CHEMICAL BONDING AND MOLECULAR STRUCTURE [PART: 3], CLASS 11 CHEMISTRY

CHEMICAL BONDING IS THE FORCE THAT HOLDS ATOMS TOGETHER IN MOLECULES AND SUBSTANCES. IT IS CAUSED BY ELECTROSTATIC ATTRACTION BETWEEN OPPOSITELY CHARGED PARTICLES.
WHEN ATOMS APPROACH EACH OTHER, THEIR ELECTRONS AND NUCLEI INTERACT. THE ELECTRONS AND NUCLEI ARRANGE THEMSELVES IN A WAY THAT LOWERS THE TOTAL ENERGY OF THE SYSTEM. IF THE TOTAL ENERGY IS LOWER THAN THE SUM OF THE ENERGIES OF THE INDIVIDUAL ATOMS, THE ATOMS BOND TOGETHER. DIFFERENT TYPES OF CHEMICAL BONDS ARE THERE: IONIC, COVALENT, METALLIC AND HYDROGEN. CHEMICAL BONDS ARE ESSENTIAL FOR CHEMICAL REACTIONS, MATERIAL FORMATION, AND THE CREATION OF MOLECULES AND SUBSTANCES THAT MAKE UP THE WORLD AROUND US. MOLECULAR GEOMETRY IS THE THREE-DIMENSIONAL ARRANGEMENT OF THE ATOMS THAT CONSTITUTE A MOLECULE. IT INCLUDES 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.

1. CLASS 11 CHEMISTRY CHEMICAL BONDING & MOLECULAR STRUCTURE PART 3

2. Q. What is meant by the shape of a molecule? A. The definite relative arrangement of bonded atoms in a molecule is called geometry or shape of molecule. Q. Why the shape of a molecule is important? A. The shape of a molecule is important in determining its physical and chemical properties. For example, methane[CH 4 ] and water[H 2 O] have nearly equal molecular weights but methane boils at 112K and water boils at 373K. Much of this great difference in boiling point is due to the difference in shapes of these molecules. Similarly, physico-chemical properties of the biologically important DNA molecule are partly due to its double spiral shape. 2

3. Q. Why do the molecules have a definite shape? A. Each molecule has a definite geometry because the atoms combine to form a molecule in order to attain a state of minimum energy. Since, there can be only one arrangement of bonded atoms[or electron pairs] around the central atom corresponding to a state of minimum energy, molecules can have only fixed shape or definite geometry. 3

4. Q. What does geometry of molecules explain? A. Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes general shapes of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that determine the position of each atom. Q. What is the geometry of a covalent molecule? A. The geometry of covalent molecules is regular if the central atom is surrounded by all bond pairs of electrons with similar atoms. The repulsive interactions in the bond pairs mutually balance each other. 4

5. Q. What is the basis of ‘VSEPR’ theory? A. The ‘VSEPR’ theory [valence shell electron pair repulsion theory] is based on the fact that there is a repulsion between the pairs of valence electrons in all the atoms, and the atoms will always try to arrange themselves in a manner in which the electron pair repulsion is minimized. Q. Name the scientists who were behind ‘VSEPR’ theory. A. The basic idea of ‘VSEPR’ proposed by Sidgwick and Powell[1940] and finally developed by Gillespie and Nyholm. 5

6. Q. What are the main postulates of ‘VSEPR’ theory? A.1.The unpaired electrons in the valence shell of central atom form bond pairs with the unpaired electrons of surrounding atoms while paired electrons remain as lone pairs. 2. Being similarly charged, the bond pairs as well as the lone pairs tend to repel one another. 3. Electron pairs around the central atom can adjust themselves with with respect to one another in such a way that the force of repulsion among them is minimum. Contd. 6

7. 4. The force of repulsion is minimum when the electron pairs are far away from one another. 5. The force of repulsion among the bond pairs and the lone pairs is not the same. The order of force of repulsion is as follows: Lone pair-Lone pair repulsion > Lone pair-Bond pair repulsion > Bone pair-bond pair repulsion. 7

