Bonding Ch 8. Objectives SWBAT identify the bond type of a molecule by using electronegativity differences. SWBAT identify the polarity in small molecules.

BondingBonding

Ch 8Ch 8

ObjectivesObjectives

SWBAT identify the bond type of a SWBAT identify the bond type of a molecule by using electronegativity molecule by using electronegativity differences.differences.

SWBAT identify the polarity in small SWBAT identify the polarity in small molecules.molecules.

Types of BondingTypes of Bonding

IonicIonic

CovalentCovalent

MetallicMetallic

Octet RuleOctet Rule

Atoms tend to gain, lose or share Atoms tend to gain, lose or share electrons until they are electrons until they are surrounded by 8 valence surrounded by 8 valence electrons.electrons.

Ionic bondingIonic bonding

Transfer of electrons from the metal to Transfer of electrons from the metal to the non-metal.the non-metal.Formation of cations and anionsFormation of cations and anions

Na + Cl Na+ + Cl-

Ionic BondingIonic Bonding

When an extremely electronegative atom, When an extremely electronegative atom, like fluorine, bonds with an electropositive like fluorine, bonds with an electropositive atom, like sodium, the resulting bond is atom, like sodium, the resulting bond is ionic due to the huge difference in ionic due to the huge difference in electronegativity (difference > ~1.7). electronegativity (difference > ~1.7).

Ionic BondingIonic Bonding

The electronegative atom's pull on the The electronegative atom's pull on the bonding electrons is so strong that it pulls bonding electrons is so strong that it pulls the bonding electron off the electropositive the bonding electron off the electropositive atom resulting in two oppositely charged atom resulting in two oppositely charged ions which are held together by ions which are held together by electrostatic attraction (an electrostatic attraction (an ionic bondionic bond).).

Energetics of Ionic Bond FormationEnergetics of Ionic Bond Formation

If you look in Appendix C (If you look in Appendix C (∆H∆Hff values) you values) you will find that the heat of formation for ionic will find that the heat of formation for ionic compounds is exothermic.compounds is exothermic.

The removal (or loss) of electrons is The removal (or loss) of electrons is always an endothermic process. always an endothermic process.

When a non-metal gains an electron the When a non-metal gains an electron the process is generally exothermic.process is generally exothermic.

Lattice EnergyLattice Energy

The principal reason that ionic compounds The principal reason that ionic compounds are stable is the attraction between ions of are stable is the attraction between ions of unlike charge.unlike charge.

Lattice energy – is the energy required to Lattice energy – is the energy required to completely separate a mole of a solid ionic completely separate a mole of a solid ionic compound into its gaseous ions.compound into its gaseous ions.

ΔH°ΔH°ff [NaCl(s)] = ΔH° [NaCl(s)] = ΔH°ff [Na(g)] + ΔH° [Na(g)] + ΔH°ff [Cl(g)] [Cl(g)]

+ I+ I11(Na) + E(Cl) - ΔH(Na) + E(Cl) - ΔHlatticelattice

Lattice Energy ProblemsLattice Energy Problems

Look at the example on B&L page 267Look at the example on B&L page 267

We will go through the example as a We will go through the example as a group.group.

Try problems # 19 and 20 at the end of the Try problems # 19 and 20 at the end of the chapter.chapter.

New Book P. 308New Book P. 308

Try 27, 28Try 27, 28

Covalent BondingCovalent Bonding

Bonding between non-metals consists of Bonding between non-metals consists of two electrons shared between two atoms. two electrons shared between two atoms.

In covalent bonding, the two electrons In covalent bonding, the two electrons shared by the atoms are attracted to the shared by the atoms are attracted to the nucleus of both atoms. Neither atom nucleus of both atoms. Neither atom completely loses or gains electrons as in completely loses or gains electrons as in ionic bonding.ionic bonding.

http://www.elmhurst.edu/~chm/vchembook/152Apolar.htmlhttp://www.elmhurst.edu/~chm/vchembook/152Apolar.html

Types of Covalent BondingTypes of Covalent Bonding

Polar Covalent BondingPolar Covalent Bonding

results when two results when two different non-metals different non-metals unequally shareunequally share electrons between them. electrons between them.

Non-Polar Covalent BondingNon-Polar Covalent Bonding

results when two results when two identical non-metals identical non-metals equally shareequally share electrons between them. electrons between them.

Non-polar covalent bondsNon-polar covalent bondsIdentical non-metallic atoms have identical Identical non-metallic atoms have identical electronegativityelectronegativity

For example in an H-HFor example in an H-H bond both H atoms have the bond both H atoms have the same electronegativity so the bond is non-polar same electronegativity so the bond is non-polar

Different atoms can have the same electronegativity, Different atoms can have the same electronegativity, such as, such as, N-Cl.N-Cl.

