11th Chemistry Volume · Part

add represented by – node anti-bonding molecular orbital antibonding node atomic orbital bonding molecular orbital atomic orbital 1s 1s σ * σ energy ↿⇂ Fig . Linear Combination of atomic orbitals . . Bonding in some Homonuclear di-atomic molecules: Molecular orbital diagram of hydrogen molecule (H ) Electronic configuration of H atom is 1s Electronic configuration of H molecule σ s N b – N a Bond order = – Molecule has no unpaired electrons.

Hence, it is diamagnetic . Atomic orbitals of hydrogen Molecular orbitals of H Atomic orbitals of hydrogen Energy s s σ * s σ s Atomic orbitals of lithium Molecular orbitals of Li Fig . MO Diagram for Li molecule Atomic orbitals of lithium Energy s s σ * s σ s Molecular orbital diagram of lithium molecule (Li ) Electronic configuration of Li atom is 1s 2s Electronic configuration of Li molecule σ s , σ * s , σ 2s N b – N a Bond order = – = Molecule has no unpaired electrons. Hence it is diamagnetic .

Atomic orbitals of boron Molecular orbitals of B Atomic orbitals of boron Fig . MO Diagram for B molecule Energy π * py π 2py π 2pz π * pz p x p x p y p y p z p z s s σ * px σ * s σ s σ px Molecular orbital diagram of boron molecule (B ) Electronic configuration of B atom is 1s 2s 2p Electronic configuration of B molecule σ s , σ * s , σ 2s , σ * 2s , π 2py , π 2pz N b – N a Bond order = – Molecule has two unpaired electrons. H ence it is paramagnetic. Fig .

MO Diagram for H molecule Molecular orbital diagram of carbon molecule (C ) Electronic configuration of C atom is 1s 2s 2p Electronic configuration of C molecule σ s , σ * s , σ 2s , σ * 2s , π 2py , π 2pz N b – N a Bond order = – Molecule has no unpaired electrons. Hence, it is diamagnetic. Atomic orbitals of carbon Molecular orbitals of C Atomic orbitals of carbon Energy π * py π 2py π 2pz π * pz p x p x p y p y p z p z s s σ * px σ * s σ s σ px Fig . MO Diagram for C molecule Atomic orbitals of nitrogen Molecular orbitals of N Atomic orbitals of nitrogen Energy π * py π 2py π 2pz π * pz p x p x p y p y p z p z s s σ * px σ * s σ s σ px Molecular orbital diagram of nitrogen molecule (N ) Electronic configuration of N atom is 1s 2s 2p Electronic configuration of N molecule σ s , σ * s , σ 2s , σ * 2s , π 2py , π 2pz σ px N b – N a Bond order = – Molecule has no unpaired electrons.

Hence, it is diamagnetic. Fig . MO Diagram for N molecule Molecular orbital diagram of oxygen molecule (O ) Electronic configuration of O atom is 1s 2s 2p Electronic configuration of O molecule σ s , σ * s , σ 2s , σ * 2s , σ px π 2py , π 2pz , π * 2py , π * 2pz , N b – N a Bond order = – Molecule has two unpaired electrons. Hence, it is paramagnetic .

Atomic orbitals of oxygen Molecular orbitals of O Atomic orbitals of oxygen Energy π * py π * pz p x p x p y p y p z p z s s σ * px σ * s σ s σ px π 2py π 2pz Fig . MO Diagram for O molecule . . Bonding in some Heteronucleardi-atomic molecules Molecular orbital diagram of Carbon monoxide molecule (CO) Electronic configuration of C atom is 1s 2s 2p Electronic configuration of O atom is 1s 2s 2p Electronic configuration of CO molecule σ s , σ * s , σ 2s , σ * 2s , π 2py , π 2pz , σ 2px N b – N a Bond order = – Molecule has no unpaired electrons.

Hence, it is diamagnetic . Atomic orbitals of carbon Molecular orbitals of CO Atomic orbitals of oxygen Energy π * py π 2py π 2pz π * pz p x p x p y p y p z p z s s σ * px σ * s σ s σ px Fig . MO Diagram for CO molecule Molecular orbital diagram of Nitric oxide molecule (NO) Electronic configuration of N atom is 1s 2s 2p Electronic configuration of O atom is 1s 2s 2p Electronic configuration of NO molecule σ s , σ * s , σ 2s , σ * 2s , π 2py , π 2pz , σ 2px π * 2py N b – N a Bond order = – . Molecule has one unpaired electron.

