Chapter 8 Honors Chemistry(partial)

Covalent Bonding

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Electronegativity

• In a covalent bond, we have seen that electron pairs are shared between two nonmetals

• Rarely are these electrons shared equally as one of the atoms has a stronger “desire” to have those electrons

• How can we measure which atoms wants the electrons more?

• Electronegativity (EN) !!!!!• It is a measure of an atoms ability to attract a

pair of electrons in a molecule

2

F is the most electronegative

element and is given a value of

4.0 and all elements E.N. values

are in comparison to this

Left to right across a period =

in EN

Down a group in EN or stays

about the same3

ElectronegativityElectronegativity

The higher the EN value, the more the

atom will attract shared electrons to it

Depending upon how great the difference

in electronegativity is between the atoms

the bond can have highly positive and

negative regions

This is called a polar bond4

• The only bond that is purely 100% covalent where the electrons are equally shared is one in which the EN = 0

• This only occurs when the electrons are shared by identical atoms, like H2, or any of the diatomic molecules

5

ElectronegativityElectronegativity

Polarity

• A bond is considered to be non-polar covalent if the EN is 0 – 0.4

• A bond is considered to be moderately polar-covalent if the EN is 0.5 – 1.0

• A bond is considered to be very polar-covalent if the EN is 1.0 – 1.7

• Any bond with EN that is ≥ 1.7 is considered to be ionic

6

Electronegativities of the Elements

Using the chart of Electronegativities (Pg. 177),

determine the type of bond formed between the

following pairs of atoms:

C and O

Fe and O

N and Br

C and H

Na and F

Cl and Cl

ElectronegativityElectronegativity

C and O EN = 1.0 polar

Fe and O EN = 1.7 ionic

N and Br EN = 0.2 nonpolar

C and H EN = 0.4 nonpolar

Na and F EN = 3.1 ionic

Cl and Cl ΔEN = 0 nonpolar

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• In a molecule of H2O, a pair of electrons are shared between each O and H

• The EN of O = 3.5 and H = 2.1• EN = 1.4 – therefore is a polar covalent bond• This means that O attracts the electrons

towards it and so will become slightly negative while the electrons move away from each H atom and they become a bit positive

10

ElectronegativityElectronegativity

• This means the electrons are not shared evenly and that one area is slightly positive, the other negative.

• This is called a polar molecule• Indicated using small delta (δ).

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Dipole Moments

• A molecule with a center of negative charge and a

center of positive charge is a DIPOLE

• (two poles),

• or has a dipole moment.

• Center of charge doesn’t have to be on an atom.

• Will line up in the presence of an electric field.

How It is drawn

H - F+ -

H - F+ -

H - F+ -

H - F

+-H - F+

-

H - F

+-

H - F +-

H - F+-

H - F

+-

H - F

+-

H - F+ -

H - F

+-H - F+

-

H - F

+-

H - F +-

H - F+-

H - F

+-

H - F

+-

+-

H - F+ -

H - F+ -

H - F+ - H - F

+ -

H - F+ -

H - F+ -

H - F+ -

H - F+ -

- +

Which Molecules Have Dipole Moments?

• Any two atom molecule with a polar bond.

H2O or FBr

• With three or more atoms there are two considerations.

1. There must be a polar bond.

2. Geometry can’t cancel it out (more about geometry later)

CH4

CO2

SO2

Ionic vs. Molecular Compounds

There are two types of forces involved in chemistry

Intermolecular forces are those between molecules

and are responsible for holding these molecules

together (inter = between)

Intramolecular forces are those between atoms

inside the actual molecule and are responsible for

holding the molecule together (intra = within)

These two forces explain many of the properties of

ionic and covalent compounds

19

• Ionic compounds are formed of positive and negative ions and these forces are very strong

• Each ion is held in place by at least 6 other ions and so both the inter and the intra molecular forces are strong

20

Ionic vs. Molecular CompoundsIonic vs. Molecular Compounds

• Covalent compounds have strong intramolecular forces holding the atoms together to form a molecule, but rather weak intermolecular forces holding the adjacent molecules together

• Because the intermolecular forces are weak, covalent compounds have low boiling and melting points (little energy is needed to move molecules apart from a solid to liquid to gas)

• Many are gases at room temp

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• Solid ionic compounds do not conduct electricity

as the ions are held tightly, but when in the liquid

state (called molten) the ions are free to move and

so can conduct electricity

• Ionic compounds dissolve easily in water as water

is a polar molecule and water molecules surround

the ions and pull them apart into the solution

(process called solvation)22

Ionic vs. Molecular CompoundsIonic vs. Molecular Compounds

Metallic Bonding

• How are metal atoms held in place?

• Most metals have 1, 2 or 3 valence electrons

• The metal atoms are relatively close to each other and their valence

energy levels overlap

• This allows the valence electrons to move freely from one metal

atom to those it overlaps with

• These electrons are not bonded to one particular metal atom and are

called delocalized electrons

• This is often referred to as the “Electron Sea Model” of metallic

bonding

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Ionic Bond, A Sea of Electrons

2424

Metallic BondingMetallic Bonding

Sea of Electrons

• Metals conduct electricity.

• Electrons are free to move through the

solid.

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Hydrogen Bonding• This is a type of bonding involving

hydrogen and either F, O or N• When hydrogen bonds with either of these

elements there is a large ΔEN• This results in a very polar molecule with

large dipoles• This produces relatively high inter

molecular forces to adjacent molecules they are held together “tightly”

• This accounts for the relatively high boiling and melting point of H2O compared to other covalent compounds

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Hydrogen Bond

11.2

The The hydrogen bondhydrogen bond is a special dipole-dipole is a special dipole-dipole interaction between the hydrogen atom in a polar interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N-H, O-H, or F-H bond and an electronegative O, N, or F atom.N, or F atom.

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