8. Q. Give reasons: Lone pair-Lone pair repulsion > Lone pair-Bond pair repulsion > Bone pair-bond pair repulsion. A. Lone pair of electrons is that pair which is present in the valence shell of the central atom. It does not take part in the bonding [except, when required] and is influenced by the nucleus of the central atom only. Bond pair of electrons is that pair which is formed by the sharing of electrons between two bonded atoms. The shared pair of elec trons is under the influence of two nuclei. Contd. 8

9. Being under the attractive influence of two nuclei, the electron cloud of the shared pair ‘contracts’ and occupies a smaller volume. The electron cloud of the lone pair being under the attractive influence of only ‘one’ nucleus ‘spreads out’ and occupies a larger volume. Thus, lone pairs due to their larger electron clouds are more nearer to other lone pairs. This leads to maximum repulsion between lone pairs. Due to the small size of electron cloud of bond pairs, the repulsive force between the two bond pairs will be the least. The repulsion between a lone pair and a bond pair wouild be in between the two extreme cases. 9

10. Q. What is the ‘VSEPR’ theory of geometry of shape? A. The valence shell electron repulsion [VSEPR] theory is a model used to predict 3-D molecular geometry based on the number of valence shell electron bond pairs among the atoms in a molecule or ion. This model assumes that electron pairs will arrange themselves to minimize repulsion effects from one another. 10

11. Q. What is the significance between the shape of the molecules and the number of electron pairs around the central atom? Explain with an example. A. According to the ‘VSEPR’ theory, the shape of a molecule depends upon the repulsive interactions among the electron pairs around the central atom. The electron pairs arrange themselves around the central atom in such a way that the force of repulsion among them is minimum. Therefore, the shape of a molecule is decided by the number of electron pairs [bond pairs as well as lone pairs] around the central atom. Contd. 11

12. As for example, in case of [Image Source: Socratic] There are only two electron pairs around the central Be atom. According to VSEPR theory, these electron pairs will arrange themselves to be as far apart as possible, so that the repulsions between them are minimum. When there are two electron pairs in the valence shell, the repulsive force will be minimum if these electron pairs are located on opposite sides of the nucleus. In BeCl 2 , the chlorine atoms are attached to the Be atom by the sharing of these electron pairs. This means that two chlorine atoms must be placed where the electron pairs are and the molecule should, therefore have the line structure as Cl : Be : Cl. Contd. 12

13. In terms of VSEPR theory, a pair of electrons in the valence shell, either bonding or lone pair, influences the structure of the molecule. These electron pairs are called stereoactive because they are effective in determining the the steric of structural arrangement of atoms in a molecule. The total number of electron pairs in the valence shell of the central atom is called the steric number [SN]. However, the lone pairs of electrons due to their more diffuse[spread out] electron clouds cause larger repulsions. Due to this, bond pairs move away from them and come closer to each other. Contd. 13

14. This results in the decrease in bond angles from the normal expected values. Examples are CH 4 , NH 3 and water. All of them have SN=4, but in NH 3 and water the bond angles are 107 ° and104.5° respectively as compared to he tetrahedral value of 109.5°[109°- 28′] in CH 4 . Q. What is meant by regular geometry of a molecule? A. A molecule have a regular geometry if the repulsive interactions between the electron pairs are of equal magnitude. 14

15. Q. What are the conditions of regular geometry of a molecule? A.1.The central atom of the molecule should be bonded to all similar atoms,e.g. CH 4 , CCl 4 . 2. The central atom should be surrounded by bonded pair of electrons and should have no lone pair of electrons. 3. All the bond lengths between the central atom and the bonded atoms should be the same. 4. The central atom should be bonded to other atoms by single covalent bonds [or same type of bonds]. 15

