Chemical Bonding – Chemistry Optional PYQs (2000–2026)

Here are all 78 Chemical Bonding questions asked in UPSC Chemistry Optional Paper I from 2000 to 2026, arranged under the 3 topics of the official syllabus. Within each topic the latest questions come first.

Each question ends with a code such as [2024/15m/3b] – the year, the marks, the word limit (where UPSC set one) and the question number in the paper.

2.1] Ionic bonding

10 questions

1. Construct the Born-Haber cycle for the formation of sodium chloride crystal at 298 K from the elements in their normal states of existence. Mention the names of the involving processes. Indicate which of them are energy demanding and which are energy evolving. [2025/5m/1b]

2. Sodium bromide and sodium iodide have higher lattice energies than expected from theoretical calculations. Justify. [2024/5m/1a(ii)]

3. The enthalpy of formation of an ionic compound can be calculated with accuracy by Born-Haber cycle. Predict, giving valid reasons, the possibility of formation of $NaC{l}_2$ salt. The following thermodynamic data are given for NaCl and $NaC{l}_2$:

NaCl	$NaC{l}_2$
$U_0=-757kJmo{l}^{-1}$	$U_0=-2155kJmo{l}^{-1}$
$\Delta H_{\left(IE\right)}=+495kJmo{l}^{-1}$	$\Delta H_{\left(I{E}_2\right)}=+4561kJmo{l}^{-1}$
$\Delta H_{\left(EA\right)}=-348kJmo{l}^{-1}$
$\frac{1}{2}\Delta H_{\left(diss\right)}=+121kJmo{l}^{-1}$
$\Delta H_{\left(sub\right)}=+108kJmo{l}^{-1}$

[2021/10m/1b]

4. In which of the following compounds do the bonds have greater covalent character: (a) NaBr or (b) $MgB{r}_2$? Give reasons to support your answer. [2019/5m/1a(i)]

5. Order the following compounds according to their expected lattice energies: LiCl, KCl, KBr, $MgB{r}_2$. Give reasons to support your answer. [2019/5m/1a(ii)]

6. Deduce the Born-Landé equation for ‘lattice energy’. What modifications are imposed on this for better results? [2014/10m/2a]

7. Set up Born – Haber cycle for determining the enthalpy of solvation of $M{g}^{2+}$ ions from $MgC{l}_2$ by water. [2012/12m/1e]

8. What is lattice energy? Describe how lattice energy of an ionic solid is evaluated by Born Haber cycle. [2008/10m/2b(ii)]

9. Explain and bring out the significance of radius ratio rule, lattice energy. Madelung constant and Born-Haber cycle. [2004/20m/1d]

10. Derive Born-Lande equation for the lattice energy of an ionic compound. [2002/20m/3b]

2.2] Covalent bond and valence bond theory

32 questions

1. Distinguish between cis- and trans-dichloroethylene by using dipole moments. [2026/5m/5b]

2. Provide the resonance structures with formal charges for the nitrate ion. [2026/5m/5c]

3. Predict the Lewis structures and geometry of the following as per VSEPR theory: (1) $S{O}_2$, (2) $I_3^{-}$, (3) $S{F}_4$, (4) $Br{F}_5$, (5) $Xe{F}_4$ [2026/10m/6a(i)]

4. Arrange the following molecules in the ascending order of their dipole moment values. Justify your answer. $N{H}_3$, $N{F}_3$ and $H_{2}O$ [2025/5m/1f]

5. Draw all the possible structural dispositions of $Cl{F}_3$ molecule. Establish logically which will be the most favoured disposition. Comment on the shape of $Cl{F}_3$ molecule. [2024/10m/8b(i)]

6. Draw Lewis dot structure of ${\left[B{r}_3\right]}^{-}$ and $H_{2}NCSN{H}_2$ (central C atom is bonded to both the N atoms and to the S atom). Does thiourea contain polar bonds? If yes, which is the most polar bond? [2023/10m/1b]

7. Assign a geometry and hybridization to each carbon atom present in cytosine and thymine nucleotide bases. [2023/10m/5c]

8. Identify the least stable ion of the following ions and justify your answer. $OC{N}^{-}$ $ON{C}^{-}$ $SC{N}^{-}$ [2022/10m/3a]

