Bonding & Molecular ShapesBonding & Molecular Shapes

Dr. Ron RusayDr. Ron Rusay

Spring 2003Spring 2003

© Copyright 2003 R.J. Rusay© Copyright 2003 R.J. Rusay

Chemical BondsChemical Bonds

Definition:Definition:

Attractive forces which hold atoms Attractive forces which hold atoms together and provide a particular together and provide a particular molecular arrangement of atoms molecular arrangement of atoms with new chemical properties.with new chemical properties.

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)

Octet Rule: General CommentsOctet Rule: General Comments 2nd row elements C, N, O, F 2nd row elements C, N, O, F observe the octet observe the octet

rulerule..

2nd row elements B and Be often have fewer 2nd row elements B and Be often have fewer than 8 electrons around themselves - they are than 8 electrons around themselves - they are very reactive.very reactive.

3rd row and heavier elements 3rd row and heavier elements CANCAN exceed the exceed the octet rule using empty valence d orbitals.octet rule using empty valence d orbitals.

When writing Lewis structures, When writing Lewis structures, satisfy octets satisfy octets firstfirst,, then place electrons around elements then place electrons around elements having having available d orbitalsavailable d orbitals..

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

Isoelectronic IonsIsoelectronic Ions

Ions containing the the same number Ions containing the the same number of electrons are isoelectronic with a Noble of electrons are isoelectronic with a Noble GasGas

(O(O22, F, F, Na, Na++, Mg, Mg2+2+, Al, Al3+3+)) But the ion sizes are not all the sameBut the ion sizes are not all the same

OO22> F> F > Na > Na++ > Mg > Mg2+2+ > Al > Al3+3+

Ionic BondsIonic Bonds

Result from electrostatic attractions of Result from electrostatic attractions of closely packed, closely packed, oppositely charged ionsoppositely charged ions..

Form when an atom which can easily Form when an atom which can easily lose electrons lose electrons reacts with one which has reacts with one which has a a high electron affinityhigh electron affinity, that is, it can , that is, it can easily gain electrons.easily gain electrons.

Mg and Cl; K and OMg and Cl; K and O

Covalent Bond LengthsCovalent Bond Lengths Interatomic Interatomic distance.distance. It is the distance where the It is the distance where the

bond energy is at a minimum value, bond energy is at a minimum value, and which is the most stable atomic and which is the most stable atomic form.form.

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