CHEMICAL BONDING & MOLECULAR STRUCTURE [PART 2], CLASS 11 CHEMISTRY

A CHEMICAL BOND IS AN ATOM-TO-ATOM ATTRACTION. THIS ATTRACTION CAN BE EXPLAINED BY DIFFERENCES IN THE OF BEHAVIOUR OF ATOMS' OUTERMOST OR VALENCE ELECTRONS. THESE ACTIONS BLEND EFFORTLESSLY INTO ONE ANOTHER IN A VARIETY OF SITUATIONS, LEAVING NO VISIBLE DISTINCTION BETWEEN THEM.
MOLECULAR STRUCTURE IS THE THREE-DIMENSIONAL ARRANGEMENT OF ATOMS IN A MOLECULE. IT INCLUDES THE GENERAL SHAPE OF THE MOLECULES, BOND LENGTHS, BOND ANGLES, AND TORSIONAL ANGLES.

1. CLASS 11 CHEMISTRY CHEMICAL BONDING & MOLECULAR STRUCTURE [PART 2]

2. Q. Distinguish between ionic bond and covalent bond. 1. Ionic bond is formed by transfer of an electron from one atom to another. Covalent bond is formed by sharing of electrons between two atoms. 2. Ionic compound has the high melting point. Covalent compound has the low melting point. 3. In ionic bond formation, participating atoms have a high electronegativity difference. In covalent bond formation, participating atoms have almost the same electronegativity. 4. In case of ionic bond, formation of ion is necessary. No cation-anion formation takes place in case of covalent bond. 2

3. Q. What are the conditions for the formation of an ionic bond? A. The the conditions for the formation of an ionic bond between two elements are: 1. The ions of two elements must be of opposite charge. 2.One element must have less ionisation enthalpy[cation]. 3. Another element must have high negative electron gain enthalpy[anion]. 4. Difference of electronegativity between two elements must be greater than or equal to 1.7. 5. High lattice energy of the compound formed. 3

4. Q. What is the difference between an ionic bond and co-ordinate bond? A.1. Ionic bond is created due to electrostatic attraction of oppositely charged ions in a chemical compound. This bond is also formed when the electrons from the valence shell transfer permanently into the other shells. 1. Co-ordinate bond is a type of covalent bond where only one atom donates its electrons to form the bond. When a co-ordinate bond is formed, it is unidentifiable from a covalent bond because there is also an overlapping between the atoms like covalent bonds. Contd. 4

5. Basically, the main difference is that the formation process of these two bonds is utterly different. However, they do have many other differences in different aspect: 2. Element type : Co-ordinate bonding takes place between non-metals, whereas ionic bonding takes place between metals and non-metals. 3. Melting point : Compounds formed by co-ordinate bonding have low melting point [mostly below 300 °C], whereas, compound formed by ionic bonding have high melting point [mostly above 300°C]. Contd. 5

6. 4. Physical state : Compounds formed by co-ordinate bonding may be solid, liquid or gas. Compounds formed by ionic bonding are solid. 5. Water solubility : Water solubility of co-ordinate compounds varies from high to low. On the other hand, water solubility of ionic compounds are mostly high. 6. Electrical conductivity : Co-ordinate compounds shows poor Electrical conductivity, whereas ionic compounds are good conductor. 6

7. Q. What is the difference between metallic bond and ionic bond? A.1. Metallic bond is formed between two similar atoms. Ionic bond is always formed between two dissimilar atoms. 2. Metallic compounds are good conductors even in the solid state. Ionic compounds conduct only in molten state or in their solutions. 3. Metallic bond is a weak bond. Ionic bond is a strong bond. 4. Metallic compounds are malleable. Compounds having ionic bonds are brittle. 7

8. Q. What is the difference between metallic bond and covalent bond? A.1. Valence electrons are spread all over the crystal. Therefore, metallic bond is non-directional. Valence electrons are localised. Therefore, covalent bond has directional character. 2. In metallic bond,valence electrons are weakly attracted towards the nucleus. Hence, it is a weak bond. Valence electrons are strongly attracted towards the nucleus. Hence, covalent bonds are strong bonds. 3. Metallic compounds are good conductors of heat and electricity. Covalent compounds are bad conductors. 4. Metallic bond exists between similar atoms. Covalent bond can be formed between similar as well as dissimilar atoms. 8

