Chapter I - Chapter I: Chemical Bonding and Chemical...

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Unformatted text preview: Chapter I: Chemical Bonding and Chemical Structure Organic Chemistry •  Organic chemistry is a discipline within chemistry within chemistry which involves the scien;fic study of the structure, proper;es, composi;on, reac;ons, and prepara;on of chemical compounds that carbon. Allotropic forms C 6 Wohler Synthesis (1828) Friedrich Wöhler •  Atoms •  Period Table (group, period) Basic Concepts Revisited (1): Atoms, electrons, period table An illustra;on of the helium atom Basic Concepts Revisited (2): Electrons •  Valence Shell •  Valence electrons •  Octet Rule The tendency of atoms to gain or lose valence electrons to from ions with the noble gas configura;on has been called the octet rule. Problem 1.1; 1.2 (Page 4) Ionic Bond: Crystal Structure of KCl An ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net posi;ve or nega;ve electrical charge. An ionic bond is a type of chemical bond formed by the aSrac;on between two oppositely charged ions. Covalent bond consists of an electron pair that is shared between bonded atoms. Covalent Bond Polar covalent bond In polar covalent bonds, such as that between hydrogen and oxygen atoms, the electrons are not transferred from one atom to the other as they are in an ionic bond. Instead, some outer electrons merely spend more ;me in the vicinity of the other atom. The effect of this orbital distor;on is to induce regional net charges that hold the atoms together, such as in water molecules. (1) The total number of electrons represented in a Lewis structure is equal to the sum of the numbers of electrons on each individual atom. (2) Non‐valence electrons are not represented in Lewis structures. (3) The octet rule states molecules with eight electrons in their valence shell will be stable, regardless of whether these electrons are Gilbert Newton Lewis bonding or nonbonding. (MIT, Berkeley) Lewis structure for oxygen atoms and molecules Unshared electron pair The formula of the nitrite ion is NO2−. 1: Nitrogen is the least electronega;ve atom, so it is the central atom by mul;ple criteria. 2: Count valence electrons. Nitrogen has 5 valence electrons; each oxygen has 6, for a total of (6 × 2) + 5 = 17. The ion has a charge of −1, which indicates an extra electron, so the total number of electrons is 18. 3: Place ion pairs. Each oxygen must be bonded to the nitrogen, which uses four electrons — two in each bond. The 14 remaining electrons should ini;ally be placed as 7 lone pairs. Each oxygen may take a maximum of 3 lone pairs, giving each oxygen 8 electrons including the bonding pair. The seventh lone pair must be placed on the nitrogen atom. 4: Sa;sfy the octet rule. Both oxygen atoms currently have 8 electrons assigned to them. The nitrogen atom has only 6 electrons assigned to it. One of the lone pairs on an oxygen atom must form a double bond, but either atom will work equally well. We therefore must have a resonance structure. 5: Tie up loose ends. Two Lewis structures must be drawn: one with each oxygen atom double‐bonded to the nitrogen atom. The second oxygen atom in each structure will be single‐bonded to the nitrogen atom. Place brackets around each structure, and add the charge (−) to the upper right outside the brackets. Draw a double‐headed arrow between the two resonance forms. Formal Charge (Page 7) Nitrogen in NO2‐: FC = 5 ‐ 2 ‐ 6/2 = 0 double bonded oxygen in NO2‐: FC = 6 ‐ 4 ‐ 4/2 = 0 single bonded oxygen in NO2‐ FC = 6 ‐ 6 ‐ 2/2 = ‐1 Polar Covalent Bond Water (H2O) NH3 HF Bond Order Single bond Double bond Polarity Vector: a quan;ty that has magnitude and direc;on and that is commonly represented by a directed line segment whose length represents the magnitude and whose orienta;on in space represents the direc;on Molecular Geometry Different shapes of molecules Bond Length, Bond Angle, dihedral angle Structure of covalent compounds Effect of atomic size on bond length Effect of bond order on bond length Dihedral angel Problem 1. 13 VSEPR (Valence‐Shell Electron‐Pair Repulsion Theory) •  Bonds and electron are arranged about a central atom so that the bonds are as far apart as possible. Problem 1.10 Tetrahedral geometry of CH4 Molecular models of methane Ball‐and‐s;ck Wire‐frame Space‐filling Wave nature of electrons •  •  •  •  Electron have wavelike proper;es; Heisenberg uncertainty principle: the accuracy with which we can determine the posi;on and velocity of a par;cle is inherently limited; Wavefunc;on Quantum numbers (n, l, ml, ms) 1.  n (principal quantum number) Integral numbers, determine the electron energy (the electron energy is quan;zed) and size 2.  l (angular momentum quantum number) depend on n, from 0 to n‐1 leSer code (s, p, f, d) determine the shape The orbital shape 3. ml (magne;c quantum number) depend on l determine the direc;onality 3. ms (Spin projec;on quantum number) For an electron, either ½ or – ½ Counter‐clockwise) determine the spin of electron (½ clockwise, – ½ S orbitals (l=0) Flute analogy (page 24‐5) 1s 2s p orbitals (l=1) 2p 3p Read Further Explora;on 1.2 “Electron Density Distribu;on in Orbitals” Orbital shapes Electronic structure of more complex atoms Hund’s rules: (1)  To distribute electrons among iden;cal orbitals of equal energy, single electrons are placed into separate orbitals before the orbitals are filled; (2)  The spins of these unpaired electrons are the same. Pauli Exclusion principle: For electrons in a single atom, it states that no two electrons can have the same four quantum numbers, that is, if n, l, and ml are the same, ms must be different such that the electrons have opposite spins. Bonding: MO orbital overlap Rules for forming MO (Page 34) Bonding and an;bonding MO Orbital interac;on diagram σ bond π‐bond More complex MOs HOMO: highest occupied molecular orbital and LUMO: lowest unoccupied molecular orbital H3C CH3 Hybrid orbitals in methane (CH4) Hybrid orbital of ammonia (NH3) Key Points •  •  •  •  •  •  •  •  •  •  •  •  •  Ionic bond vs covalent bond Octet rule Formal charge Polar bond and electronega;vity Bond length and bond angel Dihedral angel Line‐and‐wedge structures VSEPR theory Resonance Structures Atomic orbitals of hydrogen (wave func;ons, nodes) Sigma bond Rules of forming molecular orbitals Hybrid orbitals ...
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This note was uploaded on 08/24/2010 for the course CHEM 3351 at Colorado.

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