ATOMIC BONDINGATOMIC BONDING

Atomic Bonding

Primary BondsSecondary

Bonds

Ionic Covalent Metallicvan der Waals

1. IONIC BONDING1. IONIC BONDING

1111Na & Na & 1717Cl These two ions are attracted Cl These two ions are attracted to eachother by the electrostatic force to eachother by the electrostatic force developed b/w them and an ionic developed b/w them and an ionic compound (NaCl) forms.compound (NaCl) forms.

The ionic bonding b/w the two atoms The ionic bonding b/w the two atoms results from the transfer of an electron results from the transfer of an electron from an electropositive atom to an from an electropositive atom to an electronegative one, so a strong electronegative one, so a strong electrostatic attraction is set up b/w the electrostatic attraction is set up b/w the ions.ions.

Anion

Cation

Coulombic interaction

Cl-

Na+

Note the relative sizes of ions Na+ shrinks and Cl- expands

Ionic Bonds are nondirectional !

Properties of Ionic BondingProperties of Ionic Bonding

1.1. Force of attraction is electrostatic Force of attraction is electrostatic (coulombic)(coulombic)

2.2. Bond is non-directional (each + ion is Bond is non-directional (each + ion is surrounded by as many – ions as possible)surrounded by as many – ions as possible)

3.3. Bond is strong, stable, brittleBond is strong, stable, brittle4.4. High melting point (as the # of eHigh melting point (as the # of e-- involved in involved in

the bond increases, melting point increases)the bond increases, melting point increases)5.5. Poor electrical conductivityPoor electrical conductivity6.6. Forms between atoms of different Forms between atoms of different

electronegativity values (one high, one low)electronegativity values (one high, one low). . An obvious limitation is that it can form only An obvious limitation is that it can form only b/w different atoms.b/w different atoms.

2. COVALENT BONDING2. COVALENT BONDING Covalent bondCovalent bond is the bond in which e is the bond in which e-- are are

shared b/w atoms.shared b/w atoms.

The elements showing covalent bond obey (8-The elements showing covalent bond obey (8-N) rule.N) rule.

(8-N) rule: The number of the closest (8-N) rule: The number of the closest neighbors to each atom is equal to (8-N)neighbors to each atom is equal to (8-N)

N is the valance eN is the valance e--.. When N=7, such as Cl When N=7, such as Cl

8-7=1 8-7=1 → the atoms pair off as diatomic → the atoms pair off as diatomic molecules.molecules.

Cl + Cl Cl Cl

When N=6 such as SWhen N=6 such as S

16S : 1s2 2s2 2p6 3s2 3p4 6

8-6=2

each atom has two closest neighbors so they form long chains.

S

S

S

S

O, Se, Te behave like S.

When N=5, such as

33As : 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3

8-5=3 → They require 3 closest neighbors so they form sheets of atoms.

When N=4, such as 6C : 1s2 2s2 2p2

8-4=4 → They form 3-D structures.

Ethylene molecule

Properties of Covalent Properties of Covalent BondingBonding

1.1. It is based on electron sharing.It is based on electron sharing.

2.2. Bond is directional (each atom is surrounded Bond is directional (each atom is surrounded by a definite amount of other atoms)by a definite amount of other atoms)

3.3. Bond is hard and strong (slightly less than Bond is hard and strong (slightly less than ionic)ionic)

4.4. Very high melting point.Very high melting point.

5.5. Poor electrical conductivity.Poor electrical conductivity.

6.6. Forms bForms b//w atoms with w atoms with high high electronegativityelectronegativity. Covalent bonding is not . Covalent bonding is not limited to elements; many compounds are limited to elements; many compounds are covalent, like HCl, Hcovalent, like HCl, H22O.O.

3. METALLIC BONDING3. METALLIC BONDING

Covalent bonding occurs in electronegative Covalent bonding occurs in electronegative atoms where they want to give away atoms where they want to give away electrons.electrons.

Metallic bond can be considered as a Metallic bond can be considered as a special type of covalent bond in which special type of covalent bond in which instead of sharing particular valance instead of sharing particular valance electrons, general sharing of valance eelectrons, general sharing of valance e-- is is responsible for the bond.responsible for the bond.

