11Regents Chemistry U10 L01

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Ionic Bonds involve a force of attraction that developsthrough…

ELECTRON TRANSFER

In covalent bond formation the bond is formed byELECTRON SHARING

Both lead to the formation of the octet. However, Theforce of attraction in Covalent compounds involves eachatom donating an electron to form a shared pair.

What is a covalent bond?What is a covalent bond?

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Covalent BondsCovalent BondsMany of the compounds we encounter in our dailyLives are covalently bonded compounds

- Antibiotics- Food- Plants- Plastics- Clothing (Natural & Synthetic)- Biochemicals- What we breath and what we are- And so many other things its impossible to list them all!

ampicillin

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Molecular BondsMolecular Bonds

Covalent Bonds fall into several sub-categories:

Non-polar CovalentPolar CovalentCoordinate CovalentNetwork Covalent

We’ll start our discussion with non-polar covalentbonds

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2 hydrogen AtomsApproach each other

1s orbitals overlap

Each atom shares itsElectron with theOther atom but…

Each atom thinks the shared electrons are theirs alone!

Formation of a non-polar Covalent Bond

EN < 0.4)

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This sharing of electrons creates a new bondBetween the atoms.

Electron density is concentrated in the region betweenThe atoms.

When 2 atoms approach the pt. of maximum attractiona SINGLE COVALENT BOND forms

Covalent Bond

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If electron density is equally shared across both atomsOne side of the molecule has no more electron densitythan the other side so no “poles “ develop

Non-polar

This type of bond is called…

Non-polar covalentNon-polar covalent

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Electronegativity differences Electronegativity differences Recall that bonds formed by atoms with electronegativityRecall that bonds formed by atoms with electronegativitydifferences are differences are greater than 1.7greater than 1.7 are said to be are said to be IONICIONIC

2 atoms with an electronegativity difference that is 2 atoms with an electronegativity difference that is lesslessthan 1.7than 1.7 will form a will form a COVALENTCOVALENT bond bond

When 2 atoms have an EN difference that is really low,Less than or equal to 0.4 these bonds are said to be…

Non-Polar CovalentNon-Polar Covalent

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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

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Electronegativity differencesElectronegativity differences

Between 0.4 and 1.7 there is a large number of Compounds having bonds known as

Polar Covalent

For example H2O 3.4 - 2.1 = 1.3

We’ll talk more about Polar compounds later.

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Multiple atom electron sharingMultiple atom electron sharing

In water, each hydrogen shares 1 electron with oxygen

Since oxygen initially has 6 electrons it has 2 pairsOf electrons that remain unshared

Unshared electron pairs

All atoms now have an octetAll atoms now have an octet

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Unshared electron pairs are called LONE PAIRS

Shared electron pairs are called SHARED PAIRS or BONDING PAIRS

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Nitrogen has a non-bonding pair of electrons But Methane has no non-bonding pairs

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Depicting bondsDepicting bonds

The most common way to depict bonds is not by Lewis dot structures.

Instead, a line is drawn for each pair of sharedelectrons

: =

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H

CH H

H

CH4

METHANE

Carbons S and P orbitalsOverlap with Hydrogen’sS orbital to form aHybrid orbital known as a Sigma bond. Its shape is Unique – the tetrahedron

4 sigma bonds(4 single covalent bonds)

How many bonding pairs andLone pairs are there in CH4

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**P

Cl ClCl

Lone pairs are notConnected to theParent ion with a line.

The electrons areindicated using 2 “dots”

Phosphorus’ lone pair

How would you draw theWater molecule?

Lewis dot structure

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Multiple bondsMultiple bonds

So far we have looked at atoms sharing one pair ofElectrons to form a single covalent bond

It is however possible for atoms to share more than a single pair of electrons

Oxygen (O2), for example, does this. Let’s see how.

Oxygen: 6e-

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Multiple bondsMultiple bondsOxygen’s oxidation number isThis means it has electrons in it’s valence shell

To achieve the octet it needs .

1.1.A pair ofA pair ofElectrons from Electrons from oxygen #1are sharedoxygen #1are shared

2. Oxygen #2 then 2. Oxygen #2 then Shares one of itsShares one of itsPair of electrons toPair of electrons toComplete the octetComplete the octet

#1 #2

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8So how does it do this?

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The sharing of 2 pairs of electrons is known as a DOUBLE BOND. They can be formed by Moleculeshaving other than just two of the same atoms (like O2 )

Formaldehyde is an example Propene is another.

Do the atoms in these moleculeshave an Octet?

Some compounds may evenhave several double bonds.

:O = C = O:. . . .

H C = O: H

H H C = C H H

Carbon Dioxide

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Triple BondsTriple Bonds

Nitrogen is an example of a compound that is formedBy sharing three pairs of electrons.

This is called a TRIPLE BOND

How manyElectrons doesEach atom have?

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Triple bondsTriple bonds

Another example of a Triple Bond is acetylene

H - C : : : C - H

Electronsfrom carbon 1

ElectronsFrom carbon 2

H - C C - HAcetylene

Oxy-acetylene torch

Bond EnergyBond EnergyThe strength of a Chemical BondThe strength of a Chemical Bond

Energy required to Energy required to breakbreak a chemical bond a chemical bond

Measured in kJ/moleMeasured in kJ/mole

A A largelarge bond dissociation energy corresponds to a bond dissociation energy corresponds to a strongstrong covalent bond. covalent bond.

Covalent Bond StrengthCovalent Bond Strength: Triple > Double > Single: Triple > Double > Single

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Bond Dissociation EnergyBond Dissociation Energy

The energy necessaryThe energy necessaryto breakto break

A bond in a givenA bond in a givenCompound Compound varies.varies.

Different kinds of Different kinds of Bonds take Bonds take differentdifferent

EnergiesEnergies to break to breakthem apart.them apart.

It would take It would take 436 kJ/mol436 kJ/mol to break to breaka hydrogen bond.a hydrogen bond.

Bond length

Bond Strength

Bond EnergiesBond Energies

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Covalent vs. Ionic PropertiesCovalent vs. Ionic PropertiesPROPERTY IONIC COMPOUND COVALENT COMPOUND

Simplest Component

Formula UnitFormula Unit

Anion & CationAnion & CationSingle MoleculeSingle Molecule

Structure CrystallineCrystalline Amorphous solids (not Amorphous solids (not Crystalline) can also be Crystalline) can also be gas/liquidgas/liquid

Hardness HardHard Solids relatively softSolids relatively soft

Workability Shatters if workedShatters if worked Solids easily break apartSolids easily break apart

Electrical Conductivity

Good Conductor ONLY if Good Conductor ONLY if molten or in solutionmolten or in solution

Non-conductors even if Non-conductors even if moltenmolten

Melting Point High (NaCl – 801 deg C)High (NaCl – 801 deg C) LowLow

Boiling Point High (NaCl – 1413 deg C)High (NaCl – 1413 deg C) LowLow

Ease of Phase Change

Difficult – Low VP / High MP Difficult – Low VP / High MP & BP’s / High H& BP’s / High Hff

Easy – High vapor pressureEasy – High vapor pressure

Solubility in Water

Soluble – Covers range from Soluble – Covers range from very soluble to very slightly very soluble to very slightly solublesoluble

NaCl = 37.5g/100g waterNaCl = 37.5g/100g water

Non-polar: insoluableNon-polar: insoluable

Polar: Soluble to slightly Polar: Soluble to slightly soluble soluble