16. Q. What are the conditions of a irregular or distorted geometry of a molecule? A. A molecule will have irregular geometry if the repulsive interactions between the electron pairs [around the central atom] are not of equal magnitude. Q. What are the conditions of a irregular or distorted geometry? A.1. The central atom of the molecule should not be bonded to all the similar atoms e.g., CHCl 3 , CHBr 3 . 2. The central atom should have one or more lone pair of electrons in addition to one or more bond pair of electrons. Contd. 16

17. The presence of lone pairs on the central atom causes lone pair-lone pair and lone pair-bond pair repulsive interactions in addition to the bond-bond pair repulsive interactive actions. The additional repulsive interactions distort the regular geometry of the molecule. 3. The central atom may be bonded to similar atoms but should have different bond lengths. 4. The central atom should be bonded to other atoms by single as well as multiple bonds, e.g., SO 3 . 17

18. Q. What is hybridization in chemistry? A.Hybridization, in chemistry, is defined as the concept of mixing two orbitals to give rise to a new type of hybridized orbitals. This intermixing usually results in the formation of hybrid orbitals having entirely different energy, shapes, etc. Q. What are the types of hybridization? A. Depending on the types of orbitals included in the mixing, hybridization could be characterized as sp 3 , sp 2 , sp, sp 3 d, sp 3 d 2 or sp 3 d 3 . 18

19. Q. What is sp hybridization ? A. sp hybridization is a type of hybridization in which one s and one p orbital in the same main shell of an atom mix to form two new equivalent orbitals. The new orbitals formed are called sp hybridized orbitals. It forms linear molecules with an angle of 180 °. 1. This type of hybridization involves the mixing of one ‘s’ orbital and one ‘p’ orbital of equal energy to give a new hybrid orbital known as an sp hybridized orbita l. 2.The sp hybridization is also called diagonal hybridization. Contd. 19

20. 3. Each sp hybridized orbital has an equal amount of s and p characters -- 50%s and 50%p characters. Examples of sp hybridization: A] All compounds of Beryllium, like BeF 2 , BeH 2 , BeCl 2 etc. B] All compounds of Carbon-containing triple bond, like C 2 H 2 . 20

21. sp Hybridization: Image Source: AdiChemistry 21

22. Q. What is sp 2 hybridization? A. The sp 2 hybridization is the mixing of one s and two p atomic orbitals , which involves the promotion of one electron in the s orbital to one of the 2p atomic orbitals. The combination of these atomic orbitals creates three new hybrid orbitals equal in energy-level. The sp 2 hybridization is observed when one s and two p orbitals of the same shell of an atom mix to form three equivalent orbitals. The new orbitals formed are called sp 2 hybrid orbitals. Contd. 22

23. 1. The sp 2 hybridization is also called trigonal hybridization. 2. It involves the mixing of one ‘s’ orbital and to ‘p’ orbitals of equal energy to give a new hybrid orbital known as sp 2 . 3. A mixture of s and p orbital formed in trigonal symmetry and is maintained at 120 °. 4. All three hybrid orbitals remain in one plane and make an angle of 120° with one another. Each of the hybrid orbitals formed has a 33.33%’s’ character and 66.66% ‘p’ character. Contd. 23

24. 5. The molecule in which the central atom is linked to 3 atoms and is sp 2 hybridized have a triangular planar shape. Examples of sp 2 hybridization: 1. All the compounds of Boron, i.e., BF 3 and BH 3 . 2. All the compounds of Carbon, containing a carbon-carbon double bond, ethylene [C 2 H 4 ]. 24

25. sp 2 hybridization: Image Source: AdiChemistry 25

26. Q. What is sp 3 hybridization? A. sp 3 hybridization involves mixing of one ‘s’ orbital and ‘three ‘p’ orbitals of equal energy to give a new hybrid orbital known as sp 3 . A mixture of s and p orbitals formed is in tetrahedral symmetry and is maintained at 109.28 °. When one ‘s’ orbital and three ‘p’ orbitals belonging to the same shell of an atom mix together to form four new equivalent orbitals, the type of hybridization is called a tetrahedral hybridization or sp 3 . The new orbitals formed are called sp 3 hybrid orbitals. Contd. 26

