Representing Molecules. Bonding Chemical bonds are forces that cause a group of atoms to behave as a unit. Bonds result from the tendency of a system

Representing Representing MoleculesMolecules

BondingBonding

Chemical bonds are forces that cause Chemical bonds are forces that cause a group of atoms to behave as a unit. a group of atoms to behave as a unit.

Bonds result from the tendency of a Bonds result from the tendency of a system to seek its lowest possible energy.system to seek its lowest possible energy.

Bond breaking always requires Bond breaking always requires energy, and bond formation always energy, and bond formation always releases energy.releases energy.

Types of BondsTypes of Bonds

The type of bonding depends upon The type of bonding depends upon the nature of the atoms that are the nature of the atoms that are combined.combined.

A metal and a non-metal will form A metal and a non-metal will form ionic bondsionic bonds when electrons are when electrons are transferred from the metal to the non-transferred from the metal to the non-metal. The resulting attraction between metal. The resulting attraction between oppositely charged ions forms a stable oppositely charged ions forms a stable crystal. crystal.


When metals bond with each When metals bond with each other, the valence electrons are other, the valence electrons are shared by the atoms in the entire shared by the atoms in the entire crystal. The electrons are no longer crystal. The electrons are no longer associated with a specific nucleus, associated with a specific nucleus, and are free to move throughout the and are free to move throughout the sample.sample.

Lewis StructuresLewis Structures

Lewis StructuresLewis Structures, also known as , also known as Lewis dot diagrams, show how the Lewis dot diagrams, show how the valence electrons are arranged valence electrons are arranged among the atoms in the molecule. among the atoms in the molecule.

For ionic compounds, it shows For ionic compounds, it shows the end result when the metal loses the end result when the metal loses its electrons to the non-metal.its electrons to the non-metal.

Covalent compounds exist as discrete molecules, whereas ionic compounds consist of an aggregate of cations and anions.

Covalent BondsCovalent Bonds

When two (or more) non-metals When two (or more) non-metals form bonds, electrons are shared. form bonds, electrons are shared. The result is a The result is a covalent bondcovalent bond..

Covalent bonds form because Covalent bonds form because the attraction of electrons for the the attraction of electrons for the nuclei in the atoms is greater than nuclei in the atoms is greater than the electron-electron repulsion or the electron-electron repulsion or the nucleus-nucleus repulsion.the nucleus-nucleus repulsion.


There is There is usually an usually an optimum bond optimum bond length or length or internuclear internuclear distance where distance where attractions attractions between electrons between electrons and the nuclei are and the nuclei are optimized and optimized and repulsions are repulsions are minimized.minimized.

Covalent BondingCovalent Bonding

Bond EnergyBond Energy

Bond formation releases energy, and bond breaking requires energy.

Types of Covalent BondsTypes of Covalent Bonds

Atoms bonded together may Atoms bonded together may share one, two or three pairs of share one, two or three pairs of electrons to make single, double or electrons to make single, double or triple bonds. triple bonds.

Double and triple bonds are Double and triple bonds are stronger and shorter than single stronger and shorter than single bonds between the same atoms. bonds between the same atoms.


When atoms of the same element When atoms of the same element form a covalent bond, the electrons are form a covalent bond, the electrons are shared equally. Such a bond is called shared equally. Such a bond is called non-polarnon-polar, or , or pure covalentpure covalent..

All homonuclear diatomic All homonuclear diatomic molecules contain non-polar bonds. molecules contain non-polar bonds.

ElectronegativityElectronegativity

An An electronegativity scaleelectronegativity scale, , developed by Linus Pauling (1901-developed by Linus Pauling (1901-1995), is used to predict the 1995), is used to predict the direction of the polarity of bonds. direction of the polarity of bonds.

Electronegativity is a relative Electronegativity is a relative scale that reflects the ability of an scale that reflects the ability of an atom to attract the electrons in a atom to attract the electrons in a bond.bond.


The small atoms in the upper The small atoms in the upper right corner of the table, having high right corner of the table, having high values of Zvalues of Zeffeff, also have high , also have high electronegativity values. Fluorine has electronegativity values. Fluorine has the highest value at 4.0.the highest value at 4.0.

The noble gases generally do not The noble gases generally do not form compounds, and are not given form compounds, and are not given electronegativity values.electronegativity values.

ElectronegativityElectronegativityThe large metal atoms in the lower The large metal atoms in the lower

left hand corner of the periodic table left hand corner of the periodic table have the lowest electronegativity values.have the lowest electronegativity values.


ElectronegativityElectronegativityNote that hydrogen has an Note that hydrogen has an

electro-negativity value of 2.1, electro-negativity value of 2.1, consistent with non-metals.consistent with non-metals.


