounds are produced by complete transfer of an electron from one atom to another Dr Seemal Jelani 4/13/2015 4 • Covalent compounds are produced by sharing of one or more pairs of electrons by two atoms. •The valence capacity of an atom is the atom’s ability to form bonds with other atoms. •The more bonds the higher the valence. Dr Seemal Jelani 4/13/2015 5 The valence of an atom is not fixed, but some atoms have typical valences which are most common: Carbon: valence of 4 Nitrogen: valence of 3 Oxygen: valence of 2 Fluorine: valence of 1 Dr Seemal Jelani 4/13/2015 6 Covalent bonding and Lewis structures • Covalent bonds are formed from sharing of electrons by two atoms • Molecules possess only covalent bonds • The bedrock rule for writing Lewis structures for the first full row of the periodic table is the octet rule for C, N, O and F Dr Seemal Jelani 4/13/2015 7 • C, N, O and F atoms are always surrounded by eight valence electrons • For hydrogen atoms, the doublet rule is applied: H atoms are surrounded by two valence electrons. Dr Seemal Jelani 4/13/2015 8 The Lewis Model of Chemical Bonding In 1916 G. N. Lewis proposed that atoms combine in order to achieve a more stable electron configuration. Maximum stability results when an atom is isoelectronic with a noble gas. An electron pair that is shared between two atoms constitutes a covalent bond. Dr Seemal Jelani 4/13/2015 9 Covalent Bonding in H2 Two hydrogen atoms, each with 1 electron, H. .H can share those electrons in a covalent bond. H: H Sharing the electron pair gives each hydrogen an electron configuration analogous to helium. Dr Seemal Jelani 4/13/2015 10 Example Combine carbon (4 valence electrons) and four fluorines (7 valence electrons each) . .. . C. . : .. F. to write a Lewis structure for CF4. .. .. : ..F: .. : : F : ..F: C .. .. : ..F: The octet rule is satisfied for carbon and each fluorine. Dr Seemal Jelani 4/13/2015 11 Example It is common practice to represent a covalent bond by a line. We can rewrite .. .. : ..F: .. : : ..F: C .. : ..F : ..F: .. : F: as .. : .. F C .. ..F: : ..F: Dr Seemal Jelani 4/13/2015 12 Double Bonds and Triple Bonds Dr Seemal Jelani 4/13/2015 13 Inorganic examples .. .. : O: : C : : O: .. :O C .. O: C N: Carbon dioxide H : C : :: N: H Hydrogen cyanide Dr Seemal Jelani 4/13/2015 14 Organic examples H .. H .. H: C : : C:H H Ethylene H C C H H H : C : :: C:H Acetylene H Dr Seemal Jelani C 4/13/2015 C H 15 Formal Charges Revision Dr Seemal Jelani 4/13/2015 16 Formal charge is the charge calculated for an atom in a Lewis structure on the basis of an equal sharing of bonded electron pairs. Dr Seemal Jelani 4/13/2015 17 Nitric acid Formal charge of H H .. O .. .. O: N :O .. : We will calculate the formal charge for each atom in this Lewis structure. Dr Seemal Jelani 4/13/2015 18 Nitric acid Formal charge of H H .. O .. .. O: N :O .. : Hydrogen shares 2 electrons with oxygen. Assign 1 electron to H and 1 to O. A neutral hydrogen atom has 1 electron. Therefore, the formal charge of H in nitric acid is 0. Dr Seemal Jelani 4/13/2015 19 Nitric acid Formal charge of O H .. O .. .. O: N :O .. : Oxygen has 4 electrons in covalent bonds. Assign 2 of these 4 electrons to O. Oxygen has 2 unshared pairs. Assign all 4 of these electrons to O. Therefore, the total number of electrons assigned to O is 2 + 4 = 6. Dr Seemal Jelani 4/13/2015 20 Nitric acid Formal charge of O H .. O .. .. O: N :O .. : Electron count of O is 6. A neutral oxygen has 6 electrons. Therefore, the formal charge of O is 0. Dr Seemal Jelani 4/13/2015 21 Nitric acid Formal charge of O H .. O .. .. O: N :O .. : Electron count of O is 6 (4 electrons from unshared pairs + half of 4 bonded electrons). A neutral oxygen has 6 electrons. Therefore, the formal charge of O is 0. Dr Seemal Jelani 4/13/2015 22 Nitric acid Formal charge of O H .. O .. .. O: N :O .. : Electron count of O is 7 (6 electrons from unshared pairs + half of 2 bonded electrons). A neutral oxygen has 6 electrons. Therefore, the formal charge of O is -1. Dr Seemal Jelani 4/13/2015 23 Nitric acid Formal charge of N H .. O .. .. O: N – : :O .. Electron count of N is 4 (half of 8 electrons in covalent bonds). A neutral nitrogen has 5 electrons. Therefore, the formal charge of N is +1. Dr Seemal Jelani 4/13/2015 24 Nitric acid Formal charges H .. O .. .. O: N+ – : :O .. A Lewis structure is not