Atomic Structure and Chemical Bonds: Important MCQs

COMPETITIVE EXAM MCQs SERIES of LIFE SCIENCES for UGC-CSIR NET/JRF, SLET, GATE, and other entrance tests – MOLECULES AND THEIR INTERACTION RELEVANT TO BIOLOGY – Atomic Structure and Chemical Bonds.

Syllabus Outline

1. Basic structure of an atom – subatomic particles, orbitals, quantum numbers
2. Electron configuration and the Aufbau principle
3. Periodic properties – electronegativity, ionisation energy, electron affinity
4. Types of chemical bonds – covalent, ionic, hydrogen, coordinate
5. Bond properties – bond length, bond energy, bond polarity
6. Molecular orbital theory and bonding in simple diatomic molecules
7. Hybridisation and molecular geometry (VSEPR theory)
8. Intermolecular vs. intramolecular bonding
9. Polar vs. non-polar molecules and dipole moment

This quiz contains concept-based, most frequently asked 25 MCQs of “MOLECULES AND THEIR INTERACTION RELEVANT TO BIOLOGY – Atomic Structure and Chemical Bonds”. Each question has a single correct/most appropriate answer.

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1. Which one of the following statements regarding the quantum numbers of an electron in a hydrogen atom is INCORRECT?

A) The principal quantum number (n) determines the energy level and the size of the orbital.

B) The azimuthal quantum number (l) determines the shape of the orbital, ranging from 0 to n−1.

C) For a given value of l, the magnetic quantum number (m_l​) determines the orientation of the orbital in space and can have 2l+1 values.

D) The spin quantum number (m_s​) can assume values of +1/2, −1/2, or 0, depending on the external magnetic field.

2. Consider the following hypothetical electron configuration for an atom in its ground state:

1s²2s²2p_x²​2p_y¹​2p_z⁰​

This configuration violates which fundamental rule(s) of electronic structure?

I – Aufbau Principle

II – Hund’s Rule of Maximum Multiplicity

III – Pauli Exclusion Principle

A) Only I and II

B) Only II

C) Only I and III

D) Only II and III

3. Which property shows a general decrease as one moves from left to right across a period in the modern periodic table?

A) Effective nuclear charge (Z_eff​)

B) Electronegativity (χ)

C) Atomic radius (r)

D) First Ionisation Energy (IE1​)

4. The Pauling scale electronegativity (χ) values for three elements X, Y, and Z are 1.0, 3.5, and 4.0, respectively. If a biological system relies on a strong, highly directional intermolecular force between an X-H bond and Z, which statement regarding this interaction is most accurate?

A) The X-H bond is polar covalent, and the interaction with Z is a weak dipole-dipole force.

B) The interaction is a hydrogen bond, and its strength is high due to the large electronegativity difference between H and Z.

C) The X-H bond is predominantly ionic, and the interaction with Z is a strong ion-dipole interaction.

D) The interaction with Z will be a salt bridge, which is generally weaker than a hydrogen bond in an aqueous environment.

5. In biological systems, the phosphate group (PO₄³⁻​) is highly prevalent. The geometry of the central phosphorus atom, based on Valence Shell Electron Pair Repulsion (VSEPR) theory, is key to its function in DNA and ATP. What is the electron-domain geometry and the molecular geometry of the phosphate ion?

A) Electron-domain: Trigonal planar; Molecular: Trigonal planar

B) Electron-domain: Tetrahedral; Molecular: Tetrahedral

C) Electron-domain: Trigonal bipyramidal; Molecular: Trigonal pyramidal

D) Electron-domain: Tetrahedral; Molecular: Trigonal pyramidal

6. The free energy (ΔG) contributions of non-covalent interactions to the stability of double-stranded DNA (dsDNA) are critical. Given the following data (relative to GC H-Bonds = 0 kJ/mol):

I) H-Bonds (AT): +2.1 kJ/mol

II) π-Stacking: -6.3 kJ/mol

Based on this, if a dsDNA region has 10 AT base pairs and 10 GC base pairs, and 20 π-stacking interactions, which statement is most likely correct about the major stabilising force?

A) Hydrogen bonding provides the primary stability due to its additive nature in base pairing.

B) The π-stacking interactions contribute significantly to the overall thermodynamic stability (ΔG is more negative).

C) The AT base pairs destabilise the helix by contributing a positive ΔG value.

D) The net contribution of hydrogen bonding (AT + GC) is 2.1 kJ/mol, making it stronger than the stacking forces.

