Atomic bonding

Atomic Bonding Interatomic forces present in atomic bonding can predict many physical properties of materials ( i.e. melting temperature, elasticity, thermal expansion, and strength).

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As two ions approach each other, each atom exerts forces on the other.

Magnitude of Forces is a function of interatomic distance

Interatomic Forces

- Attractive Force, Fa

- Repulsive Force, Fr

rFr

FaAnionCation

Energy

Bonding Energy, E0

RAN EEE

FdrE

Net Potential Energy between two adjacent ions

drFdrFEr r

RAN

Bonding EnergyIn assessing the performance of materials, it’s important to draw attention to the relationship between material properties and structural elements.

Finding the minimum of a Potential Energy Curve corresponds to the energy released when two ions form a bond, Bonding Energy.

Bonding Energy and shape of Potential Energy Curve vs. R varies from material to material. Number of material properties depend on E0.

Co-efficient Of Thermal Expansion

Atomic bondingBonding is association of two atoms with in each other. Interatomic bonding can be categorized as:◦ Primary bonding ◦Secondary bonding

Primary bonds are also knows as chemical bonds.

Primary bonding necessarily involves the valence electrons for its types.

The nature of bonds depends up on the electronic structure of constituents atoms.

Primary bonding arises from the tendency of atoms to assume the stable electronic structure like inert gases.

Primary Bonding

▪Ionic bond▪Covalent bond▪Coordinate covalent bond▪Metallic bond

Types Of Primary Bonding

»Ionic bond is due to electrostatic (columbic) forces of attraction between oppositely charged ions.

» For two oppositely charged isolated ions, the attractive energy EA is a function of inter atomic distance.

EA = - A/rWhere A = [1/4-€)] [Z1e x Z2 e]

€ =Permittivity of vacuum

Ionic Bond

»Ionic bonds are formed where the difference in electro -negativities of the constituent’s atoms is considerable. » That= s reason ionic bonds are observed between the elements situated at the horizontal extremities of the periodic table. »In ionic bonds, bond energies generally range between 600 and 1500 kj/mole (3 and 8 eV/atom). »NaCl is example of ionic bond.

Ionic Bond Cont……

» Covalent Bonds are formed by sharing of electrons between adjacent atoms.» Two atoms of covalent bonds will contribute at least one electron each, to the bond and chaired pair of electrons may be considered to belong to both of them.»Covalent bonds are favored between similar atoms i.e. non-metallic elemental molecules (H2,O2,Cl2 etc.) 

Covalent Bond

Covalent Bond Cont….»Covalent bonds are also observed in molecules containing dissimilar atoms such as CH4, H2O, HNO3 etc.»Covalent bonds are found in diamond, silicon germanium and right hand side of periodic table such as GaAs,SiC etc.»The numbers of covalent bond that are possible for an atom are determined by valence electrons.

Covalent Bond Cont…» Covalent bonds may be very strong as in case of diamond (melting point >3550 ºC).» Covalent bond may be very week in case of Bismuth (melting point ~270 ºC).»It is found in polymers where basic molecular structure is a long chain of carbon atoms which are covalently bonded with two of their available 4 bonds per atom.»e.g. in CH4.

Co-ordinate Covalent Bond» A bond formed b/w two atoms when the shared pair of electrons is donated by one of the bonded atoms is called Co-ordinate Covalent Bond.» Atom which donates pair of electron called donor atom» Atom which accepts pair of electron called accepter atom

» e.g. bonding b/w BF3 & NH3.

♦Metallic Bonding is found in metals and their alloys.♦Metallic atoms have one ,two or three valence electrons.♦These valence electrons are not bound to any particular atoms in metals.♦They are more or less free to drift thought the entire metal.

Metallic Bonding

Metallic Bonding Cont…♦These electrons belong to the metals as a whole forming a sea of electrons around different nuclei

♦ These nuclei also called ion cores having a net positive charge which is equal in magnitude to the total valence electrons charge per atom.

♦Free electrons act as a binder to hold the ion cores together.

♦Metallic bond is non directional in nature and bonding energy in metallic bond may be strong or weak.

♦ For example, bonding energy of mercury is 68 kj/mole and Tungston has a bonding energy 850 kj/mole.

Secondary Bonding♦Secondary bond also known as physical bonds.♦These are very weaker as compared to primary bonds.♦Bonding energy are of order of 10 kj/mole.♦ It is more significant in noble gases which have stable electron structures.

a) Fluctuating induced dipole bonds b) Polar molecule induced dipole bondc) Dipole dipole bond

Secondary Bonding Cont…

Fluctuating Induced Dipole Bond♦ Also known as London Dispersion Forces. ♦ An atom is normally electrically symmetric means over all distribution of electrons is symmetric.♦ As we know in constant vibration motion that can cause instantaneous and short lived disturbance in electrical symmetry for some atoms or molecules.♦ Resulting in creation of dipole.♦ Such dipole produces a displacement of electron distribution of adjacent atoms or molecules which creates a dipole on second atom

♦

Fluctuating Induced Dipole Bond Cont…♦ Resulting weak attractive forces between the two, these forces are temporary and short lived.♦ These are the weakest bonds.

Polar Molecule Induced Dipole Bond♦ Some molecules, have separation between negatively and positively charged regions due to asymmetrical arrangement.♦ These molecules are termed as polar molecule.♦ These permanent dipoles arise due to the difference in the electro negativities of the two atoms.♦ Polar molecules may also induced dipoles in adjacent non-polar molecule.♦ So bond will formed as a result of attractive forces between the dipoles on two molecules.

Dipole Dipole bond♦Vander Waal forces will also exist between polar molecules.♦ These forces called Dipole Dipole Bonds or Permanent Dipole Bonds.♦ The associated bonding energy are significantly greater than for bonds involving induced dipole.♦Hydrogen bonding is a special case of permanent dipole bond.♦Bonding energy incase of hydrogen bonding is 51 kj/mole.