A group, G is called 'Point group'. Why? How to assign point group for a molecule?

 

Alagar Kovil
The structure across the central line or a point (which is fixed) mirrors the same

A group is usually titled a Point Group. Evidently, all symmetry elements intersect at a point that is always fixed (Illustration: Ammonia). Point group is the representation or the condensed version of symmetry elements.


Purpose: To assign the point group for molecules.

Step1 Construction of molecules with the fullest perfection is only made possible by 'Valence Shell Electron Pair Repulsion Theory' abbreviated as VSEPR theory (for molecules), Valence Bond theory_VBT(for complexes), and hybridization. Fit the molecule in a cartesian axis in three dimensions. 

Step2 Follow the below-given sequence of symmetry elements and start figuring them out.

E

All symmetrical and asymmetrical molecules (every molecule) possess ‘E’

i

See for the existence of mirrors across a fixed point

Cn and its operations

Find out the angle between two similar atoms. n=360˚/ θ

Subsidiary axis

The axis perpendicular to Cn

𝜎h

The plane perpendicular to Cn

𝜎v

Plane containing Cn

𝜎d

𝜎v bisecting a pair of C2 axis of symmetry

Sn and its operations

Cn. 𝜎h

Step3 Read this statement: 'nC2 axis perpendicular to the principal axis of symmetry (Cn).'
If this statement becomes true for a molecule, then it belongs to the dihedral group 'D'. If not, simply proceed with the letter 'C'.

Step4 Mention the order of axis to the subscript of the letter D or C.

Step5 Mention the plane of symmetry supporting the priority order (𝜎h>𝜎v>𝜎d)

Please Note: From step3 to step5, all the symmetry elements are condensed which is termed as 'Point Group'.

50 Plus Excercise (Exclusively awaiting for you to assign them with point group)
  1. Carbon dioxide
  2. Hydrochloric acid
  3. Hydrogen cyanide
  4. Nitric oxide
  5. Nitrogen dioxide
  6. Ethyne or acetylene
  7. Chloroacetylene
  8. Dichloroacetylene
  9. Propyne
  10. Borane
  11. Chloroform
  12. Benzene
  13. Chlorobenzene
  14. o-Dichlorobenzene
  15. m-Dichlorobenzene
  16. p-Dichlorobenzene
  17. 1,2,3-Trichlorobenzene
  18. 1,2,4-Trichlorobenzene
  19. 1,3,5-Trichlorobenzene
  20. 1,2,4,5-Tetrachlorobenzene
  21. 1,2,3,4,5-Pentachlorobenzene
  22. 1,2,3,4,5,6-Hexachlorobenzene
  23. 1-Fluoro-4-iodobenzene
  24. 1-Fluoro-3-iodobenzene
  25. 1,4-Dichloro-2,5-difluoro-3,6-diiodo-benzene
  26. 1,3-Dichloro-4,6-difluoro-2,5-diiodo-benzene
  27. 1-Bromo-2-chloro-3-fluoro-4-iodo-benzene
  28. 1,4-dihydroxy-benzene
  29. 1,3,5-Trihydroxy-benzene or Pyrogallol
  30. Staggered Ethane
  31. Eclipsed Ethane
  32. Ethylene
  33. Monochloro-ethylene
  34. Dichloroethylene
  35. Cis-dichloroethylene
  36. Trans-dichloroethylene
  37. Trichloroethylene
  38. Tetrachloroethylene
  39. Methane
  40. Carbon Tetrachloride
  41. Boric acid
  42. Ammonia
  43. Water
  44. Nitrosyl Chloride
  45. Hypochlorous acid
  46. Phosphane
  47. Thionyl Chloride
  48. Mesitylene
  49. Nickel tetracarbonyl
  50. Tetrachloro-platinate(II)
  51. Dinitrogen difluoride
  52. Hydrogen peroxide
  53. Fluorine dioxide
  54. Pyridine
  55. Naphthalene
  56. Phenanthrene
  57. Chyrsene
  58. Cis-butadiene
  59. Nitrogen trichloride
  60. Allene
  61. Staggered Ferrocene
  62. Eclipsed Ferrocene
  63. Azide anion
  64. Triphenylphosphine
  65. Sulfur tetrafluoride
  66. Xeon tetrafluoride
  67. Sulfuryl fluoride
  68. Phosphorous oxy-chloride
  69. Xeon oxy tetrafluoride
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