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Mr. Shields Regents Chemistry U10 L02

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The COVALENT range between 0 & 1.7 can also beFurther Segmented:

If the difference is ≤0.5 the bond is NON-POLAR

If the difference is 0.6 – 1.7 the bond is said to be POLAR

Bond vs. Electronegativity DifferencesBond vs. Electronegativity Differences

We know that if the EN difference between 2 atoms in a bond is less than 1.7 then the bond is covalent

C H EN = 0.5covalent

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3.2

2.1

Ionic

Most Ionic –Least covalent

Least Ionic -Most covalent

Molecules with differences0.5 or less are said to be Non-polar Covalent

0.5

Polar Covalent RegionPolar Covalent Region

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A hydrogen molecule forms a covalent bond inA hydrogen molecule forms a covalent bond inWhich the electrons are Which the electrons are equally sharedequally shared by by bothbothatomsatoms

This is the basis of a This is the basis of a non-polar bondnon-polar bond

Non-polar covalentNon-polar covalent

Electron cloudElectron cloud

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Polar BondPolar Bond

When one atom in a bond is much more electronegativeThan the other it pulls electron density towards it

DistortedDistortedElectron cloudElectron cloud

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The atom that pulls electron density towards it morestrongly becomes partially negative while the otheratom becomes partially positive

2.1 - 3.0 = 0.9

Br

A Polar Covalent Compound

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Polarity of WaterPolarity of Water

The electronegativity of O = 3.5The electronegativity of H = 2.1

The difference is 1.4; the O-H bond is Polar covalent

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WaterWaterWater is polar because electron density is pulled moreStrongly towards the Oxygen atom than the Hydrogen atom.

This makes OxygenPartially negativeAnd the less Electronegative H Partially positive so…

There are 2 POLAR Covalent Bonds formed

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Bond Polarity vs. Molecular polarityBond Polarity vs. Molecular polarity

If two atoms form a polar bond you might assumethe molecule is polar.However this is not always the case! Let’s see why…

Recall that a polar bondMust have 2 ends withOpposite charge.

Let’s look at the waterMolecule first…

Negative end

Positive end

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Polarity vs. SymmetryPolarity vs. Symmetry

Negative end

Positive end

Water is polar due to 2 effects:

1)It has polar covalent O-H bonds

2) The molecule has a BENT geometric shape.

The bent shape gives it anAsymmetry that allowstwo opposite polar endsTo develop.

Polar bondsPolar bondsPolar moleculePolar molecule

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Carbon DioxideCarbon Dioxide

The 2 C = O bonds in COThe 2 C = O bonds in CO22 are in fact are in fact polar covalentpolar covalent,,Just like water (what’s the electronegativity difference?)Just like water (what’s the electronegativity difference?)

Now let’s look at Now let’s look at Carbon DioxideCarbon Dioxide..

Because the electronegativity difference is >0.5 weBecause the electronegativity difference is >0.5 weexpect the COexpect the CO22 molecule to be polar … molecule to be polar …

But it isn’t. How Come ????But it isn’t. How Come ????

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CO2 is actually a linearmolecule.

As a linear molecule itHas a symmetry throughCarbon.

That means that the endsOf the molecule are BOTHNegative and there is NODefined Positive END

It’s this symmetry that makesThe molecule NON-POLAR Instead of POLAR.

:O = C = O:. . . .

- + -

VECTORS CANCELVECTORS CANCEL

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Other examples of symmetry Other examples of symmetry related non-Polar moleculesrelated non-Polar molecules

What is the electronegativity difference between C What is the electronegativity difference between C and Cl?and Cl?

Would it form a polar or non-polar Bond?Would it form a polar or non-polar Bond?

Based on this is Based on this is CClCCl44 a polar or non-polara polar or non-polarMolecule (it has 4 C-Cl bonds)?Molecule (it has 4 C-Cl bonds)?

CClCCl44 is in fact a non-polar molecule. Let’s see why is in fact a non-polar molecule. Let’s see why

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CClCCl44 has the shape of a has the shape of a TetrahedronTetrahedron (Like many C-H (Like many C-HContaining Compounds such as methane)Containing Compounds such as methane)

Notice the Notice the symmetrysymmetry of these molecules. When each of these molecules. When eachIs rotated is looks the Is rotated is looks the samesame from any angle. from any angle.

Due to this symmetry there is no negative end, noDue to this symmetry there is no negative end, noPositive end and the molecule is Positive end and the molecule is NON-POLARNON-POLAR..

