Basic aspects of crystal structure in X-ray analysis MCQs With Answer

Understanding basic aspects of crystal structure in X-ray analysis is essential for B.Pharm students studying solid-state drug properties and polymorphism. This concise introduction covers lattice types, unit cells, Bragg’s law, Miller indices, reciprocal lattice, diffraction patterns, atomic scattering factors, and symmetry elements relevant to X-ray crystallography. Knowledge of crystal packing, interplanar spacing, and interpretation of diffraction data helps predict drug stability, solubility, and bioavailability. Emphasis is on practical concepts used in pharmaceutical analysis, crystal engineering, and quality control. Familiarity with these keywords and principles prepares students for laboratory work and advanced topics in structural characterization. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is a unit cell in crystallography?

• The smallest repeating structural unit that builds the crystal lattice
• The largest repeating motif in an amorphous solid
• A molecular cluster found only in polymers
• A void between crystal planes

Correct Answer: The smallest repeating structural unit that builds the crystal lattice

Q2. How many Bravais lattices exist in three-dimensional crystals?

• 7
• 14
• 32
• 230

Correct Answer: 14

Q3. Which equation relates X-ray wavelength, interplanar spacing, and diffraction angle?

• Scherrer equation
• Bragg’s law
• Planck’s equation
• Laue condition

Correct Answer: Bragg’s law

Q4. For a cubic crystal, the d-spacing for planes (h k l) is given by which expression?

• d = a / sqrt(h^2 + k^2 + l^2)
• d = a * sqrt(h^2 + k^2 + l^2)
• d = a / (h + k + l)
• d = 2a * sin(theta)

Correct Answer: d = a / sqrt(h^2 + k^2 + l^2)

Q5. What do Miller indices (h k l) represent?

• The orientation of crystal faces or lattice planes
• The atomic coordinates inside the unit cell
• The temperature dependence of diffraction peaks
• The chemical composition of the crystal

Correct Answer: The orientation of crystal faces or lattice planes

Q6. Which of the following is a common cause of peak broadening in powder XRD?

• Instrumental optics only
• Microstrain and small crystallite size
• Perfectly ordered large crystals
• Absence of thermal vibrations

Correct Answer: Microstrain and small crystallite size

Q7. The Scherrer equation is used to estimate what property of a sample?

• Unit cell parameters
• Average crystallite size
• Electron density map
• Chemical purity

Correct Answer: Average crystallite size

Q8. Which factor directly affects the intensity of an X-ray diffraction peak?

• Structure factor (F)
• Ambient humidity only
• Color of the sample
• Magnification of microscope

Correct Answer: Structure factor (F)

Q9. What information does the structure factor contain?

• Positions and scattering power of atoms in the unit cell
• Only the unit cell dimensions
• Thermal conductivity of the crystal
• Number of defects per unit volume

Correct Answer: Positions and scattering power of atoms in the unit cell

Q10. Systematic absences (extinctions) in diffraction patterns are used to determine what?

• Space group and centering type
• Molecular weight of a drug
• Solubility in water
• Melting point

Correct Answer: Space group and centering type

Q11. Which technique is most suitable for identifying polymorphs of a pharmaceutical compound?

• Powder X-ray diffraction (PXRD)
• Optical microscopy only
• NMR spectroscopy only
• pH titration

Correct Answer: Powder X-ray diffraction (PXRD)

Q12. What is the reciprocal lattice used for in X-ray crystallography?

• Transforming diffraction data into a convenient mathematical lattice
• Measuring physical distances between atoms directly
• Counting electrons in the unit cell
• Calculating melting point

Correct Answer: Transforming diffraction data into a convenient mathematical lattice

Q13. In a face-centered cubic (FCC) lattice, which set of planes is allowed as the first strong reflection?

• (100)
• (110)
• (111)
• (200)

Correct Answer: (111)

Q14. The atomic scattering factor depends primarily on which variable?

• Type of detector used
• Scattering angle (or sinθ/λ) and atomic number
• Sample color
• Ambient pressure only

Correct Answer: Scattering angle (or sinθ/λ) and atomic number

Q15. What is the phase problem in X-ray crystallography?

• Inability to measure diffraction intensities
• Loss of phase information when measuring amplitudes, preventing direct electron density calculation
• Difficulty in measuring unit cell edges
• Uncertainty in sample mass

Correct Answer: Loss of phase information when measuring amplitudes, preventing direct electron density calculation

Q16. Which method can help solve the phase problem?