complete unless formal charges (if any) are shown. Dr Seemal Jelani 4/13/2015 25 Formal Charge An arithmetic formula for calculating formal charge. Formal charge = group number number of number of – – in periodic table bonds unshared electrons Dr Seemal Jelani 4/13/2015 26 "Electron counts" and formal charges in NH4+ and BF4- 1 H + H 4 N H H .. : F: .. – .. : .. F B ..F: : ..F: Dr Seemal Jelani 7 4 4/13/2015 27 CONDENSED STRUCTURAL FORMULAS Dr Seemal Jelani 4/13/2015 28 Condensed structural formulas Lewis structures in which many (or all) covalent bonds and electron pairs are omitted. H H H H C C C H : O: H H H can be condensed to: CH3CHCH3 or (CH3)2CHOH OH Dr Seemal Jelani 4/13/2015 29 Bond-line formulas CH3CH2CH2CH3 is shown as CH3CH2CH2CH2OH is shown as OH Omit atom symbols. Represent structure by showing bonds between carbons and atoms other than hydrogen. Atoms other than carbon and hydrogen are called heteroatoms. Dr Seemal Jelani 4/13/2015 30 Bond-line formulas H Cl Cl C H2C CH2 H2C CH2 is shown as C H H Omit atom symbols. Represent structure by showing bonds between carbons and atoms other than hydrogen. Atoms other than carbon and hydrogen are called heteroatoms. Dr Seemal Jelani 4/13/2015 31 Constitutional Isomers Dr Seemal Jelani 4/13/2015 32 Constitutional isomers Isomers are different compounds that have the same molecular formula. Constitutional isomers are isomers that differ in the order in which the atoms are connected. An older term for constitutional isomers is “structural isomers.” Dr Seemal Jelani 4/13/2015 33 A Historical Note NH4OCN Ammonium cyanate O H2NCNH2 Urea In 1823 Friedrich Wöhler discovered that when ammonium cyanate was dissolved in hot water, it was converted to urea. Ammonium cyanate and urea are constitutional isomers of CH4N2O. Ammonium cyanate is “inorganic.” Urea is “organic.” Wöhler is credited with an important early contribution that helped overturn the theory of “vitalism.” Dr Seemal Jelani 4/13/2015 34 Examples of constitutional isomers H H C H .. O: H N+ :O .. H – : Nitromethane C .. O .. N .. .. O: H Methyl nitrite Both have the molecular formula CH3NO2 but the atoms are connected in a different order. Dr Seemal Jelani 4/13/2015 35 Shapes Dr Seemal Jelani 4/13/2015 36 Dr Seemal Jelani 4/13/2015 37 Methane tetrahedral geometry H—C—H angle = 109.5° Dr Seemal Jelani 4/13/2015 38 Methane tetrahedral geometry each H—C—H angle = 109.5° Dr Seemal Jelani 4/13/2015 39 Valence Shell Electron Pair Repulsions The most stable arrangement of groups attached to a central atom is the one that has the maximum separation of electron pairs (bonded or nonbonded). Dr Seemal Jelani 4/13/2015 40 Water bent geometry H—O—H angle = 105° H H : O .. but notice the tetrahedral arrangement of electron pairs Dr Seemal Jelani 4/13/2015 41 Ammonia trigonal pyramidal geometry H—N—H angle = 107° H H N : H but notice the tetrahedral arrangement of electron pairs Dr Seemal Jelani 4/13/2015 42 Boron Trifluoride F—B—F angle = 120° trigonal planar geometry allows for maximum separation of three electron pairs Dr Seemal Jelani 4/13/2015 43 Formaldehyde: CH2=O H—C—H and H—C—O angles are close to 120° trigonal planar geometry H C O H Dr Seemal Jelani 4/13/2015 44 Figure 1.12: Carbon Dioxide O—C—O angle = 180° linear geometry O Dr Seemal Jelani C 4/13/2015 O 45 Polar Covalent Bonds and Electronegativity Dr Seemal Jelani 4/13/2015 46 Electronegativity is a measure of an element to attract electrons toward itself when bonded to another element. An electronegative element attracts electrons. An electropositive element releases electrons. Dr Seemal Jelani 4/13/2015 47 Pauling Electronegativity Scale Li Be B C N O F 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Na Mg Al Si P S Cl 0.9 1.2 1.5 1.8 2.1 2.5 3.0 Dr Seemal Jelani 4/13/2015 48 Electronegativity increases from left to right in the periodic table Electronegativity decreases going down a group. Dr Seemal Jelani 4/13/2015 49 Dr Seemal Jelani 4/13/2015 50 Generalization H—H .. : .. F .. F: .. :N N: Nonpolar bonds connect atoms of the same electronegativity Dr Seemal Jelani 4/13/2015 51 Generalization The greater the difference in electronegativity between two bonded atoms; the more polar the bond. d+ H .. dF: .. d+ H d.. d+ O H .. d- d+ :O C .. dO .. : polar bonds connect atoms of different electronegativity Dr Seemal Jelani 4/13/2015 52