Both N and Cl have an electronegativity of 3.0 so the Both N and Cl have an electronegativity of 3.0 so the bonding electrons will be shared equally between the bonding electrons will be shared equally between the two atoms resulting in a non-polar covalent bond. two atoms resulting in a non-polar covalent bond.

Polar Covalent BondsPolar Covalent Bonds

When atoms of similar, but different, When atoms of similar, but different, electronegativities (a difference < ~1.7) bond, electronegativities (a difference < ~1.7) bond, the more electronegative atom has a greater the more electronegative atom has a greater share of the bonding electrons than the less share of the bonding electrons than the less electronegative atom. electronegative atom.

The more electronegative atom has a partial The more electronegative atom has a partial negative charge, and the less electronegative negative charge, and the less electronegative atom has a partial positive charge. The resulting atom has a partial positive charge. The resulting covalent bond is called a covalent bond is called a polar covalent bondpolar covalent bond. .

Show the ClassShow the Class

Show how to draw a partial positive and Show how to draw a partial positive and partial negative charge symbol.partial negative charge symbol.

Multiple BondsMultiple Bonds

The length of the bond decreases with The length of the bond decreases with increasing numbers of bonds.increasing numbers of bonds.

Comparison of Bonding TypesComparison of Bonding Types

Compare Ionic, Polar and Compare Ionic, Polar and Non-Polar BondingNon-Polar Bonding

ElectronegativityElectronegativity

ElectronegativityElectronegativity is the relative tendency of a is the relative tendency of a bonded atom to attract electrons to itself.bonded atom to attract electrons to itself.

An atom with extremely low electronegativity, An atom with extremely low electronegativity, like a Group I metal, is said to be like a Group I metal, is said to be electropositiveelectropositive since its tendency is to lose since its tendency is to lose rather than to gain, or attract, electrons. rather than to gain, or attract, electrons.

Non-metals are more electronegative than Non-metals are more electronegative than metals. metals. http://www.ausetute.com.au/bondpola.htmlhttp://www.ausetute.com.au/bondpola.html

Electronegativity Values Electronegativity Values and Bond Typesand Bond Types

Electronegativity values are useful in Electronegativity values are useful in determining if a bond is to be classified as determining if a bond is to be classified as non-polar covalent, polar covalent or ionic. non-polar covalent, polar covalent or ionic.

What you should do is look only at the two What you should do is look only at the two atoms in a given bond. Calculate the atoms in a given bond. Calculate the difference between their electronegativity difference between their electronegativity values. Only the absolute difference is values. Only the absolute difference is important. important. http://dbhs.wvusd.k12.ca.us/webdocs/Bonding/Electroneg-Bond-Polarity.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Bonding/Electroneg-Bond-Polarity.html

Electronegativity Electronegativity and Associated Bond Typeand Associated Bond Type

< 0.5 < 0.5 non-polar covalentnon-polar covalent0.5 – 1.90.5 – 1.9 polar covalent **polar covalent **2.0 or greater2.0 or greater ionicionic

Note: Note: To find the electronegativity To find the electronegativity values of many common values of many common elements, look at the chart printed on the elements, look at the chart printed on the bottom of your orbital diagram.bottom of your orbital diagram.

** there are some exceptions** there are some exceptions

Calculating Calculating Electronegativity ValuesElectronegativity Values

Calculate the electronegativity and determine Calculate the electronegativity and determine the bond type of an O-H bondthe bond type of an O-H bond

O has an electronegativity of 3.5O has an electronegativity of 3.5H has an electronegativity of 2.1H has an electronegativity of 2.1

The difference in electronegativity is:The difference in electronegativity is:3.5 - 2.1 = 1.43.5 - 2.1 = 1.4

1.4 is less than 1.7, so the resulting bond is polar 1.4 is less than 1.7, so the resulting bond is polar covalent.covalent.

AP Students Use This SlideAP Students Use This SlideHere are the rules: Here are the rules: 1.1. If the electronegativity difference (usually called If the electronegativity difference (usually called

ΔΔEN) is less than 0.5, then the bond is nonpolar EN) is less than 0.5, then the bond is nonpolar covalent.covalent.

2.2. If the If the ΔΔEN is between 0.5 and 1.6, the bond is EN is between 0.5 and 1.6, the bond is considered polar covalentconsidered polar covalent

3.3. If the If the ΔΔEN is greater than 2.0, then the bond is EN is greater than 2.0, then the bond is ionic. That, of course, leaves us with a problem. ionic. That, of course, leaves us with a problem. What about the gap between 1.6 and 2.0? So, rule What about the gap between 1.6 and 2.0? So, rule #4 is: #4 is:

4. If the 4. If the ΔΔEN is between 1.6 and 2.0 and if a metal is EN is between 1.6 and 2.0 and if a metal is involved, then the bond is considered ionic. If only involved, then the bond is considered ionic. If only nonmetals are involved, the bond is considered nonmetals are involved, the bond is considered polar covalent. polar covalent.

http://dbhs.wvusd.k12.ca.us/webdocs/Bonding/Electroneg-Bond-Polarity.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Bonding/Electroneg-Bond-Polarity.html

AP ExampleAP Example

Sodium bromide Sodium bromide

(formula = NaBr; EN(formula = NaBr; ENNaNa = 0.9, EN = 0.9, ENBrBr = 2.8) has a = 2.8) has a DEN = 1.9DEN = 1.9

Hydrogen fluoride Hydrogen fluoride

(formula = HF; EN(formula = HF; ENHH = 2.1, EN = 2.1, ENFF = 4.0) = 4.0)

has the same DEN. has the same DEN.

