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Chem
istry CHAPTER 01
COVALENT BONDING AND SHAPES OF MOLECULES
CHEM 240: Fall 2019
Prof. Greg Cook
cook.chem.ndsu.nodak.edu/chem240
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Chem
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Chapter 01 Topics
• Mostly a review of general chemistry
• Atomic Structure
• Lewis Models and Bonding
• Bonding and Shapes of Molecules
• Resonance
• Covalent Bonding and Hybridization 2
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Chem
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Structure
• The sequence of connections that defines a molecule, including the spatial orientation of these connections
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ATOMIC STRUCTURE
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Chem
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Subatomic Particles
• Nucleus is made up of Protons and Neutrons
• mass of Proton = 1.6726 x 10-27 kg
• mass of Neutron = 1.6760 x 10-27 kg
• Surrounding the Nucleus are electrons
• mass of Electron = 9.1096 x 10-31 kg << proton
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Chem
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Atomic Structure
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+-
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Chem
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Atomic Structure
• Electrons surround nucleus in orbitals
• Atomic Number (Z) = # protons in nucleus
• Mass Number (A) = # protons + # neutrons
• Atomic Weight = average mass of a large number of atoms
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+
-
XAZ H11 C12612.01071.0079
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Chem
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Wave Function
• Electrons have properties of both Particles and Waves
• Quantum Mechanics help us understand the structure and behavior of electrons
• Schrödinger Wave Equation
• Describes the energy of an electron in an atom
• Wave functions ψ (psi) 8
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Heisenberg uncertainty principle
• We can’t tell exactly where an electron is
• but we can tell where it will most likely be
• Probability of finding an electron at a particular spot relative to the nucleus is given by ψ2 (psi)2
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Chem
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Orbitals
• Wave functions are also called orbitals
• each orbital characterized by 3 quantum numbers
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• n : principle quantum number
• l : angular momentum quantum number
• ml : magnetic quantum number
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Quantum Numbers
• principle quantum number n
• an integer
• determines major part of orbital energy - the shell
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Chem
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s orbitals
• s orbitals are spherical in shape
• energy increases with n
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Quantum Numbers
• Angular momemtum l determines the shape of the orbital
• for a given value of n : l = 0, 1, 2, , , n - 1
• l = 0 : s
• l = 1: p
• l = 2 : d
• l = 3 : f 13
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p Orbitals
• p orbitals are shaped like dumbells with a node in between the lobes (n = 2 and higher)
• Three orbitals with the same energy
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Chem
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4th Quantum Number - Spin
• Each electron also has a spin quantum number ms
• +½ and -½
• Pauli Exclusion Principle - two electrons may occupy the same orbital only when they have opposite or “paired” spins.
• No orbital can contain more than 2 electrons
• H, He, Li 15
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Chem
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Electron Configuration
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Chem
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Periodic Table - Periods
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Chem
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Second Period Electron Configurations
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Z
Li 3
1s 2s 2p
C 6
N 7
O 8
Ne 10
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Chem
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Second Period Electron Configurations
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Z
Li 3
1s 2s 2p
C 6
N 7
O 8
Ne 10
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Chem
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BONDING
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Chem
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Ions
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1s22s22p63s1 1s22s22p63s23p5
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Chem
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Ionization
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1s22s22p63s1 1s22s22p63s23p5
Na Cl• ••• •• •
•
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Ionization
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1s22s22p63s0 1s22s22p6
Na Cl• ••• •• ••+ -
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Ionic Bond
• An ionic bond is a force of attraction between oppositely charged species (ions).
• Ionic bonds are common in inorganic compounds but are more rare in organic compounds.
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Na Cl• ••• •• ••+ -
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Ionization
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1s22s22p2
1s22s22p63s1
1s22s22p63s23p5
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Chem
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Covalent Bonds
• 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.
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1s22s22p63s0 1s22s22p6
Na Cl• ••• •• ••+ -
He Ne
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Covalent Bonding in H2
• Two hydrogen atoms each have one electron
• Instead of ionizing, they come together to share both electrons between them
• Sharing the electron pair allows both hydrogen atoms to have a filled orbital
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•H • H
•H • H
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Covalent Bonding in F2
• Two fluorine atoms each have 7 valence electrons
• They can share them in a covalent bond
• Each fluorine atom has the same electron configuration as Ne (8 electrons) 28
•F • F
F •• F••••
• •• •
• •• •
••
• •• • ••
• •• •
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Chem
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Octet Rule
• When forming compounds, atomswill gain, lose or share electrons to give a stable electron configuration characterized by 8 valence electrons
• The octet rule is most useful in cases involving covalent bonds to C, N, O and F
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F •• F••••
• •• •
• •• •
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Chem
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Example with CF4
• Carbon has 4 valence electrons, F has 7
• The Lewis structure for CF4
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•C • F•
•• ••
• •• •
•C • F•
•• ••
• •• •
•• F••
• ••
•• F•••
• •
•• F•
• •• •
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Chem
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Example with CF4
• It is more common to represent a covalent bond (shared pair of electrons) with a line
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•C • F•
•• ••
• •• •
•• F••
• ••
•• F••
•• •
•• F•
• •• • F C F
F
F=
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Chem
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Example with CF4
• We often don’t write the lone pairs (Kekulé structures)
• You should know how many lone pairs an atom should have! 32
C FF
FF
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Structure Representation
• Lewis Dot Structures - all electrons are represented by dots around the atoms
• Kekulé Structures - atoms are drawn and lines represent shared electrons (covalent bonds)2-chloro-4-methylpentane
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C C C CH
H
C
H
H
H
HC
Cl
HHH
Kekulé StructureHH
HH
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Structure Representation
• Condensed Structure - No bonds are shown and side groups are attached to the carbon before it Condensed Kekulé Structures
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CH CH2 CH CH3 Condensed Kekulé StructureCH3
CH3
Cl
CH3CH(CH3)CH2CHClCH3 Condensed Structure
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Structure Representation
• Skeletal Line Structure - Carbons and Hydrogens are not shown. Each end of a line and intersection between lines represent a carbon atom. All other atoms are drawn.