9. Q. What is the difference between hydrogen bond and ionic bond? A.1.The key difference between hydrogen bond and ionic bond is that ionic bonding exists between permanent anions and cations, whereas hydrogen bond exist between partial positive and partial negative charges. 2. Hydrogen bonds are intermolecular attractions, while ionic bonds are attractive electrostatic forces. 3. Hydrogen bonds occur when there is a hydrogen atom and an electronegative atom, while ionic bonds occur between any metal and non-metal atom. 4. Hydrogen bonds are easy to break. But, due to strong chemical bonds, ionic bonds are difficult to break. 9

10. Q. What is the difference between hydrogen bond and covalent bond? A.1. Hydrogen bonding arises due to electrostatic force of attraction, whereas covalent bond is formed by mutual sharing of pairing of electrons. 2. Hydrogen bond only affects the physical properties while there is a change in chemical properties of the constituents of a covalent compound. 3. Hydrogen bond is much a weaker bond as compared to the covalent bond. For example, the bond strength of hydrogen bond in H-F is about 40kJmol -1 as compared to 433kJmol -1 for H-H bond. 10

11. Q. What is meant by polarity of bond? A. Polarity is the distribution of electrical charge among the atoms connected by a chemical bond. By the term bond polarity, we mean the separation of the charges i.e. the positive and the negative charges along the chemical bond. The separation of positive and the negative charges results in the formation of the electric dipole or simply we can say the dipole moment. 11

12. Q. What is polar and non-polar molecules? A. Polar molecules are those which have a net dipole moment due to the differences in the electronegativities of the atoms present in the molecule which do not allow to cancel the dipoles. Non-polar molecules are those which net dipole moment as zero as the dipoles of the atoms are cancelled out within the molecule. 12

13. Q. What is non-polar covalent bond? A. If two similar atoms come close to each other and form a bond by sharing their electrons, the shared electrons are equally attracted by the two atoms as the electronegativity of the atoms is same. Hence, no poles are developed. This leads to the formation of completely non-polar bonds. If two hydrogen atoms form a bond, the electron pair will lie exactly in the middle between the two atoms. The electron cloud is completely symmetrical and there is no charge separation at all. For example: Cl 2 , O 2 , N 2 , F 2 etc. 13

14. Q. What is polar covalent bond? A. When two dissimilar atoms, having different electronegativities combine together to form a covalent bond, the shared pair of electrons does not lie at equal distance from the nuclei of both the bonded atom but shift towards the atom having greater electronegativity. The more electronegative atom attracts the electrons more strongly. Hence, the distribution of electrons get distorted i.e. the electron cloud is displaced more towards the more electronegative atom. One end of the molecule, having more electronegative atom becomes slightly negatively charged while the other end acquire slightly positive charge. Positive and negative poles are developed and this type of bond is called polar covalent bond. 14

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16. Q. What is dipole moment? A. Due to polarity, polar molecules are known as dipole molecules and they possess dipole moment. Dipole moment is defined as the product of magnitude of the positive or negative charge and the distance between the charges. Dipole moment[ μ] = Charge[Q] * Distance of separation [D]. It is expressed in Debye units [D], 1D = 3.33564 * 10 -30 cm. Dipole moment is a vector quantity. It is depicted by a small arrow. It is depicted by a small arrow with tail on positive centre and pointing towards the negative centre. With the help of dipole moment, the degree of polarity of bonds can be expressed. 16

17. Q. What are the applications of dipole moment? A. Applications of the dipole moment are the followings: 1. Determination of the polarity of the molecules. 2. Finding of the shape of the molecules. For example, the molecule with zero dipole moment will be linear or symmetrical. Those molecules which have unsymmetrical shapes will be either bent or angular. 3. Calculation of the percentage of ionic character of polar bonds. 17

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19. Q. What was the reason behind the development of valence bond theory? A. The octet rule fails to explain the structure of many molecules, forces of interaction, bond energies and geometry of molecules. Hence, an alternative theory known as ‘valence bond theory’ was developed by Heitler and London in 1927. Pauling and Slater modified this theory to explain the directional character of covalent bonds. According to Heiteler and London,“The presence of one or more unpaired electrons [half-filled orbitals] in the valence shell of an atom is the cause of the chemical bonding”. Contd. 19

20. Noble gases do not take part in bonding because these do not have unpaired electrons. The combining capacity or covalency of an atom is equal to the number of unpaired electrons present in its ground or excited state. Q. What are the postulates of ‘Valence Bond Theory’? A.1.Every atom has a tendency to pair up its unpaired electrons with the oppositely spinned electrons of the other atoms. Hence, these get bonded to one another. Contd. 20