9. Determine the percentage of ionic character (bond polarity) of BrCl and comment on the nature of Br–Cl bond. (Dipole moment of $BrCl=1.42\times {10}^{-30}$ cm, bond length, $d_{Br\mathrm{\text{–}}Cl}=214\times {10}^{-14}$ m, charge of an electron $=1.6\times {10}^{-19}$ C) [2022/10m/4e]

10. Draw the geometrical arrangements for the following hybridized systems and identify the type of $d$-orbitals involved in each system: $s{p}^{3}d$, $s{p}^3{d}^2$, $ds{p}^2$, $s{d}^3$ [2021/20m/2b]

11. Why $Si{H}_3-NCS$ is linear, whereas $C{H}_3-NCS$ is bent? [2020/10m/3c]

12. What type of hybridization is expected for $S{O}_3^{-}$ and $P{F}_5$? Explain the shapes of these molecules. [2020/10m/3d]

13. Of the two compounds NOF and $N{O}_{2}F$, one has a resultant dipole moment of $\mu =1.81$ D, and the other, $\mu =0.47$ D. Which dipole moment do you predict for each compound? Explain. Also write resonance structures of $N{O}_{2}F$ and identify the dominant structure. [2019/20m/2a]

14. Explain the following on the basis of VBT and hybridization: $Be{F}_2$ is linear whereas $S{F}_2$ is angular. [2018/5m/1d(i)]

15. P forms two chlorides $PC{l}_5$ and $PC{l}_3$ whereas N forms only one $NC{l}_3$. [2018/5m/1d(ii)]

16. What is resonance energy? What are the conditions which permit structures to have resonance? Discuss resonance taking the example of $C{O}_3^{2-}$ ion. [2017/10m/1a]

17. Using valence bond theory justify the presence of two lone electron pairs in $Xe{F}_4$. [2017/15m/4c]

18. The bond angles in $C{H}_4$, $N{H}_3$ and $H_{2}O$ molecules are 109.5°, 107° and 104.5° respectively. Explain. [2016/5m/1a(i)]

19. Comment on the dipole moments of $B{F}_3$ and $P{F}_3$ molecules. [2016/5m/1a(ii)]

20. Chlorine trifluoride, $Cl{F}_3$, is one of the most reactive compounds known. It can be made by heating $C{l}_2$ and $F_2$ in a closed container. (i) Draw the electron-dot structure of $Cl{F}_3$. (ii) What is the electron-pair geometry for $Cl{F}_3$? (iii) Knowing that the molecule is polar, what can you conclude about the molecular geometry? (iv) Calculate the standard enthalpy of formation of gaseous $Cl{F}_3$, given the following bond energies: F—F: $159kJmo{l}^{-1}$; Cl—Cl: $243kJmo{l}^{-1}$; Cl—F: $255kJmo{l}^{-1}$ [2015/20m/6c]

21. Which of the following pairs is expected to have the larger bond angle and why? (1) $H_{2}O$ and $N{H}_3$ (2) $S{F}_2$ and $Be{F}_2$ (3) $B{F}_3$ and $B{F}_4^{-}$ (4) $P{H}_3$ and $N{H}_3$ (5) $N{H}_3$ and $N{F}_3$ [2013/10m/2a(ii)]

22. Using hybridization, explain what is the shape of $Xe{O}_3$. Explain how VSEPR theory accounts for the distortion. [2012/10m/3c(ii)]

23. Using VSEPR theory, predict the shapes of $PC{l}_2{F}_3$ and $S{F}_4$. Indicate the state of hybridisation in each case. [2010/15m/1b]

24. State the essential conditions for hybridisation. Show diagrammatically the possible geometries from hybridisation of s, p and d orbitals. Give examples. [2010/10m/2b(i)]

25. Define dipole moment. Explain the following: (i) Why is dipole moment of $B{F}_3$ zero? (ii) Why is dipole moment of $N{H}_3$ greater than that of $N{F}_3$? (iii) Why does $S{O}_2$ have dipole moment while $C{O}_2$ does not? [2009/20m/2b]

26. How does the principle of ‘maximum overlap’ explain the stability of a chemical bond? Enumerate the essential condition. [2008/20m/1b]

27. What is the essential difference between ionic bond and covalent bond? Explain why water molecule is non-linear while methane molecule is tetrahedral. [2008/10m/2b(i)]

28. What are the main features of valence-bond theory? Point out its shortcomings. [2007/20m/7c]

29. Explain why $C{O}_2$ has a linear structure whereas $S{O}_2$ is a bent molecule. [2005/20m/1b]

30. Justify that s-orbitals are non-directional whereas p- and d-orbitals have directional character. Derive wave functions of $s{p}^3$ hybridized orbitals. [2005/20m/3a]