27. 1.These orbitals are directed towards the four corners of a regular tetrahedron and make an angle of 109 °28′ with one another. 2. Each sp 3 hybrid orbital has 25% ‘s’ character and 75% ‘p’ character. 3. Examples of sp 3 hybridization are ethane [C 2 H 6 ] and methane. 27

28. sp 3 Hybridization: Image Source: Madoverchemistry 28

29. Q. What is sp 3 d hybridization? A. The term ‘sp 3 d hybridization’ involves the mixing of 1s orbital, 3p orbitals and 1d orbital to form 5 sp 3 d hybridized orbitals of equal energy. They have trigonal bipyramidal geometry. 1. The mixture of s,p and d orbital trigonal forms bipyramidal symmetry. 2. Three hybrid orbitals lie in the horizontal plane inclined at an angle of 120 ° to each other, known as the equatorial orbitals. Contd. 29

30. 3. The remaining two orbitals lie in the vertical plane at 90 degrees plane of the equatorial orbitals, known as axial orbitals.Example:phosphorus pentachloride[PCl 5 ]. Image Source: AdiChemistry 30

31. Q. What is sp 3 d 2 hybridization? A. sp 3 d 2 hybridization has 1s, 3p, and 2d orbitals, that undergo intermixing to form 6 identical sp 3 d 2 hybrid orbitals. These 6 orbitals are directed towards the corners of an octahedron. They are inclined at an angle of 90 degrees to one another. Example: sulphur hexafluoride. Image Source: AdiChemistry 31

32. Q. What is sp 3 d 3 hybridization? A. The overlapping of one s-orbital, three p-orbitals, and three d-orbitals form the sp 3 d 3 hybridized orbitals. That is, seven hybridized orbitals are formed. The molecule with sp 3 d 3 hybridization possesses pentagonal bipyramidal geometry. Five bonds are created in the X-Y plane, while the remaining two are produced along the Z-axis, one above and one below the plane. The angle between every five bonds in the X-Y plane is 72 °, whereas the angle between bond along Z-axis and X-Y plane is 90 °. Example of sp 3 d 3 hybridization is IF 7 . 32

33. sp 3 d 3 hybridization: Image Source: Madoverchemistry 33

34. Q. What is homonuclear diatomic molecules? A. Molecules formed upon the bonding of two same elements are known as homonuclear diatomic molecules. For example, dihydrogen[H 2 ],dinitrogen[N 2 ]. Q. What are the postulates of molecular orbital theory? A. Molecular orbital theory was developed by F. Hund and R. S. Muliken in 1932. Main postulates of this theory are: 1. Atomic orbitals of comparable energy and proper symmetry combine together to form molecular orbitals. Contd. 34

35. 2. The moment of electrons in a molecular orbitals is influenced by all the nuclei of combining atoms. 3. The number of molecular orbitals formed is equal to the number of combining atomic orbitals. When atomic orbitals combine together two molecule orbitals are formed. One molecule orbital possess higher energy than corresponding atomic orbital and is called anti bonding molecular orbital and the other has lower and is called bonding molecular orbital. Contd. 35

36. 4. In molecules, electrons are present in molecular orbitals. The electron filling is in accordance with Pauli’s exclusion principle, Aufbau principle and Hund’s rule. Q. What is hydrogen bond? A. Whenever a molecule contains a hydrogen atom linked to a highly electronegative atom, this atom attracts the shared pair of electrons more and so this end of the molecules becomes slightly negative while the other end become slightly positive. The negative end of one molecule attracts the positive end of the other and as a result , a weak bond is formed between them. This bond is called hydrogen bond. As a result of hydrogen bonding, a hydrogen atom links the two electronegative atoms simultaneously, one by a covalent bond and the other by a hydrogen bond. 36

