SHAPES OF MOLECULES - Saving the world one molecule at a...

istry CHAPTER 01

COVALENT BONDING AND SHAPES OF MOLECULES

CHEM 240: Fall 2019

Prof. Greg Cook

cook.chem.ndsu.nodak.edu/chem240

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

Structure

• The sequence of connections that defines a molecule, including the spatial orientation of these connections

ATOMIC STRUCTURE

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

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

XAZ H11 C12612.01071.0079

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

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

Orbitals

• Wave functions are also called orbitals

• each orbital characterized by 3 quantum numbers

• n : principle quantum number

• l : angular momentum quantum number

• ml : magnetic quantum number

Quantum Numbers

• principle quantum number n

• an integer

• determines major part of orbital energy - the shell

s orbitals

• s orbitals are spherical in shape

• energy increases with n

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

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

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

Electron Configuration

Periodic Table - Periods

Second Period Electron Configurations

1s 2s 2p

Second Period Electron Configurations

1s 2s 2p

BONDING

1s22s22p63s1 1s22s22p63s23p5

Ionization

1s22s22p63s1 1s22s22p63s23p5

Na Cl• ••• •• •

Ionization

1s22s22p63s0 1s22s22p6

Na Cl• ••• •• ••+ -

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.

Na Cl• ••• •• ••+ -

Ionization

1s22s22p2

1s22s22p63s1

1s22s22p63s23p5

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.

1s22s22p63s0 1s22s22p6

Na Cl• ••• •• ••+ -

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

•H • H

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••••

• •• •

••

• •• • ••

• •• •

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

F •• F••••

• •• •

Example with CF4

• Carbon has 4 valence electrons, F has 7

• The Lewis structure for CF4

•C • F•

•• ••

• •• •

•C • F•

•• ••

• •• •

•• F••

• ••

•• F•••

• •

•• F•

• •• •

Example with CF4

• It is more common to represent a covalent bond (shared pair of electrons) with a line

•C • F•

•• ••

• •• •

•• F••

• ••

•• F••

•• •

•• F•

• •• • F C F

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

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

C C C CH

Kekulé StructureHH

• Condensed Structure - No bonds are shown and side groups are attached to the carbon before it Condensed Kekulé Structures

CH CH2 CH CH3 Condensed Kekulé StructureCH3

CH3CH(CH3)CH2CHClCH3 Condensed Structure

• 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.

Line StructureCl

Line Structure Examples

CH CH2 CH3CH3

Br CH2 CH2 BrCH3=

OHCH CH3CH3

hydrocortisone

beta carotene

HYBRID ORBITALS - MOLECULAR SHAPES

Methane Structure

109.5°

C-H bond length1.1Å (110 pm)

• Valence Shell Electron Pair Repulsion - The electron repulsion between bonds and lone pairs.

Hybridization in Water?

sp3 Hybridization

Carbon with 4 valence electronsatomic electron configuration

sp3 Hybrid electron configuration

Bonding in Methane

Bonding in Ethane

Structures of Methane, Ethane and Propane

Double and Triple Bonds

O C OO C O

H C NH C N

Organic Examples

C C HH

Ethylene (Ethene)

Acetylene (Ethyne)

sp2 Hybridization

sp2 Hybrid electron configuration

Alkenes

• Compounds that contain double bonds.

HEthylene (Ethene)

Structure and Bonding

• A double bond is sp2 hybridized (pi bond)

Structure of Alkynes

C CH H

sp Hybridization

sp Hybrid electron configuration

POLAR COVALENT BONDS

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

Pauling Electronegativity Scale

Electronegativity

• Increases from left to right and bottom to top (decreases going down a group)

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

H H Cl Cl O O

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+

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.

H Fδ+ δ-

Polar vs Ionic Bonds

A A A B A Bδ+ δ-

Covalent Ionicpolar covalent

δEN >2 IONIC BONDSδEN <2 COVALENT BONDS

RESONANCE

Delocalization of Anion

• Spreading out charge is a stabilizing affect - through pi-bonds

• Spreading out charge is a stabilizing affect - resonance has a large affect on acidity

More examples of resonance

Arrows are not Arrows

• In chemistry we use different kinds of arrows to indicate different things

Reaction Arrow A B

Equilibrium Arrow A B

Resonance Arrow A B

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

A BA B +

Dissociation and Combination

• Curved arrows can be used in a variety of situations

HH O +

H+ H Cl

FUNCTIONAL GROUPS

Functional Groups

• Reactive parts of a molecule that undergo chemical reactions

Functional Groups

• Alcohols, Amines, Aldehydes, Ketones, Carboxylic Acids and derivatives

SHAPES OF MOLECULES - Saving the world one molecule at a...

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METHANE MOLECULE