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CHEMICAL BONDING

Chemical bond: attractive force holding two or more atoms together.

Type of Chemical BondsInteratomic bondsIntermolecular forces Ionic (electrovalent) bonds Covalent bond(dative covalent bond) Metallic bonds

Van der Waals forces permanent dipole-dipole interactions hydrogen bondCovalent bond results from sharing electrons between the atoms. Usually found between nonmetals. Ionic bond Results from the transfer of electrons from a metal to a nonmetal.It is the electrostatic force of attraction between two oppositely charged ions formed.Metallic bond: attractive force holding pure metals together.IONIC BONDNaCl

MgO

CaCl2

Na2ODOT-AND CROSS DIAGRAMSIONIC BONDIonic Bonding

COVALENT BONDH2O2Cl2HClCO2CH4 (methane)C2H4 (ethene)DOT-AND-CROSS DIAGRAMSCOVALENT BONDDOT-AND CROSS DIAGRAMScovalent BONDFORMATION OF HYDROGEN (H2)

- Has one electron in the valence shell- 2 hydrogens share electrons to form a covalent bond and a stable duplet structure of He.Dot-and-cross diagram :Formation of Oxygen (O2) :- Oxygen has 6 valence electrons- Shortage of two electrons to form octet structure- Hence, each atom shares 2 electrons each to form 2 covalent bonds (double bond) to achieve a stable octet structure of Ne.

Dot-and-cross diagram :NO

NO2COMPOUNDS WITH ODD NUMBER OF ELECTRONSBeCl2

BF3

AlCl3COMPOUNDS WITH LESS THAN 8 ELECTRONS..PCl5

SF6

BrF5COMPOUNDS WITH MORE THAN 8 ELECTRONS..elements of Period 3 and beyondDATIVE BOND OR CO-ORDINATIVE BONDWhen one atom from a covalent bond shares 2 electrons to form a bond, that bond is called a co-ordinative or dative bond.

But, when the dative bond is formed, it is difficult to differentiate between covalent and dative bond. Hence, it doesnt matter from where the electrons come from.

The conditions required for this bond to form are : - Donor group should have a lone pair in the outermost shell to donate 2 electrons - Acceptor group must have a vacant orbital in the outermost shell to accept the 2 electrons from the donor.

NH4+

Al2Cl6

BF3.NH3NH4+ - H+ ion accepts a lone pair electrons and achieves stable duplet structure-dative bond is formed.Al2Cl6 - This is called a dimer because it is formed by 2 AlCl3 . Al ion is shortage of 2 electrons and hence accepts a lone pair of electrons from Cl from another AlCl3 to form a dative bond which has a stable octet structure.BF3.NH3- B has 6 electrons in the valence shell. So, B accepts 2 electrons to form a dative bond and thus achieves the octet structure.

VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) In a molecule two kind of electron pairs exist. Bond pairs and lone pairs.

Lone pairs electrons repel more than bonding pairs.

Lone pair-lone pair > lone pair bond pair > bond pair-bond pairThe repulsion increases by the increase of electronegativity.Examples :

CH4 molecule (109.50)

NH3 molecule (1070)

H2O molecule (1050)Dot-and-cross method and VSEPR do not explain why a bond forms.What are the orbitals that are involved in bonding?

We use Valence Bond Theory:

Bonds form when orbitals on atoms overlap.There are two electrons of opposite spin in the orbital overlap.Covalent Bonding and Orbital OverlapHYBRIDISATION ALKANESALKENESBENZENEHYBRIDISATION OF ORBITALS IN ALKANES

The four orbitals (an s and three ps) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.

Because one s and three p orbitals are used, it is called sp3 hybridisation

2s22p22s12p3 4 x sp3

In ALKANES, the four sp3 orbitals of carbon repel each other into a TETRAHEDRAL arrangement with bond angles of 109.5.

Each sp3 orbital in carbon overlaps with the 1s orbital of a hydrogen atom to form a C-H bond.THE STRUCTURE OF ALKANES

109.5HYBRIDISATION OF ORBITALS - ALKENES

Alternatively, only three orbitals (an s and two ps) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged.2s22p22s12p3 3 x sp2

2p

In ALKANES, the four sp3 orbitals repel each other into a tetrahedral arrangement.

HOWEVER...In ALKENES, the three sp2 orbitals repel each other into a planar arrangement and the 2p orbital lies at right angles to themTHE STRUCTURE OF ALKENEStwo sp2 orbitals overlap to form a sigma bond between the two carbon atomsORBITAL OVERLAP IN ETHENE - REVIEW

two 2p orbitals overlap to form a pi bond between the two carbon atoms

s orbitals in hydrogen overlap with the sp2 orbitals in carbon to form C-H bondsthe resulting shape is planar with bond angles of 120

The two 2p orbitals also overlap to form a second bond. This is known as a PI () bond.

THE STRUCTURE OF ALKENES

PAI BONDS CLOUD FORM AT THE ABOVE AND BOTTOMTHE STRUCTURE OF BENZENESNotice that the p electron on each carbon atom is overlapping with those on both sides of it. This extensive sideways overlap produces a system of pi bonds which are spread out over the whole carbon ring. Because the electrons are no longer held between just two carbon atoms, but are spread over the whole ring, the electrons are said to be delocalised.

STRUCTURE OF BENZENEBENZENES HAVE THE SAME HYBRIDISATION AS FOR ETHENEEach carbon undergoes sp2 hybridisation.6 carbons 6 hydrogens (C6H6)The unhybridised p orbital develops the double bond in the ring. The pai electrons are delocalised and do not belong particularly to one carbon.Distributed through the entire pai system.The term Resonance.Benzene molecule is planar with a bond angle of 1200

STRUCTURE OF BENZENE - REVIEWConfigure the number of valence electrons in central atom.Add one electron for each atom joined to central atom.Add electron if particle is negatively chargedSubtract if particle is positively charged.Divide by two to get the number of electron pairs.

RULES TO DETERMINE THE NUMBER OF ELECTRON PAIRS AROUND CENTRAL ATOM.

HyperChemChemical Bonds Bond length & Bond strengthBond Type Single Double Triple# of es 2 4 6Notation = Bond order 1 2 3Bond strengthIncreases from Single to TripleBond lengthDecreases from Single to Triple

Covalent Bonds Bond length

Chemical Bonds bond polarityFormed when 2 atoms have difference in terms of electronegativities.Representation of polarity Polarisation unequal sharingExample HClA polar molecule has dipole momentCompare individual dipole moment between H2O, CCl4, CO2

Van Der Waals forceFactors that affect VDW force number of electrons and shape of molecule.

Dipole-dipole forcesHydrogen bonds (Intermolecular & Intramolecular)

INTERMOLECULAR FORCESElectrostatic force of attraction between delocalised electron cloud and the metal ions.

METTALIC BONDING

Size of cation increase, strength of mettalic bond decreases.

Charge on the cation increase, strength increase.Draw the dot-and-cross diagrams for CO2, ethene and methane, NO2, BF3, AlCl3, SF6 and BrF5.Questions