When atoms of different elements When atoms of different elements form a covalent bond, the electrons form a covalent bond, the electrons often are not shared equally. The often are not shared equally. The electrons in the bond may spend, on electrons in the bond may spend, on average, more time on one of the atoms. average, more time on one of the atoms. This atom will have a slightly negative This atom will have a slightly negative charge, indicated by the symbol charge, indicated by the symbol δδ––. .

The other atom will be slightly The other atom will be slightly positive, indicated as positive, indicated as δδ++. .


Covalent bonds with unequal sharing Covalent bonds with unequal sharing of the electrons in the bond are called of the electrons in the bond are called polar bonds. polar bonds.

An example is the molecule HF. The An example is the molecule HF. The electrons spend more time on fluorine than electrons spend more time on fluorine than on hydrogen. As a result, HF is a polar on hydrogen. As a result, HF is a polar molecule.molecule.

H FH F

δ+ δ-

Dipole MomentDipole MomentPolar molecules have a Polar molecules have a dipole momentdipole moment. . This is a measure of the tendency of a This is a measure of the tendency of a molecule to line up in an electric field.molecule to line up in an electric field.

Dipole MomentDipole Moment

Dipole moment, μ, depends Dipole moment, μ, depends upon the size of the partial charges upon the size of the partial charges and the distance between the and the distance between the charges. It is measured in Debye charges. It is measured in Debye (D). A positive charge and a (D). A positive charge and a negative charge separated by 100 negative charge separated by 100 pm has a dipole moment of 4.80 D.pm has a dipole moment of 4.80 D.

Polarity of MoleculesPolarity of Molecules

Molecules with polar bonds Molecules with polar bonds maymay be be polar, having a permanent dipole moment. polar, having a permanent dipole moment. Both the polarity of the bonds and the Both the polarity of the bonds and the shape of the molecule must be considered.shape of the molecule must be considered.

Once the Lewis structure (dot Once the Lewis structure (dot diagram) for the molecule has been diagram) for the molecule has been determined, it is possible to predict the determined, it is possible to predict the shape of the molecule and its polarity.shape of the molecule and its polarity.

Polar MoleculesPolar Molecules

Lewis Structures – Covalent Lewis Structures – Covalent MoleculesMolecules

Once the Lewis structure has been Once the Lewis structure has been obtained, the Valence Shell Electron Pair obtained, the Valence Shell Electron Pair Repulsion approach can be used to predict Repulsion approach can be used to predict the shape of the molecule and its polarity.the shape of the molecule and its polarity.

COCO22 has polar carbon to oxygen bonds. has polar carbon to oxygen bonds. Even though the bonds are polar, the Even though the bonds are polar, the molecule is non-polar because of its linear molecule is non-polar because of its linear shape.shape.


Since COSince CO22 is a linear molecule, is a linear molecule, the dipoles cancel out, and the the dipoles cancel out, and the moleculemolecule is non-polar. is non-polar.


Water also has polar bonds, and a bent Water also has polar bonds, and a bent shape. As a result, water is a polar shape. As a result, water is a polar molecule.molecule.


The polarity of molecules has a The polarity of molecules has a profound effect on the properties profound effect on the properties and behavior of a substance. It will and behavior of a substance. It will affect solubility, melting and boiling affect solubility, melting and boiling points, and other important aspects points, and other important aspects of molecular behavior.of molecular behavior.

The Continuum of Bond The Continuum of Bond TypesTypes

Metal and a non-metal

Between non-metals


In many covalent molecules, the In many covalent molecules, the non-metals share valence electrons. non-metals share valence electrons. Electrons are shared so that each atom Electrons are shared so that each atom in the molecule has a full valence shell.in the molecule has a full valence shell.

HH.. ..HH

HH::H or HH or H....H or HH or H--H H

Each hydrogen has access to two Each hydrogen has access to two electrons, as does the noble gas helium. electrons, as does the noble gas helium.


For elements in period 2, the For elements in period 2, the non-metals generally share enough non-metals generally share enough valence electrons so that each atom valence electrons so that each atom obtains the same number of valence obtains the same number of valence electrons as neon (a total of eight electrons as neon (a total of eight electrons). electrons).

This may involve making This may involve making multiple (double or triple) bonds multiple (double or triple) bonds between atoms.between atoms.


Provide Lewis structures for Provide Lewis structures for elemental nitrogen, oxygen and elemental nitrogen, oxygen and fluorine.fluorine.

Exceptions to the Octet Exceptions to the Octet “Rule”“Rule”

The elements B and Be sometimes The elements B and Be sometimes form compounds with less than four form compounds with less than four electrons pairs on them. The are called electrons pairs on them. The are called electron deficient, and are often highly electron deficient, and are often highly reactive.reactive.

The elements in period 3 and below The elements in period 3 and below maymay accommodate more than four accommodate more than four electron pairs. electron pairs.

Exceptions to the Octet Exceptions to the Octet “Rule”“Rule”

In addition, some molecules, In addition, some molecules, such as NO or NOsuch as NO or NO22, have an odd , have an odd number of electrons and do not obey number of electrons and do not obey the octet rule.the octet rule.