21. 2. Half-filled atomic orbitals of the two atoms [having electrons with opposite spin] come in proper alignment and proceed towards one another to overlap. 3. The overlapping of two half-filled valence orbitals of two different atoms gives rise to a molecular orbital or covalent bond. The energy of the molecular orbital is less than that of the atomic orbitals overlapped. 4. Due to overlapping, the electron density between two bonded atoms increases and this gives stability to the molecule. Contd. 21

22. 4. Greater the overlapping of atomic orbitals, the stronger is the bond formed. Complete overlapping is impossible due to inter-nuclear repulsion . 5. In case the atomic orbitals possess more than one unpaired electron, more than one bond can be formed and electrons paired in the valence shell cannot take part in such a bond formation. 6. A covalent bond is directional and it is parallel to the region of overlapping atomic orbitals. Contd. 22

23. 7. Based on the pattern of overlapping, molecular orbital or covalent bond are of two types: a] sigma bond( σ) and b] pi(π) bond. The covalent bond formed by sideways overlapping of atomic orbitals is known as pi(π) bond , whereas the bond formed by overlapping of atomic orbital along the internuclear axis is known as as a sigma(σ) bond . 23

24. Q. What are the modifications of valence bond theory done by Pauling and Slater? A.1.The extent of overlapping depends upon the nature of orbitals involved in overlapping and nature of overlapping. 2. Overlapping is more if valence shells are closer to the nucleus. 3. In case of two subshells of same energy level, the subshell having more directional character shows more overlapping. Order of overlap and bond energy is: p-p > s-p > s-s 4. s-orbitals show only head-on overlapping due to their symmetrical nature. However, p-orbitals due to their directional character can show either head-on overlapping or lateral overlapping. 5. Head-on overlapping is stronger than lateral overlapping. 24

25. Q. What is sigma[ σ] bond? A. Sigma bond is formed by the head on overlapping of half-filled atomic orbitals along the inter-nuclear axis [i.e. the line passing through the centres of nuclei of the atoms.]. This type of overlap is also called axis overlap. Formation of sigma bond involves one of the following types of overlap. 1. s-s overlapping: When a partially filled s-orbital of one atom overlaps with the partially filled s-orbital of another atom then the overlapping and the bond is called s-s bond. An example of s-s overlapping is furnished by the hydrogen molecule where the two 1s orbitals of two hydrogen atoms overlap to form H 2 molecule. Contd. 25

26. 2. s-p overlapping: The bond formed by the overlap of partially filled s-orbital of one atom and p-orbital of another atom is again a σ-bond. It is known as s-p bond and this type of overlapping is known as s-p overlapping. Formation of HF is an example of s-p overlapping in which half-filled 1s-orbital of hydrogen overlaps with a half-filled 2p orbital of fluorine. 3. p-p overlapping: When the two partially filled p-orbitals overlap, the overlapping is known as p-p overlapping. The bond formed by overlapping of two partially filled p-orbitals is a p-p σ bond if the orbitals get close enough to overlap along their axis. An important example of p-p σ bond is F 2 molecule in which a half-filled 2p-orbital of one F-atom overlaps with half-filled 2p-orbital of another F-atom. 26

27. Q. What is Pi[ π] bond? A. This type of bond is formed by the sidewise overlapping of half-filled atomic orbitals in a direction perpendicular to the internuclear axis. It is a weak bond as the extent of overlapping of atomic orbitals is very small. The only type of sidewise overlapping is the p-p overlapping perpendicular to the internuclear axis. On coming closer, the two p-orbitals perpendicular to the internuclear axis, slightly overlap laterally. The upper lobes of the two p-orbitals overlap above and and lower lobes below the internuclear axis.Pi–molecular orbital possesses electron cloud above and below the internuclear axis. 27

28. Q. What is the difference between sigma[ σ] bond and pi[π] bond? A.1.Sigma bond is formed by the head on overlapping of s-s, s-p, p-p orbitals along the inter-nuclear axis. Pi- bond is formed by the side-wise overlapping of p-orbitals. 2.Sigma bond is a strong bond because orbitals overlap to a greater extent. Pi-bond is a weak bond because orbitals overlap to a less extent. 3. s-orbitals take part in the formation of sigma bond. s-orbitals do not take part in the formation of pi-bond. Contd. 28

29. 4. In case of sigma bond, overlapping of atomic orbitals takes place symmetrically along the inter-nuclear axis. In case of pi-bond, overlapping of atomic orbitals takes place symmetrically above and below the inter-nuclear axis. 5. One lobe of p-orbital taking part in σ-bond formation is stretched while the other is reduced in size. Both lobes of p-orbitals taking part in a π-bond formation remain of the same size. 6. There is a free rotation of atoms about the σ-bond. There cannot be free rotation of atoms about the π-bond. 7. Shape of a molecule is decided by the number of σ-bonds. Shape of a molecule does not depend upon the π-bonds. 29

