Elements of symmetry MCQs With Answer are essential for B. Pharm students studying stereochemistry, molecular structure and drug design. This concise introduction explains symmetry elements—rotation axes (Cn), mirror planes (σ), inversion centers (i), improper axes (Sn) and identity (E)—and their role in classifying point groups, determining chirality, and predicting physical and spectroscopic behavior of drug molecules. Understanding these concepts helps explain enantiomeric activity, drug–receptor interactions, and IR/Raman selection rules. Mastery of elements of symmetry strengthens problem-solving in medicinal chemistry, stereochemistry and quality control of pharmaceuticals. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is an element of symmetry in a molecule?
- A geometric operation that maps a molecule onto itself
- A bond type between two atoms
- A measurement of optical rotation
- A method to determine molecular weight
Correct Answer: A geometric operation that maps a molecule onto itself
Q2. Which operation is always present in every molecule?
- Center of inversion (i)
- Mirror plane (σ)
- Identity (E)
- Improper rotation (Sn)
Correct Answer: Identity (E)
Q3. What does a Cn axis represent?
- A mirror plane perpendicular to the molecular plane
- An n-fold proper rotation axis that rotates the molecule by 360°/n
- An inversion center combined with reflection
- A type of vibrational mode
Correct Answer: An n-fold proper rotation axis that rotates the molecule by 360°/n
Q4. Which symbol denotes a mirror plane that is perpendicular to the principal axis?
- σv
- σd
- σh
- σt
Correct Answer: σh
Q5. What is the defining action of an inversion center (i)?
- Rotation by 90° about an axis
- Reflection through a vertical plane
- Mapping (x, y, z) to (−x, −y, −z) through a point
- Rotation followed by translation
Correct Answer: Mapping (x, y, z) to (−x, −y, −z) through a point
Q6. What is an improper rotation axis (Sn)?
- A rotation followed by translation along the axis
- A reflection followed by rotation about an axis perpendicular to the plane
- A rotation by 360°/n followed by reflection in a plane perpendicular to the axis
- A mirror plane combined with inversion center
Correct Answer: A rotation by 360°/n followed by reflection in a plane perpendicular to the axis
Q7. Which point group does water (H2O) belong to?
- C3v
- C2v
- D3h
- Td
Correct Answer: C2v
Q8. Which point group characterizes methane (CH4)?
- D3h
- Td
- Oh
- C2v
Correct Answer: Td
Q9. Which point group is benzene (planar) classified under?
- D6h
- C6v
- Oh
- C2v
Correct Answer: D6h
Q10. A molecule is chiral if it lacks which of the following?
- Any Cn axis
- Improper rotation axes, mirror planes and inversion center
- Rotational symmetry altogether
- An identity operation
Correct Answer: Improper rotation axes, mirror planes and inversion center
Q11. Which symmetry element is responsible for optical activity when absent?
- Identity (E)
- Improper rotation axis (Sn)
- Any proper rotation axis (Cn)
- External magnetic field
Correct Answer: Improper rotation axis (Sn)
Q12. In the context of spectroscopic selection rules, what does a center of inversion (i) imply for IR and Raman activities?
- Modes are both IR and Raman active
- Mutual exclusion: modes that are IR active are Raman inactive and vice versa
- No vibrational modes exist
- Only electronic transitions are allowed
Correct Answer: Mutual exclusion: modes that are IR active are Raman inactive and vice versa
Q13. Which of the following point groups is inherently chiral (can contain chiral molecules)?
- C1
- Ci
- Cs
- C2h
Correct Answer: C1
Q14. Which element of symmetry is represented by σv?
- A vertical mirror plane that contains the principal axis
- A horizontal mirror plane perpendicular to the principal axis
- An inversion center
- An n-fold rotation axis
Correct Answer: A vertical mirror plane that contains the principal axis
Q15. For a C3 axis, what is the rotation angle for one operation?
- 360°
- 90°
- 120°
- 60°
Correct Answer: 120°
Q16. Which point group does ammonia (NH3) belong to?
- C3v
- D3h
- C2v
- Td
Correct Answer: C3v
Q17. If a molecule has an S2 improper axis, this operation is equivalent to which symmetry element?
