chemical bonding 2 by aditya abeysinghe

Chemical Bonding-2 by Aditya Abeysinghe

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Continued from Chemical Bonding-1 by Aditya Abeysinghe

For more information about the basics of bonding please visit the above mentioned presentation


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PolarityPolarity is the measurement which depicts the tendency of an atom’s, of a molecule’s or of an ion’s to separate into poles.(Here poles are the two ends of the bond in which molecules are bonded or in which atoms are self polarized as the positive and the negative poles.)There are two factors that affect the polarity of a substance:1. The size of the substance2. The distribution of the electrons around the substance


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1. Size of the substance-When the size of the substance increases, it is more

tendentious to be separated into poles. Thus the polarity of such a substance is higher than that of a smaller substance. (When the size of a substance increases, the electrons in the outermost shell show less attraction to the nucleus than in a smaller atom)2. The distribution of electrons-

When there’s an asymmetric electron distribution around the nucleus of a substance its polarity is high as compared to that in a substance with a symmetrical electron distribution (Here asymmetric means that the resultant of the forces directed towards the nucleus of a substance is not zero) E.g.: Ammonia is asymmetrical due to the presence of a lone pair of electron whereas methane is symmetrical as the resultant forces of any 3 bonds will be balanced by the fourth force to make the molecule symmetrical.

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Dipole Moment (μ)Dipole moment is the product of the charge of a pole and the distance between the two charges.The SI unit of the dipole moment is the coulomb meter. However, since the measurements of these types are so small, the dipole moment is usually calculated by a unit called Debye1 Debye (D) = 3.34 × 10-30 Cm . (Here Cm means Coulomb meter)Dipole moment is a key factor in deciding about the variation of the intermolecular attractions.Dipole moments of some compounds are as follows:



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Molecule Dipole Moment (D)

H2 0

HF 1.78

HCl 1.07

HBr 0.79

HI 0.38

ClF 0.88

BrF 1.29

BrCl 0.52

H2O 1.85

H2S 0.95

CO2 0

NH3 1.47

NF3 0.23

NCl3 0.39

CH4 0


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Intermolecular forces of attraction

The term intermolecular refers to attractions between molecules.There are different types of such attractions:1. Dipole-Dipole forces2. Dipole-ion forces3. Ion-induced dipole forces4. Dipole-induced dipole forces5. Induced dipole-Induced dipole forces6. Hydrogen bonds7. Vanderwall’s forces


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Dipole-dipole forces

The interactions between atoms through their oppositely charged poles are called dipole-dipole forces.

δ+ δ-δ + represents the positive pole of the atom while δ - represents the negative pole of the atom

Broken lines showing the intermolecular attractions- dipole-dipole forces

Due to this bond single elements cannot escape from the solution in which they exist. Thus, melting points of such substances are high.

These attractive forces are weaker than the full charges carried by ions in ionic crystals

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Dipole-ion forces

The forces between cations and anions due to the presence of an electric field between them can be summarized as follows:


+ -


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These bonds are important in forming hydrates and ammoniates of compounds. E.g.: [Mg.6H2O]2+ , [Ca.6NH3]2+

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Ion-induced dipole forces

A nonpolar molecule or atom may be polarized by an electron cloud of an ion and the induced dipole may make a bond with the ion. Such a bond is called an ion-induced dipole force.


δ+

I II-

Iodide ion Iodine (I2 )

molecule

I3- Compound

Another example- Li+ nG , where G is a noble gas and n= 1 or 2


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Dipole- Induced dipole bond

An atom or a molecule which is nonpolar may be induced by a field of a permanent dipole and the formation of a bond between the permanent dipole and the induced dipole is called a dipole-induced dipole bond.E.g.: Noble gas hydrates (G.xH2O ; maximum x=6)

H

O -2δ

Dipole (Water)

Electrically neutral substance later induced and separated into poles by the inducing action of water(dipole)

Induced dipole


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Induced dipole-Induced dipole bondsAlthough the electron cloud of a noble gas may be symmetrically arranged, it can be asymmetrical quickly due to the movement of electrons.Therefore, noble gases and nonpolar molecules act as temporary induced dipoles. Self-induced dipoles polarize more non-polar atoms or molecules. Such a bond is called an induced dipole-induced dipole bond.

DipoleInduced dipole-1 Induced dipole- 2

Induced dipole-Induced dipole bond


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London forcesLondon forces are a type of force that can be found in both polar and nonpolar molecules.A nonpolar atom/ molecule may sometimes become polar due to the constant motion of its electrons. This might even cause uneven charge distribution.When this occurs, the molecule/atom has a temporary dipole.This dipole can then cause a second atom be attached to the opposite pole of the first molecule.


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London dispersion forces are the weakest of all the electronic forces that act between atoms or between molecules.Due to these forces nonpolar substances such as noble gases and halogens condense into liquids and then freeze into solids when the temperature is lowered sufficientlyLondon force are the cause as to why liquids are composed of molecules with no permanent dipole attraction and have low boiling points compared to their molecular masses.Solids which have London dispersion attraction have rather soft crystals, are easily deformed, and vaporize easily.Because of the low intermolecular forces, the melting points are also low and evaporation takes place so easily that it may occur at room temperatureE.g.: Iodine crystals , Moth Balls (Para dichlorobenzene and naphthalene ).


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Hydrogen Bonds

A proton or hydrogen nucleus has a high concentration of positive charge when a hydrogen atom is bonded to a highly electronegative atom. Its positive charge will have an attraction for neighboring electron pairs.This special kind of dipole attraction is called hydrogen bond.


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C

Cl

H

CH3

O

Due to the presence of hydrogen bonds, there’s an increase in the melting and boiling points of elements where hydrogen bonds are present

By energy of bonds,

London Forces < Hydrogen bonds < Primary bonds (ionic, metallic, and covalent bonds)


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Double and triple bonds

To achieve the octet structure some atoms to share two or even three pairs of electrons.Sharing two pairs- Double bond (CO2 )

Sharing three pairs- Triple bond (C2H2)

In double and triple bonds the attractive force between the nuclei and the shared electrons is greater as compared to the bonds where single bonds are formed.


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ΔHD (Single Bond) < ΔHD (Double bond) < ΔHD (Triple bond)

Heat needed to break the bonds increases

Because double bonds and triple bonds tend to pull atoms close together,

Single bond > Double bond > Triple bond

Intermolecular distance increases


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Resonance Structures

It’s not always possible to represent the bonding structure of a molecule by either the Lewis dot structure or the line drawing structure.This is because data about the bond strengths and bond distance reveal that most molecules have a hybrid(resonant) structure.Thus many molecules are represented as a series of transitions of bond structures, not as an individual component.E.g.: Sulfur trioxide and Benzene.


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S

O

Resonance Structure 1



O

SThis intermediate form between different contributing structures is

called a resonance hybrid(true structure)


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The actual or the true structure is always stable than the resonance structure and the energy of the actual structure is thus lower than the resonance structures

ΔERE – Resonance energy difference

ΔH / KJmol-1

(Resonance Structures)

(Resonance Hybrid (True Structure))


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Although sulfur trioxide is an overall neutral molecule, many ions too show resonance.The charge distribution of such an actual structure is as follows:

- 2/3

*The three -2/3 charges on the three oxygen atoms add up to the total of -2 charge on the overall molecule.

chemical bonding 2 by aditya abeysinghe

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