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31. Q. What is the cause behind ‘resonance’ ? A. In case of certain molecules, a single Lewis structure cannot explain all the properties of the molecule. The molecule is then supposed to have many structures, each of which can explain most of the properties of the molecules but none can explain all the properties of the molecule. The actual structure is in between of all these constituting structure and is called resonance hybrid and different individual are called resonating structures or canonical forms. 31

32. Q. What is resonance? A. The phenomenon of existence of a molecule in many structures due to delocalization of electrons is called resonance. These structures are similar in energy, bonding and non-bonding pairs of electrons. Each form of structure is known as canonical form. Resonance stabilizes the molecule and it causes equality of bond lengths. Q. What are the characteristics of resonance? A. Resonating structures are not isomers. They differ in position of electrons, but not in position of nucleus. Real structures have less potential energy than contributing structures. 32

33. Q. Explain resonance with an example. A. If we consider the structure of ozone [O 3 ], each oxygen atom has an octet of an electron. But, this structure is unsatisfactory because it depicts the central O atom to be bonded to one O atom by a double bond and other oxygen atom by a single bond. Since, double bond is shorter than single bond, the two bond length in this molecule should be unequal. But experimental evidence not only show that the bond length are equal but also shows that the bonds are intermediate between single and double bond. Contd. 33

34. Therefore, for molecules like Ozone, a single lewis structure is unable to explain the observed facts. Hence, an alternative Lewis structure can be written in which the double and single bonds are interchanged. The actual structure is intermediate between the two Lewis structure and it is called resonance hybrid. 34

35. Image Source: ResearchGate Resonating structure of Ozone [O 3 ] molecule 35

36. Q. Why does resonance occur? A. Electron delocalization lowers the potential energy of the substance and thus makes it more stable than any of the contributing structures. The difference between the potential energy of the actual structure and that of the contributing structure with the lowest potential energy causes resonance effect. Q. What is resonance energy? A. The difference between the energy of the resonance hybrid and that of the most stable contributing structure is called as resonance energy. 36

37. Q. What are the rules for writing of resonating structure? A.1.The contributing structures should have the same position of atoms. They should differ only in the position of electrons. 2. The contributing structures should have the same number of electrons and have nearly same energy. 3. The contributing structures should have negative charge on the electronegative atom and the positive charge on the electropositive atoms. 4. In a contributing structure, like charges should not be present on adjacent atom while unlike charges should not be widely separated. 37

38. Q. What is molecular geometry? A. The study of the three-dimensional arrangement of the atoms that constitute a molecule is called molecular geometry. Molecular geometry gives information about the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that determines the position of each atom. Q.How does molecular geometry can be determined? A. The molecular geometry can be determined by various spectroscopic methods and diffraction methods. 38

39. Q.What does account for the shape of a covalent molecule? A. In covalent molecules, the bonds are of directional nature because the shared pairs of electrons remain localized in a definite space between the nuclei of the participating atoms. The space model obtained by joining the points which represent the different bonds in a covalent molecule accounts for its shape. Q.What are the different types of molecular geometry? A. 1.Linear molecular geometry : In this structure two molecules are attached to the central atom. So, they are arranged in the opposite direction in order to minimize their repulsion. The bond angle of this structure is 180 ° . Example: BeCl 2 , MgCl 2 . Contd. 39

40. 2. Trigonal planar molecular geometry: In this type of molecule, we find three molecules attached to a central atom. So, they are arranged towards the corners of an equilateral triangle in order to minimise their repulsion. The bond angle of this structure is 120 ° . Example: BF 3 ,BCl 3 , AlF 3 etc. 3. Trigonal Bipyramidal molecular geometry: In this structure, repulsion can be minimized by even distribution of electrons towards the corner of a trigonal pyramid. In a trigonal bipyramid, three positions lie along the equator of the molecule. The two positions lie along an axis perpendicular to the equatorial plane. The bond angle are 90 ° and 120 ° . Example: PF 5 40

41. 4. Tetrahedral molecular geometry: In this structure, a central atom is located at the centre with four substituents that are located at the corners of a tetrahedron. The bond angle of the structure is 109 ° 28 ′ . Example: CH 4 , CCl 4 etc. 5.Octahedral molecular geometry: Octahedral molecular geometry describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The bond angle is 90 ° . Examples: SF 6 , TeF 6 etc. 41

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