1 ATOMIC STABILITY Ion and Molecule Formation. 2 Electronegativity In a covalent bond, we have seen that electron pairs are shared between two nonmetals

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ATOMIC STABILITYATOMIC STABILITY

Ion and Molecule FormationIon and Molecule Formation

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ElectronegativityElectronegativity In a covalent bond, we have seen that electron In a covalent bond, we have seen that electron

pairs are shared between two nonmetalspairs are shared between two nonmetals Rarely are these electrons shared equally as one of Rarely are these electrons shared equally as one of

the atoms has a stronger “desire” to have those the atoms has a stronger “desire” to have those electronselectrons

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

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

of electrons in a moleculeof electrons in a molecule

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F is the most electronegative element F is the most electronegative element and is given a value of 4.0 and all and is given a value of 4.0 and all elements E.N. values are in elements E.N. values are in comparison to thiscomparison to this

Left to right across a period = Left to right across a period = in in ENEN

Down a group Down a group in EN or stays about in EN or stays about the samethe same

ElectronegativityElectronegativity

The higher the EN value, the more the atom will The higher the EN value, the more the atom will attract shared electrons to itattract shared electrons to it

Depending upon how great the difference in Depending upon how great the difference in electronegativity is between the atoms the bond electronegativity is between the atoms the bond can have highly positive and negative regionscan have highly positive and negative regions

This is called a This is called a polar bondpolar bond

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

This only occurs when the electrons are This only occurs when the electrons are shared by identical atoms, like Hshared by identical atoms, like H22, or any of , or any of the diatomic moleculesthe diatomic molecules

Every other bond will have a % ionic Every other bond will have a % ionic character and a % covalent character based character and a % covalent character based upon the upon the EEN N


A bond is considered to be non-polar covalent if A bond is considered to be non-polar covalent if the the EN is 0 – 0.3EN is 0 – 0.3

A bond is considered to be polar-covalent if the A bond is considered to be polar-covalent if the EN is 0.3 – 1.7EN is 0.3 – 1.7

Any bond with Any bond with EN > than 1.7 is ionic in EN > than 1.7 is ionic in charactercharacter

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Electronegativity Electronegativity differencedifference

00

0.30.3

> 1.7> 1.7

CovalentCovalent

Polar Polar CovalentCovalent

IonicIonic

Covalen

t Ch

aracter decreases

Covalen

t Ch

aracter decreases

Ionic C

haracter in

creasesIon

ic Ch

aracter increases

Nonpolar Nonpolar CovalentCovalent 95% Covalent 95% Covalent

5% Ionic5% Ionic

50% Covalent 50% Covalent 50% Ionic 50% Ionic

100% Covalent100% Covalent

1.71.7

Electronegativities of the Elements

Using the chart of Electronegativities, determine Using the chart of Electronegativities, determine the type or bond formed between the following the type or bond formed between the following pairs of atoms:pairs of atoms:

C and OC and O Fe and OFe and O N and BrN and Br C and HC and H Na and FNa and F


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In a molecule of HIn a molecule of H22O, a pair of electrons are O, a pair of electrons are shared between each O and Hshared between each O and H

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

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

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

This is called a This is called a polar moleculepolar molecule Indicated using small delta (Indicated using small delta (δδ).).


Dipole MomentsDipole Moments

A molecule with a center of negative charge and a A molecule with a center of negative charge and a center of positive charge is center of positive charge is dipolar dipolar (two poles),(two poles),

or has a or has a dipole moment.dipole moment. Center of charge doesn’t have to be on an atom.Center of charge doesn’t have to be on an atom. Will line up in the presence of an electric field.Will line up in the presence of an electric field.

How It is drawnHow 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?Which Molecules Have Dipole Moments?

Any two atom molecule with a polar bond.Any two atom molecule with a polar bond.Eg. NaCl or FBrEg. NaCl or FBr

With three or more atoms there are two With three or more atoms there are two considerations.considerations.

1.1. There must be a polar bond.There must be a polar bond.2.2. Geometry can’t cancel it out (more about Geometry can’t cancel it out (more about

geometry later)geometry later)

Eg. Eg. CHCH44

COCO22

SOSO22

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Ionic vs. Molecular CompoundsIonic vs. Molecular Compounds There are two types of forces involved in There are two types of forces involved in

chemistry chemistry InterIntermolecular forcesmolecular forces are those between are those between

molecules and are responsible for holding molecules and are responsible for holding these molecules together (inter = between)these molecules together (inter = between)

IntraIntramolecular forcesmolecular forces are those between atoms are those between atoms inside the actual molecule and are responsible inside the actual molecule and are responsible for holding the molecule together (intra = for holding the molecule together (intra = within)within)

These two forces explain many of the These two forces explain many of the properties of ionic and covalent compoundsproperties of ionic and covalent compounds

