The Foundations of Chemistry

a regular tetrahedron with the other atoms at the corners (HXNXH and OXSXO bond
angles109.5°).

B. VSEPR Theory

VSEPR theory predicts that four valence shell electron pairs are directed toward the
corners of a regular tetrahedron. That shape gives the maximum separation for four elec-
tron pairs around one atom. Thus, VSEPR theory predicts tetrahedralelectronic geometry
for an AB 4 molecule that has no unshared electrons on A. There are no lone pairs of elec-
trons on the central atom, so another atom is at each corner of the tetrahedron. VSEPR
theory predicts a tetrahedral molecular geometryfor each of these molecules.

O

O

O S O

^2 

H

H

HHN

NH 4 , ammonium ion SO 42 
, sulfate ion

8-7 Tetrahedral Electronic Geometry: AB 4 Species (No Lone Pairs of Electrons on A) 319

H

C

In CH 4

all H C H angles

109.5°

H

H

H

In CF 4

all F C F angles

109.5°

F

F

C

F

F