Bonding & Molecular ShapesBonding & Molecular Shapes

Dr. Ron RusayDr. Ron Rusay

Spring 2004Spring 2004

© Copyright 2004 R.J. Rusay© Copyright 2004 R.J. Rusay

Chemical BondsChemical Bonds

Definition:Definition:Attraction between atoms that hold them together. Attraction between atoms that hold them together. Bonds provide a particular arrangement of atoms in a Bonds provide a particular arrangement of atoms in a molecule which provides it with new chemical molecule which provides it with new chemical properties.properties. QuickTime™ and aGIF decompressorare needed to see this picture.

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Table salt: Sodium chloride Table salt: Sodium chloride Water Water

Ionic vs. Covalent BondingIonic vs. Covalent Bonding

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Electron Configurations Electron Configurations Noble Gases and The Rule of EightNoble Gases and The Rule of Eight

A nonmetal and a metal A nonmetal and a metal react to form an react to form an ionic compound: Valence electrons of ionic compound: Valence electrons of the metal are the metal are lostlost and the nonmetal and the nonmetal gainsgains these electrons. (Ionic Bonding) these electrons. (Ionic Bonding)

When When two nonmetals two nonmetals react: They react: They shareshare electrons to achieve a Noble Gas electrons to achieve a Noble Gas Configuration. (Covalent Bonding)Configuration. (Covalent Bonding)

Where are metals and non-metals located in theWhere are metals and non-metals located in thePeriodic Table?Periodic Table?

Lewis Electron-Dot Symbols for Elements in Periods 2 & 3

How many valence electrons does each How many valence electrons does each element have in each of the vertical columns element have in each of the vertical columns (families)?(families)?

Ionic BondsIonic Bonds

Result from electrostatic (positive-negative) Result from electrostatic (positive-negative) attractions of closely packed, attractions of closely packed, oppositely oppositely charged ionscharged ions..

Form when an atom which can easily Form when an atom which can easily lose lose electronselectrons (low electronegativity) (low electronegativity) reacts with reacts with one which can easily one which can easily gain electronsgain electrons (high (high electronegativity)electronegativity)..

Na and Cl; Fe and ONa and Cl; Fe and O

Covalent Bond LengthsCovalent Bond Lengths The The distance distance between atoms.between atoms. It is the distance that provides It is the distance that provides

the most stable arrangement of the the most stable arrangement of the atoms where the bond energy is at atoms where the bond energy is at a minimum.a minimum.

Covalent Bonding &Covalent Bonding &Bond LengthBond Length

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Bond Lengths and Covalent Radius

The Periodic TableThe Periodic TableBond Lengths : Bond StrengthsBond Lengths : Bond Strengths

Actual vs. Expected Bond LengthActual vs. Expected Bond Length

Actual = 0.127 nm Actual = 0.127 nm Expected = 0.136 nmExpected = 0.136 nm

ElectronegativityElectronegativity

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ElectronegativityElectronegativity

The ability of an atom in a The ability of an atom in a molecule to attract shared electrons molecule to attract shared electrons to itself.to itself.

= (H = (H X) X)actualactual (H (H X) X)expectedexpected

Periodic TrendPeriodic Trend

Electronegativity Differences &Electronegativity Differences &Polar Covalent BondsPolar Covalent Bonds

A molecule with a relatively high A molecule with a relatively high difference in electronegativities, such as difference in electronegativities, such as HF, has a center of positive charge and HF, has a center of positive charge and a center of negative charge. It is a center of negative charge. It is polarpolar, , having an experimentally measureable having an experimentally measureable dipole momentdipole moment..δ+ δ−FH

The spectrum of bond The spectrum of bond differences: Ionic at one differences: Ionic at one extreme : Non-polar extreme : Non-polar covalent at the other.covalent at the other.

Bond ShapesBond Shapes

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Lewis StructureLewis Structure

Shows how valence electrons are Shows how valence electrons are arranged among atoms in a molecule.arranged among atoms in a molecule.

Reflects central idea that stability of a Reflects central idea that stability of a compound relates to noble gas electron compound relates to noble gas electron configuration.configuration.

Lewis Structure of Water Lewis Structure of Water

Fundamental BondingFundamental BondingPatternsPatterns

Carbon has a total of four bonds:Carbon has a total of four bonds:• 4 single bonds4 single bonds• 2 single bonds plus 1 double bond 2 single bonds plus 1 double bond • 1 single bond plus 1 triple bond 1 single bond plus 1 triple bond

Oxygen has a total of 2 bonds plus 2 Oxygen has a total of 2 bonds plus 2 “free pairs of electrons”:“free pairs of electrons”:• 2 single bonds2 single bonds• 1 double bond 1 double bond

Fundamental BondingFundamental BondingPatternsPatterns

Nitrogen has a total of three bonds plus Nitrogen has a total of three bonds plus 1 “free pair of electrons”: 1 “free pair of electrons”: • 3 single bonds3 single bonds• 1 single bonds plus 1 double bond 1 single bonds plus 1 double bond • 1 triple bond1 triple bond

Hydrogen only has 1 single bond.Hydrogen only has 1 single bond. This is the total of almost all bonding This is the total of almost all bonding

arrangements in organic molecules. arrangements in organic molecules.

Lewis Structures of Simple Lewis Structures of Simple MoleculesMolecules

N

H

H H. .

AmmoniaC

NN

O

H H

HH

. .

. .

. .

. .

Urea

Carbonate Ion COCarbonate Ion CO332-2- : :

The carbonate polyatomic ion has two coordinate bonds, The carbonate polyatomic ion has two coordinate bonds, resonance forms and two electrons from a cation!resonance forms and two electrons from a cation!

The Lewis Structure for Sulfur trioxideThe Lewis Structure for Sulfur trioxide

Where to draw the sulfur-Where to draw the sulfur-oxygen double bond? There oxygen double bond? There are three options.are three options.

Simplified drawings without free pairs of electrons:Simplified drawings without free pairs of electrons:

Experimental data shows that each of the Experimental data shows that each of the three sulfur-oxygen bonds are the same three sulfur-oxygen bonds are the same length.length.

ResonanceResonance Occurs when more than one valid Occurs when more than one valid

Lewis structure can be written for a Lewis structure can be written for a particular molecule.particular molecule.

These are These are resonance structuresresonance structures. . The actual structure is an average of the The actual structure is an average of the resonance structures.resonance structures.

Resonance: Delocalized Electron-Pairs

Ozone : O3 ......

..

O O

O ..

........ ..OOO ....

I II

O

O

O

..

........

Resonance Hybrid Structure

One pair of electron’s resonates between the two locations!!

Ozone and CFCsOzone and CFCs

CCl2F2

Freon 12..

..

.. C

Cl

FF

Cl

......

..

.. ..

..

....

O

O

O......

VSEPR ModelVSEPR ModelValence Shell Electron Pair RepulsionValence Shell Electron Pair Repulsion

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VSEPR ModelVSEPR Model

The molecular structure which The molecular structure which surrounds a given atom is surrounds a given atom is determined determined principallyprincipally by by minimizing electron pair minimizing electron pair repulsions through maximizing repulsions through maximizing separations.separations.

Molecular ModelsMolecular Models

Computer Generated ModelsComputer Generated Models

Ball and stick models of ammonia, water and Ball and stick models of ammonia, water and methane. For many others see:methane. For many others see:

http://http://epep..llnlllnl..govgov//msdsmsds//pdbpdb//

http://http://epep..llnlllnl..govgov//msdsmsds//orgchemorgchem/Chem226/Smell-/Chem226/Smell-StereochemStereochem.html.html