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

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

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

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

Many are gases at room tempMany are gases at room temp

Ionic vs. Molecular CompoundsIonic vs. Molecular Compounds

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Ionic compounds tend to have high melting points Ionic compounds tend to have high melting points and boiling points as much energy is required to and boiling points as much energy is required to pull the ions apartpull the ions apart

Most are solids at room temperatureMost are solids at room temperature Ionic compounds are also hard and brittleIonic compounds are also hard and brittle This is because that while it is difficult to break This is because that while it is difficult to break

the ions apart (hard) if the ions get moved slightly the ions apart (hard) if the ions get moved slightly the ions line up to repel one another and force the the ions line up to repel one another and force the solid to “break”solid to “break”

It is dependent upon how the crystal is arrangedIt is dependent upon how the crystal is arranged Carbon can be arranged to form graphite (brittle) Carbon can be arranged to form graphite (brittle)

or diamonds (hard) and each had a different or diamonds (hard) and each had a different crystal arrangement using the same atomscrystal arrangement using the same atoms


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Solid ionic compounds do not conduct electricity Solid ionic compounds do not conduct electricity as the ions are held tightly, but when in the liquid as the ions are held tightly, but when in the liquid state (called molten) the ions are free to move and state (called molten) the ions are free to move and so can conduct electricityso can conduct electricity

Ionic compounds dissolve easily in water as water Ionic compounds dissolve easily in water as water is a polar molecule and water molecules surround is a polar molecule and water molecules surround the ions and pull them apart into the solution the ions and pull them apart into the solution (process called solvation)(process called solvation)


Metallic BondingMetallic Bonding How are metal atoms held in place?How are metal atoms held in place? Most metals have 1, 2 or 3 valence electronsMost metals have 1, 2 or 3 valence electrons The metal atoms are relatively close to each other The metal atoms are relatively close to each other

and their valence energy levels overlapand their valence energy levels overlap This allows the valence electrons to move freely This allows the valence electrons to move freely

from one metal atom to those it overlaps withfrom one metal atom to those it overlaps with These electrons are not bonded to one particular These electrons are not bonded to one particular

metal atom and are called delocalized electronsmetal atom and are called delocalized electrons This is often referred to as the This is often referred to as the “Electron Sea “Electron Sea

Model”Model” of metallic bonding of metallic bonding

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Ionic Bond, A Sea of ElectronsIonic Bond, A Sea of Electrons

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Metallic BondingMetallic Bonding

Because each metal atom is sort of positive and is Because each metal atom is sort of positive and is surrounded by this moving “sea” of esurrounded by this moving “sea” of e--, the atoms are , the atoms are held in place very stronglyheld in place very strongly

The bond strength is variable and depends upon the The bond strength is variable and depends upon the number of electrons each atom allows to roam in the number of electrons each atom allows to roam in the “sea” and thus the sea size is the determining factor“sea” and thus the sea size is the determining factor

Fe & Ti have large seas and so are strong metalsFe & Ti have large seas and so are strong metals The sea of eThe sea of e-- also explains the conductivity of metals also explains the conductivity of metals

as eas e-- are free to move through the metal are free to move through the metal The ductility and malleability of metals is explained The ductility and malleability of metals is explained

by the fact that metal atoms are separated easily and by the fact that metal atoms are separated easily and in all directionsin all directions

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Metallic BondingMetallic Bonding

Sea of ElectronsSea of Electrons

Metals conduct electricity.Metals conduct electricity. Electrons are free to move through the Electrons are free to move through the

solid.solid.

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Metals are MalleableMetals are Malleable Hammered into shape (bend).Hammered into shape (bend). Also Also ductileductile - drawn into wires. - drawn into wires. Both malleability and ductility explained in terms Both malleability and ductility explained in terms

of the of the mobilitymobility of the valence electrons of the valence electrons

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MalleableMalleable

+ + + +

+ + + ++ + + +

Mobile electrons allow atoms to slide by, sort of Mobile electrons allow atoms to slide by, sort of like ball bearings in oil.like ball bearings in oil.

Force

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Ionic solids are brittleIonic solids are brittle

+ - + -+- +-

+ - + -+- +-

Force

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Ionic solids are brittleIonic solids are brittle

+ - + -

+- +-+ - + -

+- +-

Strong RepulsionStrong Repulsion breaks a crystal apart, due to breaks a crystal apart, due to similar ions being next to each other.similar ions being next to each other.

Force

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Hydrogen BondingHydrogen Bonding This is a type of bonding involving hydrogen This is a type of bonding involving hydrogen

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

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

dipolesdipoles This produces relatively high inter molecular This produces relatively high inter molecular

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

This accounts for the relatively high boiling This accounts for the relatively high boiling and melting point of Hand melting point of H22O compared to other O compared to other covalent compounds (table of bp’s on page 190)covalent compounds (table of bp’s on page 190)

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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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Documents

1 ATOMIC STABILITY Ion and Molecule Formation. 2 Electronegativity In a covalent bond, we have seen that electron pairs are shared between two nonmetals