Hence, it is paramagnetic . Atomic orbitals of nitrogen Molecular orbitals of NO Atomic orbitals of oxygen Energy π * py π 2py π 2pz π * pz p x p x p y p y p z p z s s σ * px σ * s σ s σ px Fig . MO Diagram for NO molecule ) Draw the MO diagram for acetylide ion C – and calculate its bond order. ?

Evaluate Yourself Metallic bonding Metals have some special properties of lustre, high density, high electrical and thermal conductivity, malleability and ductility, and high melting and boiling points. The forces that keep the atoms of the metal so closely in a metallic crystal constitute what is generally known as the metallic bond. The metallic bond is not just an electrovalent bond(ionic bond), as the latter is formed between atoms of different electro negativities. Similarly, the metallic bond is not a covalent bond,as the metal atoms do not have sufficient number of valence electrons for mutual sharing with or neighboring metal atoms in a crystal.

So, we have to search for a new theory to explain metallic bond. The first successful theory is due to Drude and Lorentz, which regards metallic crystal as an assemblage of positive ions immersed in a gas of free electrons. The free electrons are due to ionization of the valence electrons of the atoms of the metal. As the valence electrons of the atoms are freely shared by all the ions in the crystal, the metallic bonding is also referred to as electronic bonding.

As the free electrons repel each other, they are uniformly distributed around the metal ions. Many physical properties of the metals can be explained by this theory, nevertheless there are exceptions. The electrostatic attraction between the metal ions and the free electrons yields a three-dimensional close packed crystal with a large number of nearest metal ions. So, metals have high density.

As the close packed structure contains many slip planes along which movement can occur during mechanical loading, the metal acquires ductility. Pure metals can undergo to % elongation prior to rupturing under mechanical loading. As each metal ion is surrounded by electron cloud in all directions, the metallic bonding has no directional properties. As the electrons are free to move around the positive ions, the metals exhibit high electrical and thermal conductivity.

The metallic luster is due to reflection of light by the electron cloud. As the metallic bond is strong enough, the metal atoms are reluctant to break apart into a liquid or gas, so the metals have high melting and boiling points. The bonding in metal is better treated by Molecular orbital theory. As per this theory, the atomic orbitals of large number of atoms in a crystal overlap to form numerous bonding and antibonding molecular orbitals without any band gap.

The bonding molecular orbitals are completely filled with an electron pair in each, and the antibonding molecular orbitals are empty. Absence of band gap accounts for high electrical conductivity of metals. High thermal conductivity is due to thermal excitation of many electrons from the valence band to the conductance band. With an increase in temperature, the electrical conductivity decreases due to vigorous thermal motion of lattice ions that disrupts the uniform lattice structure, that is required for free motion of electrons within the crystal.

Most metals are black except copper, silver and gold. It is due to absorption of light of all wavelengths. Absorption of light of all wavelengths is due to absence of bandgap in metals. SUMMARY In molecules, atoms are held together by attractive forces, called chemical bonds.

Kossel and Lewis are the first people to provide a logical explanation for chemical bonding. They proposed that atoms try to attain the nearest noble gas electronic configuration by losing, gaining or sharing one or more electrons during the bond formation. The noble gases contain eight electrons in their valance shell which is considered to be stable electronic configuration. The idea of Kossel – Lewis approach to chemical bond lead to the octet rule, which states that “the atoms transfer or share electrons so that all atoms involved in chemical bonding obtain electrons in their outer shell (valance shell)”.

There are different types of chemical bonds. In compounds such as sodium chloride, the sodium atom loses an electron which is accepted by the chlorine atom resulting in the formation of Na + and Cl - ions. These two ions are held together by the electrostatic attractive forces. This type of chemical bond is known as ionic bonds or electrovalent bonds.

In certain compounds, instead of the complete transfer of electrons, the electrons are shared by both the bonding atoms. The two combining atoms are held together by their mutual attraction towards the shared electrons. This type of bond is called covalent bonding. In addition, there also another bond type known as coordinate covalent bonds, where the shared electrons of a covalent bond are provided by only one of the combining atoms.