We use rule #4 to decide that NaBr has ionic We use rule #4 to decide that NaBr has ionic bonds and that HF has a polar covalent bond in bonds and that HF has a polar covalent bond in each HF molecule. each HF molecule.

Electronegativity TrendElectronegativity Trend

Electronegativity decreases down a Group in the Electronegativity decreases down a Group in the Periodic Table as the atomic radius and number Periodic Table as the atomic radius and number of inner electron shells both increase. of inner electron shells both increase.

Electronegativity increases across a Period of Electronegativity increases across a Period of the Periodic Table, in general, due to increasing the Periodic Table, in general, due to increasing nuclear charge and decreasing atomic radius. nuclear charge and decreasing atomic radius.

For the commonly encountered atoms in high For the commonly encountered atoms in high school science, the order in decreasing school science, the order in decreasing electronegativity is:electronegativity is:F > O > N ~ Cl > Br > C ~ S ~ I > P ~ H > Si F > O > N ~ Cl > Br > C ~ S ~ I > P ~ H > Si

Metallic BondingMetallic Bonding

Metallic bonding is characterized by a Metallic bonding is characterized by a

““sea of electrons.”sea of electrons.”

DipoleDipole

Polarity results from the uneven partial Polarity results from the uneven partial charge distribution between various atoms charge distribution between various atoms in a compound. in a compound. Atoms, such as nitrogen, oxygen, and Atoms, such as nitrogen, oxygen, and halogens, that are more electronegative halogens, that are more electronegative have a tendency to have partial negative have a tendency to have partial negative charges. charges. Atoms, such as carbon and hydrogen, Atoms, such as carbon and hydrogen, have a tendency to be more neutral or have a tendency to be more neutral or have partial positive charges. have partial positive charges.

DipoleDipole

Electrons in a polar covalent bond are Electrons in a polar covalent bond are unequally shared between the two bonded unequally shared between the two bonded atoms, which results in partial positive and atoms, which results in partial positive and negative charges. The separation of the negative charges. The separation of the partial charges creates a dipole. partial charges creates a dipole.

The word dipole means two poles: the The word dipole means two poles: the separated partial positive and negative separated partial positive and negative charges. A polar molecule results when a charges. A polar molecule results when a molecule contains polar bonds in an molecule contains polar bonds in an unsymmetrical arrangement.unsymmetrical arrangement.

DipoleDipole

Non-polar molecules are of two types.Non-polar molecules are of two types.

Molecules whose atoms have equal or Molecules whose atoms have equal or nearly equal electronegativities have zero nearly equal electronegativities have zero or very small dipole moments. or very small dipole moments.

A second type of nonpolar molecule has A second type of nonpolar molecule has polar bonds, but the molecular geometry is polar bonds, but the molecular geometry is symmetrical allowing the bond dipoles to symmetrical allowing the bond dipoles to cancel each other. cancel each other.

Dipole MomentDipole MomentThe dipole moment is a measure of the The dipole moment is a measure of the unevenness, or lack of symmetry, of the unevenness, or lack of symmetry, of the charge distribution in a molecule. charge distribution in a molecule. The mathematical definition of the dipole The mathematical definition of the dipole moment involves adding up the size of moment involves adding up the size of each charge in the molecule multiplied by each charge in the molecule multiplied by the average distance that charge is from the average distance that charge is from an arbitrary origin. an arbitrary origin.

www.chem.unsw.edu.au/.../dipolemoments.html

Dipole MomentDipole Moment

symbol for dipole momentsymbol for dipole moment

→→

Formal ChargeFormal Charge

This is a method for determining the most This is a method for determining the most probable structure of a molecule.probable structure of a molecule.

It is ideal to have a formal charge of 0 for It is ideal to have a formal charge of 0 for as many of the atoms as possible.as many of the atoms as possible.

Formal charge = [# of valence electrons] – Formal charge = [# of valence electrons] – [electrons in lone pairs + 1/2 the number [electrons in lone pairs + 1/2 the number of bonding electrons]of bonding electrons]

Formal ChargeFormal Charge

See page 280 in the “old” B & L textSee page 280 in the “old” B & L text

http://www.chemprofessor.com/bonding_files/image033.jpg

Try this websiteTry this website

http://www.mhhe.com/physsci/chemistry/http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch01lewis.htmlcarey/student/olc/ch01lewis.html