31. What are the factors that decide the percent ionic character in polar compounds? [2005/20m/3c]

32. Discuss the factors that decide the percent ionic character of polar compounds. [2002/20m/1a]

2.3] Molecular orbital theory

36 questions

1. Draw the molecular orbital diagram for $F_2$ molecule, and mention its bond order and magnetic behaviour. [2026/10m/6a(ii)]

2. Draw the molecular orbital (MO) diagram of NO molecule. The experimental bond dissociation energy of NO is $626kJmo{l}^{-1}$ while that of $N{O}^{+}$ is $1047kJmo{l}^{-1}$ – rationalize it. NO can also act as a reactive radical – how? [2025/20m/4a]

3. Calculate the bond order for the following: (I) Oxygen (II) Superoxide (III) Peroxide (IV) Dioxygenyl ion Which will have the highest stability? [2024/10m/2b(i)]

4. Draw the molecular orbital diagram for CO. [2024/10m/2b(ii)]

5. Consider the following atomic orbitals of $A$ and $B$ atoms in a heterodiatomic $AB$ molecule.

$A$	$B$
$2s$	$2p_y$
$2p_x$	$2p_z$

Depict the nonbonding interactions among these atomic orbitals with reason. [2022/10m/1b]

6. The dissociation energy decreases on going from $N_2$ to $N_2^{+}$ whereas it increases on going from $O_2$ to $O_2^{+}$. Explain. [2020/10m/1a]

7. What is LCAO? What are bonding, non-bonding and anti-bonding orbitals? Illustrate with a suitable diagram with an example of $H_2$ molecule within the framework of molecular orbital (MO) theory. [2020/10m/4a]

8. When an electron is removed from an $N_2$ molecule, forming $N_2^{+}$ ion, the bond between the N atoms is weakened. When an $O_2$ molecule is similarly ionized to $O_2^{+}$, the bond between O atoms is strengthened. Explain this difference. [2019/10m/7c]

9. Calculate the bond order for the following molecules/ions. Arrange them as per the increasing order of their stability. Give reasons. (i) $H_2^{+}$ (ii) $N{e}_2$ (iii) $C{N}^{-}$ (iv) NO (v) HF. [2018/15m/2c]

10. Explain the paramagnetism of NO with its molecular orbital diagram. [2017/10m/4b]

11. Draw the molecular orbital diagram of NO molecule. Show that $N{O}^{+}$, $C{N}^{-}$ and CO are isoelectronic, and the bond order of NO is lower than that of $N{O}^{+}$. [2016/15m/2a(i)]

12. Construct the MO diagram for the CO molecule. On the basis of the diagram, state whether the bonding in metal carbonyls is of the type M—C or M—O. Explain your reasoning. You may ignore the 1s electrons. [2015/10m/1e]

13. The BF molecule is isoelectronic with CO. (i) Construct an MO diagram for the BF molecule. What assumptions can be made in BF which make the MO construction for this molecule much more straightforward than for CO? (ii) Discuss the similarities and differences in the electronic structures of BF and CO in terms of $\sigma$ and $\pi$ bonding and non-bonding orbitals. What possible Lewis structures can be drawn for BF and CO, and which of these is most reasonable for each species based on your diagram? [2015/20m/4a]

14. Point out the main features of the Valence Bond and Molecular Orbital theories. For the $H_2$ molecule, write down the wave function using (i) Valence Bond theory (ii) Molecular Orbital theory Suggest improvements to the wave functions. [2014/10m/1b]

15. Draw the molecular orbital diagrams for $N_2$ and NO. Based on the diagrams, find the bond orders and comment on the magnetic properties of the two. [2014/20m/2b]

16. Compare the molecules OF, $O{F}^{-}$ and $O{F}^{+}$, discussing molecular orbitals, bond orders, bond lengths, bond energies, and paramagnetism. [2013/15m/2a(i)]

17. (i) Construct a trial LCAO – MO wavefunction for the $H_2$ molecule. (ii) The trial VB wavefunction used by Heitler and London to describe the $H_2$ molecule in terms of hydrogen-like wavefunctions is $\varphi =c_1{\psi }_a\left(1\right){\psi }_b\left(2\right)+c_2{\psi }_a\left(2\right){\psi }_b\left(1\right)$ Compare with the wavefunction you wrote in (i) and comment on the differences, if any. [2013/15m/4c]