7. According to Molecular Orbital Theory (MOT), what is the bond order and magnetic character of the superoxide anion (O₂⁻​), a common reactive oxygen species (ROS) in cellular metabolism? (Atomic number of O is 8)

A) Bond Order =2.5; Diamagnetic

B) Bond Order =1.5; Paramagnetic

C) Bond Order =2.0; Diamagnetic

D) Bond Order =1.0; Paramagnetic

8. The molecular orbital (MO) energy level diagram for heteronuclear diatomic molecules like Carbon Monoxide (CO) is different from homonuclear diatomic molecules. Which statement accurately reflects the nature of bonding in CO?

A) CO is isoelectronic with N₂​ and therefore has a bond order of 3.0 and a non-polar bond.

B) The MOs are formed by the combination of atomic orbitals of unequal energy, resulting in a net dipole moment.

C) The bonding MOs are equally shared between C and O, resulting in a small dipole moment directed toward the less electronegative carbon atom.

D) The 2s and 2p orbitals of Oxygen contribute more significantly to the bonding molecular orbitals due to its lower energy levels.

9. The dipole moment (μ) of a molecule is a vector quantity, dependent on bond polarity and molecular geometry. Which of the following molecules has a net dipole moment of zero (μ=0)?

A) H_2​O (Bent)

B) NH₃​ (Trigonal pyramidal)

C) CHCl₃​ (Tetrahedral, unsymmetrical)

D) BF₃​ (Trigonal planar, symmetrical)

10. In the context of protein structure, X-ray diffraction analysis of protein-protein interfaces showed that the geometry of interfacial hydrogen bonds is often less optimal and has a wider distribution compared to intra-chain hydrogen bonds. This difference primarily originates from:

A) The higher concentration of hydrophobic residues buried in the binding interface.

B) The greater strength of salt bridges compared to hydrogen bonds at the interface.

C) The constraint of rigid binding, where molecules are already folded, limiting available degrees of freedom for optimal bond formation.

D) The exclusion of all water molecules from the interface, which forces non-optimal donor-acceptor pairings.

11. Which one of the following compounds exhibits sp² hybridisation in all its carbon atoms, a characteristic structure essential for its function?

A) Methane (CH₄​)

B) Acetylene (C₂​H₂​)

C) Benzene (C₆​H₆​)

D) Ethanol (C₂​H₅​OH)

12. In a peptide chain, the peptide bond (amide bond) has partial double-bond character. This characteristic is best explained by:

A) The presence of a lone pair on the oxygen atom of the carbonyl group.

B) The resonance structures leading to sp³ hybridisation of the nitrogen atom.

C) The delocalisation of the nitrogen lone pair electrons into the π system of the carbonyl group, stabilising a planar geometry.

D) The short C-N bond length (approx. 1.33 A˚), which is typical of a single bond.

13. Which of the following describes the relationship between a molecule’s C≡N bond and C-N bond, as used in biological structures like cyano groups and peptide bonds?

A) C≡N is longer, weaker, and involves sp³ hybridisation on the carbon atom.

B) C≡N is shorter, stronger, and contains one σ and two π bonds.

C) C-N has a higher bond energy due to its crucial role in the protein backbone.

D) C≡N is less polar than C-N because of the symmetrical distribution of triple bond electrons.

14. In the context of intermolecular forces, which sequence correctly ranks the relative strengths of the following non-covalent interactions in an aqueous biological environment?

I – London Dispersion Forces

II – Hydrogen bonds

III Salt bridges

A) III > II > I

B) II > III > I

C) I > II > III

D) III > I > II

15. An organic molecule X has two adjacent carbon atoms, one with sp² hybridisation and the other with sp³. Which of the following is an accurate description of the bonding environment around these carbons?

A) The sp² carbon has a bond angle of approximately 109.5°, and the sp³ carbon is involved in a double bond.

B) The sp² carbon is trigonal planar, and the sp³ carbon is tetrahedral.

C) Both carbons must be part of a conjugated system due to the presence of p-orbitals.

D) The Csp²​−Csp³​ bond is stronger and shorter than a standard Csp³​−Csp³​ bond.

16. In Molecular Orbital Theory, the concept of Hückel’s Rule (4n+2 π electrons) determines the aromaticity of biological ring systems (e.g., DNA bases, porphyrins). This rule is based on:

A) The formation of degenerate, non-bonding molecular orbitals.

B) The complete filling of all bonding molecular orbitals with delocalized π electrons.

C) The presence of a linear combination of atomic orbitals that creates sp³ hybridised atoms.

D) The stability gained from maximising the number of filled anti-bonding molecular orbitals.

17. A researcher studies a novel peptide containing an unusually high concentration of arginine and glutamate residues. They hypothesise that this confers extraordinary stability in aqueous solution. Which statement best supports this hypothesis?

A) The hydrogen bonds formed by the guanidinium group of arginine are exceptionally strong.