H

CH H

H

MethaneCl-

C+Cl- Cl-

Cl-

Carbontetrachloride(w/partial charges)

CHCH44CClCCl44

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SummarySummary1.1.Molecules containing only Molecules containing only non-polar bondsnon-polar bonds are are always always Non-polarNon-polar

examples:examples: OO22, N, N22, I, I2, 2, CHCH44

2. Molecules containing 2. Molecules containing polar bondspolar bonds with with symmetricalsymmetrical chargecharge distributions are distributions are Non-polarNon-polar

examples: COexamples: CO22, CCl, CCl44

3. Molecules containing 3. Molecules containing polar bondspolar bonds with with asymmetricalasymmetrical chargecharge distributions are distributions are PolarPolar

examples: HF, Hexamples: HF, H22O, NHO, NH33, CH, CH33ClCl

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Coordinate Covalent BondsCoordinate Covalent Bonds

Recall…A covalent bond is formed between two atoms,EACH donating 1 electron to form a shared pair.

When one of the two atoms donates BOTH electronsTo form the shared pair the bond is calledCOORDINATE COVALENT.

This type of bond is found in many POLYATOMIC IONS (but not exclusively)

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Coordinate Covalent BondsCoordinate Covalent Bonds

Let’s look at some example of molecules with coordinateCovalent bonds and how they’re formed?

Carbon Monoxide

Ammonium ion

H O:H H

+

Hydronium ion

Oxygen contributes electron pair Nitrogen contributes electron pair

Oxygen contributes electron pair

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Formation of a Coordinate Covalent bond

Electron pairDonated solelyBy Oxygen

The added Proton to theWater moleculeleads toThe positive charge

A polyatomic:A polyatomic:The hydronium ionThe hydronium ion

Std Polar covalent bondsStd Polar covalent bondsIn the water moleculeIn the water molecule

HH220 + H0 + H++ H H33OO++

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2. Draw the Lewis dot structure for this compound 2. Draw the Lewis dot structure for this compound

3. What kind of covalent bonds are present?3. What kind of covalent bonds are present?

Problem: Problem:

1. Ammonium Chloride is an ionic compound. 1. Ammonium Chloride is an ionic compound. Why?Why?

NHNH44ClCl

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Network SolidsNetwork Solids

Some atoms are able to form multiple repeatingcovalent bonds with themselves.

- for example: carbon

Certain other atoms can also form multiple repeatingcovalent bonds with another atom

- for example: Si and O or B and N,

The result is a three dimensional structure known as aNETWORK SOLID

Let’s look at some examples.

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Diamond is an example of a network solid

A highly interconnected carbonCovalent Structure consistingof Hexagonal rings

1 carbon 1 carbon Bonded toBonded to4 others4 others

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- Different combinations or geometric structures of theDifferent combinations or geometric structures of the same element or molecule are known as same element or molecule are known as ALLOTROPESALLOTROPES

- - Examples of allotropes: Examples of allotropes: - Diamond, Graphite, Coal, buckyballs- Diamond, Graphite, Coal, buckyballs - O- O22, O, O3 3 These are are allotopes but These are are allotopes but

not network solidsnot network solids

- Allotropes have - Allotropes have differentdifferent physical and chemical physical and chemical properties. properties.

AllotropesAllotropes

- Carbon can covalently bond with itself in several- Carbon can covalently bond with itself in several different geometric structures different geometric structures

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GraphiteGraphite is an is an allotropeallotrope of diamond and like diamond of diamond and like diamondit consists of connected hexangonal rings. It is also ait consists of connected hexangonal rings. It is also anetwork solidnetwork solid

Unlike diamond however, these rings form layers thatUnlike diamond however, these rings form layers thatare are not connectednot connected and can slide across one another. and can slide across one another.

It’s this structure that givesIt’s this structure that givesgraphite its lubricating quality &graphite its lubricating quality & ElectricalElectrical ConductivityConductivity

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4 Other Examples of Network 4 Other Examples of Network SolidsSolids

SiCSiC

BNBN SiOSiO22

SiSi33NN44

QuartzQuartzMP 1650 CMP 1650 C

A Lubricant likeA Lubricant likeGraphiteGraphiteMP 2967 CMP 2967 C

ExtremelyExtremelyHardHardMP 2700 CMP 2700 C

MPMP1900 C1900 C

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Network solidsNetwork solids

PROPERTIES:

Extraordinarily high melting points Very hard Not soluble in polar or non-polar liquids Non conductors of electricity in either solid or liquid state

Notable Exception: Graphite does conduct electricity!