science at nano-04feb11

8/7/2019 Science at Nano-04feb11

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One more bond is there

Metallic bond

In metals all the electrons are shared by all

the atoms at one time.Metals easily conduct electricity becauseone extra electron can be added or

removed without removing it from a singlediscrete atom.


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Metal Bond

Hard, shiny, ductile, molecule-like structures.

Metals easily conduct because one extra

electron can be added or removed

without removing it from a single discreteatom.

Note:

Chemical bond, exchange of electron,asymmetric and symmetric electron densitydistribution.


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Symmetric distribution but in one particular

direction makes a covalent bond

When bonding takes place in multiple,spatial directions which furthermore is

combined with high mobility of electrons, it

is known as metal bond.

This spatial non-constraint makes an extendedbond network, hence a three-dimensional

solid.

Metal bonds facilitate adhesion to surfaces

in nanotechnology.


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0

Com

bin

interatomic distance

Ionic/covalent bond

van der Waals (weak) bond


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This effect is not influenced by other bonds,

i.e., created by exchange of atoms,therefore these are relatively weak force,known as van der Waals force.

This force is important in Nanotechnology.

This can very easily be provoked or brokeneven at room temperature.


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When two surfaces come closer to eachother, the electron shells of approachingmolecules influence only in conjunction withthe van der Waals force.

van der Waals bonds are extremelyimportant in Nanotechnology:

hydrophobic interactions

resist technology of nanolithography

living cells specially in 3-dim proteins


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van der Waals force is an intermolecularforce.

The name refers to a Dutch chemistJohannes Diderik van der Waals.

Generally refers to those forces that arisefrom the polarization of molecules intodipoles.

van der Waals force is commonly observedin noble gases.


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Three forms ofvan der Waals force are:

Dipole Dipole force

London force

Hydrogen bonding


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Dipole Dipole force

Dipoles are two opposite poles of charges.

Different from monopoles (single charge).

External electric fields can distort thecharge distribution of a dielectric material(atom/molecule) by stretching or by rotating.

When it is done by stretching, a dipole isformed.


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Some interaction is caused by dipole

moment.When you put a neutral atom in anexternal electric field, it polarizes.

Or, charge is induced in vicinity of someother charged body.

These dissimilar charges attract eachother, interaction starts.


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Bonds between two atoms could besymmetrical or asymmetrical - because ofelectronegativity of atoms that are boundtogether. Molecules of assymetrical bondexhibit an inhomogeneous electron

distribution in the surroundings causing anelectrical polarity that generates dipolemoment. Such molecules, with one or more

dipole moments, attract each other. Theintensity of polarity determines the strengthof the dipole-dipole interaction.


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If a H atom (a0 = 0.5 ) is kept between twometal plates 1 mm apart maintained at V =500 V, the relative displacement between

positive and negative charge centres wouldbe d= 2.29x10-16 m. It would require 108 V toionize this hydrogen atom.


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Molecules polarize more readily in some

direction than other.Some molecules are naturally polar, likewater molecule.

Like any charge these dipoles or polarmolecules also have their own potential andhence the electric field.


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This potential will cause an electric field

Now imagine a situation when this dipolep1

comes closer to another dipolep2of opposite

orientation.


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p2will interact with E1with an interaction

energy given by

U = -p2.E1

which can readily be expressed as

Force is the negative derivative of thisinteraction energy U.


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Hence the two dipoles will attract each otherwith a magnitude

F ~ -3 p1p2/ (4 0 r4).

where ris the separation distance betweenthese two. This is the reason why polarmolecules, such as H2O, attract each other.

Smaller the separation more is the attractionand hence closeness. This attraction betweentwo molecules encourages them to condenseto a liquid or solid phase.


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Other mechanisms of occurrence of dipole-dipole interaction:

polar molecule and a non-polar molecule(interaction in such case is weaker than thatbetween two molecules having permanentdipole moments)

polar molecule and an ion (ion-dipoleinteraction, this reduces the Coulomb field

around the ion)Molecules with heavy atoms readily tend topolarize causes adsorption on the

surface.


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London Force

Named after German-American physicist

Fritz London.

The electrons tend to wash around nucleuslike a wave. Electron density moves about amolecule probabilistically.

More electrons can be found at any point of

time at a given space. This creates chargedifference resulting in a temporary multipoleand hence the force.


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London forces exist between all atoms andmolecules.

They can affect charge of other atoms ormolecules not otherwise instantaneouslycharged.

Hence interaction starts between the two.

In nanotechnology, London forces can

be used to pick up and move atoms.


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Some interaction is caused by dipole

moment.When you put a neutral atom in anexternal electric field, it polarizes.

Or, charge is induced in vicinity of someother charged body.

These dissimilar charges attract eachother, interaction starts.


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Hydrogen bond is a typical bond whereone constituent is hydrogen atom.

Such as H2O, H2Se, ethanol, DNA.

A hydrogen bond is formed when a Hatom lies between two atoms whoseelectrons are not shared in bonding.


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O

H H

O

HH

+ +

+ +

Hin water is pulled

from one watermolecule and partlybonds with another.

His attracted to these

atoms and originalbond is weakened.

In turn, the Hatoms inthis second water

molecule are hydrogenbonded to other watermolecules.


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Phase of elements or compounds

Smaller individual molecules are normally

vapors.Water molecule (H20)

Single molecule is a gas at room temp.,becomes water droplet when clustered, and iceif cooled below 00C.

Solid, liquid and gaseous water are all made ofthe same molecule, but the molecules arepacked together in different ways.

Similar behaviour occurs with many molecules.


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Nanotechnology is largely concerned withsolid phases because the molecules or

atoms do not move around so much and

are hence easier to see.

The molecules can be arranged in regularpattern, lattice. Called Crystalline.

These can also take disorderly shape,amorphous. Called non-crystalline.


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The types of bonds are important tounderstand which phase the element or

compound will reside in.When the bonds are strong, viz. ionicbond, solids are formed even at high

temperatures.When only van der Waals forces exist,solids can be formed only at low temp. With

rise in temp., the weakest intermolecularbonds break first.

Without hydrogen bonding, water would bea as at room tem

science at nano-04feb11

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