18. Draw molecular orbital diagram for the following molecules and compare bond order, bond length and magnetic properties: CN and $C{N}^{-}$ Explain why the bonding in metal cyanides is between metal and carbon. [2012/20m/2c]

19. Draw and label an energy level diagram for molecular orbitals up to ${\sigma }^{*}2p$. Use the diagram to explain why (i) The bond length of $L{i}_2$ is much greater than that of $B_2$. (ii) $B_2$ is paramagnetic, but $C_2$ is not. (iii) $N{O}^{+}$ is more stable towards dissociation into its atoms than NO, whereas $C{O}^{+}$ is less stable than CO. [2011/20m/2a]

20. (i) Write down the Hamiltonian for the hydrogen molecule ion, $H_2^{+}$. (ii) For a trial wavefunction of the form $\varphi =c_a{\psi }_a+c_b{\psi }_b$ where ${\psi }_i$ is the normalized atomic orbital on atom i and the $c_i$ are coefficients, show that the normalization constant is $N={(2+2S)}^{-1}$ where $S=S_{ab}=S_{ba}$ is the overlap integral. Assume that the $c_i$’s are all equal and that identical normalized hydrogen-like wavefunctions are chosen. (iii) By expanding the probability density, ${\varphi }^{*}\varphi$, qualitatively confirm that $\varphi ={(2+2S)}^{-1/2}({\psi }_a+{\psi }_b)$ represents the wavefunction for a bonding orbital of $H_2^{+}$. [2011/20m/4c]

21. Draw the MO energy-level diagram for $N_2$ and $O_2$ molecules. Predict their magnetic behaviours and also calculate the bond orders. [2010/20m/2a]

22. Using the principle of LCAO, obtain the normalised wave functions for the bonding and the antibonding molecular orbitals for $H_2^{+}$ system. [2010/20m/4b]

23. Explain on the basis of MO theory, why: (i) Liquid oxygen sticks to the poles of a magnet while liquid nitrogen does not do so. (ii) $H{e}_2$ and $B{e}_2$ molecules do not exist. (iii) The bond order in oxygen is 2, while that of nitrogen is 3. (iv) Hydrogen molecule is more stable than $H_2^{+}$ ion. [2009/20m/1b]

24. Briefly discuss M.O. theory. Explain the formation of $H_2$ molecule on the basis of this theory. [2007/20m/1b]

25. Write the electronic configuration of NO molecule. (i) What is its bond-order? (ii) How many unpaired electrons are present? (iii) What will be the bond-order of $N{O}^{+}$ [?]? (iv) Will the bond-length be shorter or longer than $N{O}^{+}$? [2007/20m/1c]

26. Explain how paramagnetism of $O_2$ molecule can be accounted for by Molecular Orbital (MO) theory whereas Valence Bond (VB) theory predicts it to be diamagnetic. [2006/20m/1a]

27. Define bond order, bond strength and bond length, and differentiate between them. Explain with suitable examples as to how these are interrelated with each other. [2006/30m/2a]

28. What are the criteria to be met for the formation of molecular orbitals? Describe the bonding in $N_2$ according to MO theory. [2004/20m/1a]

29. Substantiate the statement that “Hybridization does not enter explicitly into MO theory but mixing of orbitals does occur as in VB theory”, taking the example of $Be{H}_2$. [2004/20m/1b]

30. Draw the MO energy level diagram for CO and discuss the bonding features. [2003/20m/7c]

31. Resonance is a necessity in VB theory but not so in MO theory – Discuss with reference to nitrite ion. [2002/20m/1b]

32. Draw the MO energy level diagram for $O_2$ and discuss the bonding features. [2002/20m/1c]

33. Which of the following are more stable? Explain:- (i) $F_2$ or $F_2^{+}$ (ii) $H{e}_2$ or $H{e}_2^{+}$ (iii) $N_2$ or $N_2^{+}$ (iv) $H_2^{+}$ or $H_2^{-}$ [2001/20m/1c]

34. Compare the Valence bond and Molecular orbital theory. [2001/20m/5c]

35. What is the bond order in CO molecule according to the molecular orbital theory? Also describe how the theory explains (1) coordination through carbon and (ii) negligible dipole moment of the molecule. [2000/20m/1c]

36. Apply molecular orbital theory to explain paramagnetism of $B_2$ molecule. [2000/20m/1d]

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Last updated: 30 October 2026