37. Q. What are the types of hydrogen bonding? A. There are two types of hydrogen bonds, and it is classified as the following: 1. Intermolecular Hydrogen Bonding 2. Intramolecular Hydrogen bonding Q. What is Intermolecular Hydrogen Bonding? A. When hydrogen bonding takes place between different molecules of the same or different compounds, it is called intermolecular hydrogen bonding. For example, hydrogen bonding in water, alcohol, ammonia etc. 37

38. Q. What is intermolecular hydrogen bonding? A. The hydrogen bonding which takes place within a molecule itself is called intramolecular hydrogen bonding . It takes place in compounds containing two groups such that one group contains a hydrogen atom linked to an electronegative atom, and the other group contains a highly electronegative atom linked to a lesser electronegative atom of the other group. The bond is formed between the hydrogen atoms of one group with the more electronegative atom of the other group. 38

39. Types of Hydrogen Bonding: . Image Source: Chemistry Learner 39

40. Q. What are the conditions for hydrogen bonding? A.1.The molecule must contain a highly electronegative atom linked to a hydrogen atom. The higher higher the electronegativity, more is the polarization of the molecule. 2. The size of the electronegative atom should be small. The smaller the size, the greater is the electrostatic attraction. 40

41. Q. Give few examples of hydrogen bonding: A.1. Hydrogen Fluoride Fluorine having the highest value of electronegativity forms the strongest hydrogen bond. Image Source: Meritnation 41

42. 2. Water: Water molecule contains highly electronegative oxygen atom linked to hydrogen atom. Oxygen atom attract the shared pair of electrons more and this end of the molecule becomes negative whereas the hydrogen atoms become positive. Image Source: Study Mind 42

43. Q. What are the conditions for hydrogen bonding? A.1.The molecule must contain a highly electronegative atom linked to hydrogen atom. The higher the electronegativity, more is the polarization of the molecule. 2. The size of the electronegative atom should be small. The smaller the size, the greater is the electrostatic attraction. 43

44. Q. Discuss about the strength of the hydrogen bond. A. Hydrogen bond is a very weak bond. The strength of hydrogen bond is intermediate between the weak Van der walls forces and the strong covalent bonds. The dissociation energy of the hydrogen bond depends upon the attraction of the shared pair of electrons and hence on the electro negativity of the atom. 44

45. Q. What are the effect or consequences of hydrogen bonding? A. 1 . Dissociation: In aqueous solution, HF dissociates and gives the difluoride ion instead of fluoride ion. This is due to hydrogen bonding in HF. The molecules of HCl, HBr, Hi do not form hydrogen bonding. 2.Association: The molecules of carboxylic acids exist as dimer because of the hydrogen bonding. The molecular masses of such compounds are found to be double than those calculated from the simple formula. Contd. 45

46. 3 . High melting point and boiling point: The compounds having hydrogen bonding show abnormally high melting and boiling points. It is due to the fact that some extra energy is needed to break these bonds. 4.Solubility: Lower alcohols are soluble in water because of the hydrogen bonding which can take place between water and alcohol molecule. 5.Volatility: As the compounds having hydrogen bonding between different molecules have higher boiling point, so they are less volatile. Contd. 46

47. 6. Viscosity and Surface tension: The substances which contain hydrogen bonding exist as associated molecule. So their flow becomes comparatively difficult. They have higher viscosity and surface tension. 7. Lower density of ice than water: In case of solid ice, the hydrogen bonding gives rise to a cage like structure of water molecules. As a matter of fact, each water molecule is linked tetrahedral to four water molecules. The molecules are not so closely packed as they are in liquid state. When ice melts, this case like structure collapses and the molecules come closer to each other. Thus, for the same mass of water, the volume decreases and density increases. Therefore, ice has lower density than water at 273K. That is why ice floats. 47

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