Zumdahl • DeCoste chemistry world of Chapter 12

Other Molecular Structures

• Linear structure: all atoms are in a line
• Example: Carbon dioxide (see pg. 382)
• Bond angle = 180°
• Trigonal Planar: triangular, planar with 120° bond angles
• Example: BF3 (see pg. 382)
• Tetrahedral structure: tetrahedron
• Example: methane
• Four identical triangular faces

Figure 12.12: Molecular structure of methane.

Molecular Structure: The VSEPR Model

• Structure very important to molecular properties
• Determines taste
• Biological molecules – structure change can convert cell from normal to cancerous
• Experimental methods exist for determining 3-D structure
• Useful to predict approximate structure

VSEPR Model

• Valence Shell Electron Pair Repulsion (VSEPR) model
• Used to predict molecular structure of molecules formed from nonmetals
• The structure around a given atom is determined by minimizing repulsions between electron pairs
• Bonding & non-bonding electron pairs around an atom are positioned as far apart as possible

VSEPR Model Rules (see ex pp 384-386)

• Two pairs of electrons on a central atom of a molecule are always placed at an angle of 180° to each other to give a linear arrangement
• Three pairs of electrons on a central atom in a molecule are always placed 120° apart in the same plane as the central atom – trigonal planar
• Four pairs of electrons on a central atom are always placed 109.5° apart - tetrahedral
• When every pair of electrons on central atom is shared, the molecular structure has same name as the arrangement of electron pairs
• 2 = linear, 3 = trigonal planar, 4 = tetrahedral
• When one or more of the electron pairs around the central atom are unshared, the name of the structure is different from that for the arrangement of electron pairs (see table 12.4 # 4 & 5)