• Direct methods, molecular replacement, or anomalous dispersion
• Simple peak counting
• Measuring color changes on heating
• Using NMR chemical shifts only

Correct Answer: Direct methods, molecular replacement, or anomalous dispersion

Q17. What does the Debye–Waller factor (or B-factor) describe?

• Thermal vibration attenuation of scattering from atoms
• Chemical reactivity of a drug
• Instrumental noise in detector electronics
• Magnetic susceptibility

Correct Answer: Thermal vibration attenuation of scattering from atoms

Q18. How many unique crystal systems are there (e.g., cubic, tetragonal)?

• 5
• 7
• 10
• 14

Correct Answer: 7

Q19. What information is obtained from a powder diffraction pattern peak positions?

• Interplanar spacings and unit cell parameters
• Only crystallite size
• Only sample color
• Absolute atomic positions

Correct Answer: Interplanar spacings and unit cell parameters

Q20. Which space group element combines a rotation and a translation?

• Mirror plane
• Screw axis
• Center of inversion
• Pure rotation

Correct Answer: Screw axis

Q21. In X-ray crystallography, what is an asymmetric unit?

• The smallest portion of structure that, by symmetry operations, generates the whole unit cell
• The full unit cell including all symmetry-related atoms
• A void space in the unit cell
• An impurity region

Correct Answer: The smallest portion of structure that, by symmetry operations, generates the whole unit cell

Q22. What does indexing of powder diffraction peaks accomplish?

• Assigns Miller indices (hkl) and determines unit cell parameters
• Removes instrumental errors only
• Measures thermal expansion directly
• Determines chemical formula

Correct Answer: Assigns Miller indices (hkl) and determines unit cell parameters

Q23. Which statement about single-crystal XRD vs powder XRD is correct?

• Single-crystal XRD yields full 3D atomic coordinates; powder XRD gives averaged 1D patterns
• Powder XRD always gives better atomic positions than single-crystal XRD
• Single-crystal XRD cannot determine unit cell
• Both techniques provide identical data in all cases

Correct Answer: Single-crystal XRD yields full 3D atomic coordinates; powder XRD gives averaged 1D patterns

Q24. Which reflection condition indicates body-centering (I) type lattice?

• Only h+k+l = even reflections are present
• Only h, k, l all odd present
• No reflections allowed
• All reflections with h = 0 are absent

Correct Answer: Only h+k+l = even reflections are present

Q25. What does peak intensity proportionality to |F|^2 imply?

• Intensity depends on square of structure factor magnitude
• Intensity is independent of atom types
• Intensity equals structure factor magnitude linearly
• Intensity is only a function of temperature

Correct Answer: Intensity depends on square of structure factor magnitude

Q26. Which parameter is needed to calculate crystal density from XRD unit cell data?

• Unit cell volume, molecular weight, and number of formula units (Z)
• Only the number of atoms in the unit cell
• Only the peak widths
• Only the diffraction angle

Correct Answer: Unit cell volume, molecular weight, and number of formula units (Z)

Q27. Laue equations are primarily concerned with what?

• Electron spin orientations
• Conditions for constructive interference of scattered X-rays
• Determining sample melting point
• Calculating pKa values

Correct Answer: Conditions for constructive interference of scattered X-rays

Q28. Which experimental variable is changed in a typical θ–2θ powder scan?

• Sample temperature only
• The incident and detector angles relative to sample (θ and 2θ)
• The chemical composition
• The X-ray tube current only

Correct Answer: The incident and detector angles relative to sample (θ and 2θ)

Q29. What is meant by multiplicity of a reflection?

• The number of symmetry-equivalent planes producing the same d-spacing and intensity factor
• The number of atoms in the unit cell
• The count of impurities present
• The number of detectors used

Correct Answer: The number of symmetry-equivalent planes producing the same d-spacing and intensity factor

Q30. Why is X-ray wavelength important in crystallography?

• It must be comparable to interatomic spacings to produce useful diffraction
• It determines chemical reactivity
• Shorter wavelengths prevent any diffraction
• Wavelength has no effect on diffraction angles

Correct Answer: It must be comparable to interatomic spacings to produce useful diffraction

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