Valance electrons are detached from Valance electrons are detached from atoms, and spread in an “electron cloud” atoms, and spread in an “electron cloud” that holds the ions together.that holds the ions together.

The positive metal ions are arranged The positive metal ions are arranged regularly in a regularly in a ““crystal lattice”crystal lattice” and a and a cloud of valance electrons surround cloud of valance electrons surround them.them.

Electron cloud

Metal ions

1.1. It is based on electron sharing. Electrons It is based on electron sharing. Electrons are shared among all atoms.are shared among all atoms.

2.2. NoNonn directionality - desire for the largest directionality - desire for the largest number of nearest neighborsnumber of nearest neighbors..

3.3. High thermal and electrical conductivity.High thermal and electrical conductivity.

4.4. Moderately lower melting point.Moderately lower melting point.

5.5. Weakest primary bond.Weakest primary bond.

6.6. Forms between atoms with Forms between atoms with lowlow electronegativityelectronegativity..

Properties of Metallic BondProperties of Metallic Bond

Since the valance eSince the valance e-- are not bound to any are not bound to any particular atom, they can move through particular atom, they can move through the lattice under the application of an the lattice under the application of an electric potential causing a current flow.electric potential causing a current flow.

Also by a series of collisions with Also by a series of collisions with neighboring electrons they transmit neighboring electrons they transmit thermal energy rapidly through the thermal energy rapidly through the lattice.lattice.

High thermal and electrical conductivity ?High thermal and electrical conductivity ?

SECONDARY BONDS SECONDARY BONDS ((VAN DER WAALS BONDS)VAN DER WAALS BONDS)

Secondary bonds are universal to all Secondary bonds are universal to all atoms and molecules, but as it is a very atoms and molecules, but as it is a very weak bond, it may be neglected when weak bond, it may be neglected when primary bonds exist.primary bonds exist.

It can also be termed as a physical bond It can also be termed as a physical bond as opposite to chemical bonding that as opposite to chemical bonding that involves einvolves e-- transfer. transfer.

Describes a dipolar attraction b/w Describes a dipolar attraction b/w

neutral atoms.neutral atoms.

Since electrons move around nucleus Since electrons move around nucleus (electronic charge is in motion), it is (electronic charge is in motion), it is possible for electrons to be located possible for electrons to be located unsymmetrically with respect to nucleus unsymmetrically with respect to nucleus at a moment.at a moment.

In this way a dipole will be formed.In this way a dipole will be formed. Van der Waals bonding is a result of an Van der Waals bonding is a result of an

attraction b/w opposite poles of these attraction b/w opposite poles of these dipoles.dipoles.

Dipole: Dipole: Pair of equal andPair of equal and

opposite electric charges.opposite electric charges.

Ex: HYDROGEN BONDEx: HYDROGEN BOND As the valance electrons of water molecule As the valance electrons of water molecule

spend more of its time around Oxygen atom spend more of its time around Oxygen atom than the Hydrogen atom, a dipole is formed.than the Hydrogen atom, a dipole is formed. TThe oxygen end of the he oxygen end of the moleculemolecule develops a develops a

partial negative charge (because of the negative partial negative charge (because of the negative charge on the charge on the electronselectrons).).

For the same reason, the hydrogen end of the For the same reason, the hydrogen end of the moleculemolecule develops a partial positive charge. develops a partial positive charge.

Negative end of each water molecule is Negative end of each water molecule is attracted by a positive end of another water attracted by a positive end of another water molecule.molecule.

IonsIons are not formed; however, the are not formed; however, the moleculemolecule develops a partial develops a partial electrical chargeelectrical charge across it across it called a called a dipoledipole. .

Metals:Metals: Metallic bond Metallic bond Ceramics:Ceramics: Ionic / Covalent bonds Ionic / Covalent bonds Polymers:Polymers: Covalent and Secondary Covalent and Secondary

bondsbonds Semiconductors:Semiconductors: Covalent or ovalent / Covalent or ovalent /

Ionic bondsIonic bonds