Practice Practice

Write Lewis dot diagrams for CO, Write Lewis dot diagrams for CO, NHNH33 and CO and CO22

ResonanceResonance

Some molecules may have more Some molecules may have more than one valid Lewis structure. than one valid Lewis structure. These structures differ in the These structures differ in the placement of multiple bonds. placement of multiple bonds.

In molecules with resonance, In molecules with resonance, none of the Lewis structures none of the Lewis structures accurately represents the true accurately represents the true bonding in the molecule. bonding in the molecule.

ResonanceResonance

The molecule SOThe molecule SO22 has two has two resonance structures:resonance structures:

[ O=S-O: ] ↔ [ :O-S=O][ O=S-O: ] ↔ [ :O-S=O]

The molecule has two The molecule has two equivalentequivalent bonds between sulfur and oxygen.bonds between sulfur and oxygen.

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ResonanceResonance

The sulfur-oxygen bonds are The sulfur-oxygen bonds are identical- longer than double bonds, identical- longer than double bonds, and shorter than single bonds.and shorter than single bonds.

[ O=S-O: ] ↔ [ :O-S=O][ O=S-O: ] ↔ [ :O-S=O]

The true structure of the The true structure of the molecule is in between the two Lewis molecule is in between the two Lewis structures drawn.structures drawn.

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Formal ChargesFormal Charges

Formal charge is a way to keep Formal charge is a way to keep track of the electrons in a covalent track of the electrons in a covalent molecule. The formal charges can molecule. The formal charges can also be used to determine if one also be used to determine if one Lewis structure is more valid than Lewis structure is more valid than another.another.


The formal charge on an atom is The formal charge on an atom is a comparison between the number a comparison between the number of valence electrons on each atom of valence electrons on each atom and the number of electrons it has in and the number of electrons it has in the Lewis structure.the Lewis structure.


Consider the ion SCNConsider the ion SCN-1-1. There . There are three valid Lewis structures for are three valid Lewis structures for the ion.the ion.

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:]

Formal charges can be used to Formal charges can be used to determine the major contributor(s) to determine the major contributor(s) to the actual structure of the ion.the actual structure of the ion.

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Divide the bonds in half and Divide the bonds in half and determine the number of electrons determine the number of electrons on each atom.on each atom.

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:] 6e 4e 6e6e 4e 6e 7e 4e 5e 7e 4e 5e 5e 5e

4e 7e 4e 7e

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-1

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Compare the number of electrons in Compare the number of electrons in the structure to the number of valence the structure to the number of valence electrons.electrons.

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:] 6e 4e 6e6e 4e 6e 7e 4e 5e 7e 4e 5e 5e 4e 7e 5e 4e 7e

6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e

: : ::

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-1

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The net charge is the formal charge The net charge is the formal charge on each atom.on each atom.

0 0 -10 0 -1 -1 0 0 +1 0 -1 0 0 +1 0 -2 -2

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:] 6e 4e 6e6e 4e 6e 7e 4e 5e 7e 4e 5e 5e 4e 7e 5e 4e 7e 6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e

: : ::

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The net charge is the formal The net charge is the formal charge on each atom.charge on each atom.

0 0 -10 0 -1 -1 0 0 +1 -1 0 0 +1 0 -2 0 -2

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:] The sum of the formal charges The sum of the formal charges

must equal the charge on the ion.must equal the charge on the ion.

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There are two rules used to There are two rules used to determine the most likely Lewis determine the most likely Lewis structure(s).structure(s).

1. Atoms try to achieve formal 1. Atoms try to achieve formal charges as close to zero as charges as close to zero as possible.possible.

2. Any negative formal charges 2. Any negative formal charges should reside on the most should reside on the most electronegative atoms.electronegative atoms.


The third Lewis structure is The third Lewis structure is unlikely, due to the high formal charge unlikely, due to the high formal charge on nitrogen.on nitrogen.

0 0 -10 0 -1 -1 0 0 +1 0 -1 0 0 +1 0 -2 -2

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:] Since nitrogen is more Since nitrogen is more

electronegative than sulfur, the first electronegative than sulfur, the first structure should be the major structure should be the major contributor.contributor.

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The actual molecule will be The actual molecule will be somewhere in between the first and somewhere in between the first and second structures.second structures.

0 0 -10 0 -1 -1 0 0 +1 0 -1 0 0 +1 0 -2 -2

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:]

The sulfur-carbon bond should be The sulfur-carbon bond should be slightly longer than a double bond, and slightly longer than a double bond, and the carbon-nitrogen bond should be the carbon-nitrogen bond should be slightly shorter than a double bond.slightly shorter than a double bond.

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Representing Molecules. Bonding Chemical bonds are forces that cause a group of atoms to behave as a unit. Bonds result from the tendency of a system