- Mirror plane (σ)
- Identity (E)
- Center of inversion (i)
- C2 rotation only
Correct Answer: Center of inversion (i)
Q18. Which of the following molecules is achiral despite having stereocenters due to an internal mirror plane?
- Meso-tartaric acid
- Lactic acid
- Thalidomide enantiomer
Correct Answer: Meso-tartaric acid
Q19. Which point group does ethylene (C2H4, planar) belong to?
- D2h
- Oh
- C2v
- D3h
Correct Answer: D2h
Q20. Which symmetry element is described as the ‘principal axis’ in point group assignment?
- The highest order Cn axis present in the molecule
- The lowest order Cn axis
- Any σ plane
- The inversion center only
Correct Answer: The highest order Cn axis present in the molecule
Q21. Which point group contains a horizontal mirror plane but no vertical mirror planes?
- Cnv
- Cnh
- Dnd
- Th
Correct Answer: Cnh
Q22. Which element of symmetry, when present, makes a molecule definitely achiral?
- Only a Cn axis
- Improper rotation axis, mirror plane or inversion center
- Identity (E) alone
- Multiple Cn axes of different orders
Correct Answer: Improper rotation axis, mirror plane or inversion center
Q23. What is the point group of BF3 (planar trigonal molecule)?
- C3v
- D3h
- C2v
- Td
Correct Answer: D3h
Q24. Which symmetry element would you look for first when assigning a point group?
- Any mirror plane
- Highest order rotation axis (principal axis)
- Number of atoms
- Bond lengths
Correct Answer: Highest order rotation axis (principal axis)
Q25. Which type of mirror plane bisects the angle between two C2 axes in molecules like methane derivatives?
- σh
- σv
- σd (dihedral plane)
- σt
Correct Answer: σd (dihedral plane)
Q26. For point group identification, the presence of a C2 axis and a center of inversion typically indicates which point group?
- C2v
- C2h
- Cs
- D2h
Correct Answer: C2h
Q27. Which of the following point groups commonly describes staggered ethane (C2H6)?
- D3d
- D3h
- C3v
- C2v
Correct Answer: D3d
Q28. Which symmetry element is absent in a chiral center-containing drug that shows optical activity?
- Identity (E)
- Improper axis (Sn) including mirror planes and inversion
- Proper rotation axis (Cn)
- Multiple equivalent substituents
Correct Answer: Improper axis (Sn) including mirror planes and inversion
Q29. Which point group is associated with a molecule that has only an identity operation (no other symmetry)?
- C1
- Ci
- Cs
- C2
Correct Answer: C1
Q30. In drug design, why is knowledge of molecular symmetry important?
- It determines the cost of synthesis only
- It helps predict chiral interactions, physical properties and spectroscopic behavior of drugs
- It replaces the need for biological testing
- It only affects melting point
Correct Answer: It helps predict chiral interactions, physical properties and spectroscopic behavior of drugs
Q31. Which point group contains a mirror plane but no rotation axis except the identity?
- Cs
- Ci
- C1
- Oh
Correct Answer: Cs
Q32. Which element distinguishes Cn from Sn?
- Sn involves rotation only; Cn involves reflection
- Sn is rotation followed by reflection in a plane perpendicular to the axis, while Cn is pure rotation
- Cn includes inversion centers; Sn does not
- There is no difference
Correct Answer: Sn is rotation followed by reflection in a plane perpendicular to the axis, while Cn is pure rotation
Q33. Which of the following point groups is centrosymmetric (contains an inversion center)?
- Ci
- C1
- Cs
- Cnv
Correct Answer: Ci
Q34. How does the presence of symmetry simplify vibrational spectroscopy analysis?
- By increasing the number of vibrational modes
- By allowing classification of vibrations into symmetry species and predicting IR/Raman activity
- By making all modes IR active
- By preventing any spectroscopic transitions
Correct Answer: By allowing classification of vibrations into symmetry species and predicting IR/Raman activity
Q35. Which point group does trans-1,2-dichloroethene belong to (planar, with C2 axis and inversion center)?
- C2v
- C2h
- Cs
- D2h
Correct Answer: C2h
Q36. Which symmetry element is most directly assessed by observing optical rotation of a substance?