Metallic bonding is another type of bonding which is observed in metals. Lewis theory in combination with VSEPR theory will be useful in predicting the shape of molecules. According to this theory, the shape of the molecules depends on the number of valance shell electron pair (lone pairs and bond pairs) around the central atom. Each pair of valance electrons around the central atom repels each other and hence, they are located as far away as possible in three-dimensional space to minimise the repulsion between them.

Heitler and London gave a theoretical treatment to explain the formation of covalent bond in hydrogen molecule on the basis of wave mechanics of electrons. It was further developed by Pauling and Slater. According to this theory when half-filled orbitals of two atoms overlap, a covalent bond will be formed between them. Linus Pauling introduced the concept of hybridisation.

Hybridisation is the process of mixing of atomic orbitals of the same atom with comparable energy to form equal number of new equivalent orbitals with same energy. There are different types of hybridization such as sp, sp , sp , sp d etc.. F. Hund and Robert.

S. Mulliken developed a bonding theory called molecular orbital theory. According to this theory, when atoms combines to form molecules, their individual atomic orbitals lose their identity and forms new orbitals called molecular orbitals. The filling of electrons in these orbitals follows Aufbau's principle, Pauli's exclusion principle and Hund's rule as in the case of filling of electrons in atomic orbitals.

Evaluation I. Choose the best answer. . In which of the following Compounds does the central atom obey the octet rule?

a) XeF b) AlCl c) SF d) SCl . In the molecule O A O B , the formal charge on O A , C and O B are respectively. a) - , , + b) + , ,- c) - , ,+ d) , , . Which of the following is electron deficient?

a) PH b) (CH ) c) BH d) NH . Which of the following molecule contain no л bond? a) SO b) NO c) CO d) H O . The ratio of number of sigma (σ) and pi (л) bonds in - butynal is a) / b) / c) / d) / .

Which one of the following is the likely bond angles of sulphur tetrafluoride molecule? a) , b) . c) d) , . Assertion: Oxygen molecule is paramagnetic.

Reason : It has two unpaired electron in its bonding molecular orbital a) both assertion and reason are true and reason is the correct explanation of assertion b) both assertion and reason are true but reason is not the correct explanation of assertion c) assertion is true but reason is false d) Both assertion and reason are false . According to Valence bond theory, a bond between two atoms is formed when a) fully filled atomic orbitals overlap b) half filled atomic orbitals overlap c) non- bonding atomic orbitals overlap d) empty atomic orbitals overlap . In ClF ,NF and BF molecules the chlorine, nitrogen and boron atoms are a) sp hybridised b) sp ,sp and sp respectively c) sp hybridised d) sp d, sp and sp hybridised respectively . When one s and three p orbitals hybridise, a) four equivalent orbitals at to each other will be formed b) four equvivalent orbitals at ' to each other will be formed.

c) four equivalent orbitals, that are lying the same plane will be formed d) none of these . Which of these represents the correct order of their increasing bond order. a) C < C - < O - < O b) C - < C + < O < O - c) O - < O < C - < C d) O - < C + < O < C - . Hybridisation of central atom in PCl involves the mixing of orbitals.

a) s, p x , p y , d x2 , d x2-y2 b) s, p x . p y , p xy . d x2-y2 c) s, p x , p y , p z , d x2-y2 d) s, p x , p y , d xy , d x2-y2 . The correct order of O-O bond length in hydrogen peroxide, ozone and oxygen is a) H O > O >O b) O > O > H O c) O > H O > O d) O > O > H O .

Which one of the following is diamagnetic.? a) O b) O - c) O d) None of these . Bond order of a species is . and the number of electons in its bonding molecular orbital is formd to be The no.

of electons in its antibonding molecular orbital is a) three b) four c) Zero d) can not be calculated from the given information. . Shape and hybridisation of IF are a) Trigonal bipyramidal, Sp d b) Trigonal bipyramidal, Sp d c) Square pyramidal, Sp d d) Octahedral, Sp d . Pick out the incorrect statement from the following a) Sp hybrid orbitals are equivalent and are at an angle of ' with eachother b) dsp hybrid orbitals are equivalent and bond angle between any two of them is c) All five sp d hybrid orbitals are not equivalent out of these five sp d hybrid orbitals, three are at an angle of , remainir two are perpendicular to the plane containing the other three d) none of these .