B) The side chains can form numerous stabilising salt bridges, which compete effectively with solvent interactions.

C) The hydrophobic effect is maximised by clustering the charged residues together.

D) Both side chains possess high electronegativity, promoting strong π-stacking interactions.

18. Why is the O-H bond in water significantly more polar than the N-H bond in ammonia?

A) Oxygen is less electronegative than Nitrogen, increasing the bond dipole moment.

B) The size of the oxygen atom is smaller than nitrogen, causing higher electron density.

C) The difference in electronegativity (Δχ) between O and H is greater than that between N and H

D) Ammonia has a trigonal planar geometry, cancelling out bond dipoles, while water is bent.

19. A scientist experiments on a small molecule AX₃​ and determines its molecular geometry is T-shaped. Based on VSEPR theory, how many bond pairs and lone pairs must be present on the central atom A?

A) 3 bond pairs, 0 lone pairs

B) 3 bond pairs, 1 lone pair

C) 3 bond pairs, 2 lone pairs

D) 2 bond pairs, 2 lone pairs

20. In the formation of the DNA double helix, the two major non-covalent forces are hydrogen bonding (base pairing) and π-stacking (base-base interactions). Which statement accurately compares their contributions to stability?

A) Hydrogen bonds are intramolecular forces, while π-stacking is intermolecular.

B) Hydrogen bonding dictates the sequence specificity, while π-stacking provides the major portion of the overall stability in aqueous conditions.

C) π-stacking is primarily an electrostatic interaction, while hydrogen bonding is purely covalent.

D) GC pairs form three H-bonds, making H-bonding stronger than the collective π-stacking energy.

21. The formation of a coordinate covalent bond (dative bond) is crucial in the binding of metal ions (e.g., Mg²⁺, Fe²⁺) to biological ligands (e.g., porphyrin in haemoglobin, phosphate in ATP). A coordinate bond is characterised by:

A) The equal sharing of electrons between two non-metallic atoms.

B) The complete transfer of electrons from one atom to another.

C) The sharing of a pair of electrons from the donor atom.

D) The formation of a hydrogen bridge between two highly electronegative atoms.

22. The stability of a lipid bilayer is maintained by van der Waals forces (London dispersion forces) between the hydrocarbon tails. The strength of these LDFs increases with:

A) An increase in the polarity of the hydrocarbon chain.

B) A decrease in the molar mass of the hydrocarbon chain.

C) An increase in the surface area and polarizability of the molecule.

D) The formation of π-stacking interactions between the saturated fatty acids.

23. A novel compound is an intercalator that inserts between the bases of dsDNA. If the addition of this compound results in a ΔΔG of −1.0 kJ/mol for dsDNA formation, this indicates that the intercalator:

A) Destabilises the DNA helix

B) Stabilises the DNA helix

C) Disrupt the hydrogen bonds between base pairs.

D) Reduces the melting temperature (Tm​) of the dsDNA sample by providing a negative free energy change.

24. Consider the following statements regarding Atomic Structure and Electron Configuration:

I – An electron in a 3d orbital (n=3,l=2) has lower energy than an electron in a 4s orbital (n=4,l=0) according to the Aufbau principle.

II – The maximum number of electrons that can occupy an orbital is two, governed by the Pauli Exclusion Principle.

III – Electrons always fill degenerate orbitals singly first before pairing, according to Hund’s rule.

Which combination of statements is correct?

A) I and II only

B) II and III only

C) I and III only

D) I, II, and III

25. Why is the melting temperature (Tm​) of dsDNA directly proportional to the GC content?

A) GC pairs have a larger molecular weight, which requires more energy to denature.

B) GC pairs are more susceptible to π-stacking interactions than AT pairs.

C) GC pairs are held together by three hydrogen bonds, while AT pairs have only two.

D) The free energy contribution of the GC hydrogen bond is higher than AT.

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References

1. Nelson, David L. & Cox, Michael M. (2021) Lehninger Principles of Biochemistry, W. H. Freeman, 8th Edition
2. Voet, Donald, Voet, Judith G., & Pratt, Charlotte W. (2018). Voet’s Principles of Biochemistry, Wiley, 5th Edition
3. Berg, Jeremy M., Tymoczko, John L., & Stryer, Lubert (2023). Biochemistry, W. H. Freeman, 10th Edition
4. Palmer, Trevor & Bonner, Philip L. (2007) Enzymes: Biochemistry, Biotechnology, Clinical Chemistry, Horwood Publishing, 2nd Edition
5. Upadhyay, Avinash, Upadhyay, K., & Nath, Nirmalendu (2023). Biophysical Chemistry: Principles and Techniques, Himalaya Publishing House, 4th Edition