CHEM 341 - Extended Response 1

VSEPR

valence shell electron pair repulsion

define VSEPR theory

the shape of a molecule, whose central atom is a p-block element, is determined by the electrons present in the valence shell of the central atom

consequence of electrons being negatively charged

they repel each other, thus are arranged as far as possible from each other to minimize repulsive forces

what does the geometry of a molecule depend on?

the number of electron domains around the central atom (steric number)

electron domains

areas with high electron density (bonds, lone pairs) - arrange themselves to minimize repulsion

parent shape

geometries for a molecule that depend on the number of electron domains

electron domains in order of increasing repulsivity

single bond < double bond < triple bond < lone pair

parent shape vs. coordinate geometry

parent shape considers all electron domains to be the same, coordinate geometry accounts for lone pair repulsion

why do lone pairs have the strongest repulsive forces?

lone pairs lie closer to the central atom

limitations of VSEPR

• doesn’t work for d-block transition metals

• occasionally fails with stereochemically inactive lone pairs

(fails to mention hybridization or anything in molecular orbital theory, and oversimplifies resonance)

deviation from ideal angles: lone pair vs. bonded pairs

lone pairs repel more than bonded pairs, causes bond angles to shrink 

ex:

CH₄: 109.5 (tetrahedral)

NH₃: 107 (trigonal pyramidal)

H₂0: 104.5 (bent)

deviation from ideal angles: multiple bonds

double and triple bonds have a higher electron density so they repel more strongly than single bonds, causes bond angles to shrink

deviation from ideal angles: different substituents 

if surrounding atoms differ in electronegativity or size, electron clouds are pulled unequally (the same goes for bulky substituents)

bond polarity

in a bond attaching two atoms with different electronegativities, the bond density will shift towards the more electronegative atom