- Presence of an inversion center
- Chirality (absence of improper symmetry elements)
- Presence of a σh plane
- Number of carbon atoms
Correct Answer: Chirality (absence of improper symmetry elements)
Q37. Which type of symmetry element is σd commonly associated with?
- Dihedral mirror plane that bisects angle between two C2 axes
- Horizontal mirror plane perpendicular to principal axis
- Inversion center located at the molecular center
- Translation operation in a crystal lattice
Correct Answer: Dihedral mirror plane that bisects angle between two C2 axes
Q38. Which point group does an octahedral complex like SF6 belong to?
- Td
- Oh
- D4h
- C3v
Correct Answer: Oh
Q39. In pharmaceutical stereochemistry, why can two enantiomers have different biological effects?
- Because they have different molecular weights
- Because chiral receptors interact differently with each enantiomer
- Because only one enantiomer has bonds
- Because one enantiomer lacks electrons
Correct Answer: Because chiral receptors interact differently with each enantiomer
Q40. Which point group is assigned to a molecule that has a C3 axis and three vertical mirror planes?
- C3v
- D3h
- C3h
- D3d
Correct Answer: C3v
Q41. How does symmetry affect NMR spectroscopy interpretation for small organic molecules?
- Symmetry can reduce the number of distinct NMR signals by making chemically equivalent nuclei identical
- Symmetry always increases signal complexity
- Symmetry has no effect on NMR
- Symmetry only affects mass spectrometry
Correct Answer: Symmetry can reduce the number of distinct NMR signals by making chemically equivalent nuclei identical
Q42. Which of the following is true about molecules in the Td point group?
- They always have an inversion center
- They are highly symmetric and can be chiral or achiral depending on substituents
- They always belong to D3h
- They lack any rotation axes
Correct Answer: They are highly symmetric and can be chiral or achiral depending on substituents
Q43. Which operation would you use to test for a σh plane experimentally or computationally?
- Rotate molecule 90° about principal axis
- Reflect molecule through a plane perpendicular to the principal axis and compare
- Invert coordinates through origin
- Translate the molecule along the axis
Correct Answer: Reflect molecule through a plane perpendicular to the principal axis and compare
Q44. Which point group is associated with a molecule that has an S4 improper axis but no mirror planes?
- C4v
- S4
- C4h
- D4h
Correct Answer: S4
Q45. How does the presence of multiple identical substituents on a drug molecule affect its symmetry?
- It generally increases molecular symmetry and can create additional symmetry elements
- It always makes the molecule chiral
- It decreases symmetry by introducing asymmetry
- It prevents any spectroscopic analysis
Correct Answer: It generally increases molecular symmetry and can create additional symmetry elements
Q46. Which point group is typical for a planar square molecule with inversion center and four C2 axes (e.g., square planar complex)?
- D4h
- C4v
- D4d
- Td
Correct Answer: D4h
Q47. Why is the assignment of point group important before performing a group theory vibrational analysis?
- Because point group determines the number and symmetry of normal modes and selection rules
- Because point group gives the boiling point
- Because point group replaces quantum chemical calculations
- Because point group determines the empirical formula
Correct Answer: Because point group determines the number and symmetry of normal modes and selection rules
Q48. Which point group describes a molecule with only a horizontal mirror plane and an inversion center but no other axes?
- C1
- Ci
- Cnh (with n = 1 equivalent to Cs?)
- C1h
Correct Answer: Ci
Q49. For pharmaceutical molecules, which statement about enantiomers is correct?
- They have identical interactions with chiral environments
- They rotate plane-polarized light in opposite directions but may have different biological activity
- They always have different molecular formulas
- They are always achiral
Correct Answer: They rotate plane-polarized light in opposite directions but may have different biological activity
Q50. When assigning point groups to complex drug molecules, what practical approach helps most?
- Ignore symmetry and treat each atom distinct
- Identify the highest order rotation axis, then search for perpendicular C2 axes, mirror planes and inversion
- Only consider the number of stereocenters
- Rely solely on empirical formulas
Correct Answer: Identify the highest order rotation axis, then search for perpendicular C2 axes, mirror planes and inversion
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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