The molecules having same hybridisation, shape and number of lone pairs of electons are a) SeF , XeO F b) SF , Xe F c) XeOF , TeF d) SeCl , XeF . In which of the following molecules / ions BF , NO - , H O the central atom is sp hybridised? a) NH - and H O b) NO - and H O c) BF and NO - d) BF and NH - . Some of the following properties of two species, NO - and H O + are described below.

which one of them is correct? a) dissimilar in hybridisation for the central atom with different structure. b) isostructural with same hybridisation for the Central atom. c) different hybridiration for the central atom with same structure d) none of these .

The types of hybridiration on the five carbon atom from right to left in the, , pentadiene. a) sp , sp , sp, sp , sp b) sp , sp, sp, sp, sp c) sp , sp, sp ,sp , sp d) sp , sp , sp , sp , sp . Xe F is isostructural with a) SbCl b) BaCl c) TeF d) ICl – . The percentage of s-character of the hybrid orbitals in methane, ethane, ethene and ethyne are respectively a) , , .

, b) , , . , c) , , . , d) , , , . Of the following molecules, which have shape similar to carbon dioxide?

a) SnCl b) NO c) C H d) All of these. . According to VSEPR theory, the repulsion between different parts of electrons obey the order. a) l.p – l.p > b.p–b.p> l.p–b.p b) b.p–b.p> b.p–l.p> l.p–b.p c) l.p–l.p> b.p–l.p > b.p–b.p d) b.p–b.p> l.p–l.p> b.p–l.p .

Shape of ClF is a) Planar triangular b) Pyramidal c) 'T' Shaped d) none of these . Non- Zero dipole moment is shown by a) CO b) p-dichlorobenzene c) carbontetrachloride d) water. . Which of the following conditions is not correct for resonating structures?

a) the contributing structure must have the same number of unpaired electrons b) the contributing structures should have similar energies c) the resonance hybrid should have higher energy than any of the contributing structure. d) none of these . Among the following, the compound that contains, ionic, covalent and Co- ordinate linkage is a) NH Cl b) NH c) NaCl d) none of these . CaO and NaCl have the same crystal structure and approximately the same radii.

It U is the lattice energy of NaCl, the approximate lattice energy of CaO is a) U b) 2U c) U/ d) 4U II. Write brief answer to the following questions. . Define the following i) Bond order ii) Hybridisation iii) σ - bond .

What is a pi bond? . In CH , NH and H O, the central atom undergoes sp hybridisation - yet their bond angles are different. why?

. Explain Sp hybridisation in BF . Draw the M.O diagram for oxygen molecule calculate its bond order and show that O is paramagnetic. .

Draw MO diagram of CO and calculate its bond order. . What do you understand by Linear combination of atomic orbitals in MO theory. .

Discuss the formation of N molecule using MO Theory . What is dipolemoment? . Linear form of carbondioxide molecule has two polar bonds.

yet the molecule has Zero dipolement why? . Draw the Lewis structures for the following species. i) NO – ii) SO – iii) HNO iv) O .

Explain the bond formation in BeCl and MgCl . . Which bond is stronger σ or π ? Why?

. Define bond energy. . Hydrogen gas is diatomic where as inert gases are monoatomic – explain on the basis of MO theory.

. What is Polar Covalent bond? explain with example. .

Considering x- axis as molecular axis, which out of the following will form a sigma bond. i) 1s and 2p y ii) 2P x and 2P x iii) 2p x and 2p z iv) 1s and 2p z . Explain resonance with reference to carbonate ion? .

Explain the bond formation in ethylene and acetylene. . What type of hybridisations are possible in the following geometeries? a) octahedral b) tetrahedral c) square planer.

. Explain VSEPR theory. Applying this theory to predict the shapes of IF , and SF . CO and H O both are triatomic molecule but their dipole moment values are different.

Why? . Which one of the following has highest bond order? N , N + or N – .

Explain the covalent character in ionic bond. . Describe fajan's rule. Chemical Bonding Types of Bonding Covalent Partial ionic character Partial covalent character (Fajan’s Rule) Ionic Co-ordinate Metallic Theories of Bonding Lewis theory VSEPR thoery VBT (hybridisation) MOT Bond Parameters Bond length Bond order Bond angle Bond enthalpy Dot Structure Resonance Formal Charges Electron pair (bp & lp) Shape of molecules sp,sp , sp dsp sp d, sp d , sp d MO diagram Bond order Magnetic property CONCEPT MAP