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Line StructureCl
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Chem
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Line Structure Examples
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CH CH2 CH3CH3
CH3=
Br CH2 CH2 BrCH3=
OHCH CH3CH3
OH=
O
O OH
OHHO
hydrocortisone
beta carotene
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Chem
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HYBRID ORBITALS - MOLECULAR SHAPES
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Chem
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Methane Structure
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H
CH H
H
109.5°
C-H bond length1.1Å (110 pm)
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VSEPR
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• Valence Shell Electron Pair Repulsion - The electron repulsion between bonds and lone pairs.
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Hybridization in Water?
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O
HH
105°
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Chem
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sp3 Hybridization
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s
p
Carbon with 4 valence electronsatomic electron configuration
sp3
sp3 Hybrid electron configuration
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Bonding in Methane
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C H
H
HH
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Chem
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Bonding in Methane
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C H
H
HH
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Bonding in Ethane
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C C
H
H
HH
H
H
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Structures of Methane, Ethane and Propane
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Double and Triple Bonds
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O C OO C O
H C NH C N
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Organic Examples
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C CH
H
H
H
C C HH
Ethylene (Ethene)
Acetylene (Ethyne)
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sp2 Hybridization
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s
Carbon with 4 valence electronsatomic electron configuration
p
sp2
sp2 Hybrid electron configuration
p
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Alkenes
• Compounds that contain double bonds.
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C CH
H
H
HEthylene (Ethene)
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Structure and Bonding
• A double bond is sp2 hybridized (pi bond)
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Structure of Alkynes
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C CH H
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sp Hybridization
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s
Carbon with 4 valence electronsatomic electron configuration
p
sp
sp Hybrid electron configuration
p
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Chem
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POLAR COVALENT BONDS
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Chem
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Electronegativity
• Electrons are not always shared equally between atoms in a covalent bond
• Electronegativity is a measure of the ability of an element to attract electrons toward itself when bonded to another element
• An electronegative element attracts electrons gathering negative charge
• An electropositive element releases electrons gathering a positive charge
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Pauling Electronegativity Scale
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Electronegativity
• Increases from left to right and bottom to top (decreases going down a group)
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EN Generality
• The greater the difference in EN between two bonded atoms, the more polar the bond
• Nonpolar bonds connect atoms with the same EN
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H H Cl Cl O O
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Polar Covalent Bonds
• Polar Covalent Bonds connect atoms that have different EN
• partial negative charges on atoms with higher EN and partial positive charges on atoms with lower EN
• Bond dipoles pointing from ∂+ toward ∂- 58
H F H O H O C Oδ+ δ- δ- δ- δ-δ+ δ+ δ+
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Polar Covalent Bonds
• Electrostatic potential maps show the charge distribution within a molecule
• inductive effect - the shifting of electrons in a sigma bond in response to electronegative atoms nearby.
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H Fδ+ δ-
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Polar vs Ionic Bonds
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A A A B A Bδ+ δ-
Covalent Ionicpolar covalent
δEN >2 IONIC BONDSδEN <2 COVALENT BONDS
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RESONANCE
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Delocalization of Anion
• Spreading out charge is a stabilizing affect - through pi-bonds
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Chem
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Delocalization of Anion
• Spreading out charge is a stabilizing affect - resonance has a large affect on acidity
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O
OH
16
OH
4.7
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Chem
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Delocalization of Anion
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10
OH
OH
O
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Chem
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More examples of resonance
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Chem
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Arrows are not Arrows
• In chemistry we use different kinds of arrows to indicate different things
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Reaction Arrow A B
Equilibrium Arrow A B
Resonance Arrow A B
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Chem
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Curved Arrows
• Curved arrows are used to track the flow of electrons in chemical reactions
• Arrows begin where the electrons were originally and points to where they end up
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A BA B +
A BA B +
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Chem
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Dissociation and Combination
• Curved arrows can be used in a variety of situations
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H3C O
O
H
HC
Br
H
HH O +
H
O
H+ H Cl
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Chem
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FUNCTIONAL GROUPS
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Chem
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Functional Groups
• Reactive parts of a molecule that undergo chemical reactions
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Chem
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Functional Groups
• Alcohols, Amines, Aldehydes, Ketones, Carboxylic Acids and derivatives
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