Predict and explain the shapes of and bond angles in molecules and ions using the principle of valence shell electron pair repulsion (VSEPR theory), e.g.
What is VSEPR Theory?
The valence-shell electron-pair repulsion (VSEPR) theory states that electron pairs repel each other whether or not they are in bond pairs or in lone pairs. Thus, electron pairs will spread themselves as far from each other as possible to minimize repulsion.
a) the electron-pair around the central atom repel each other
(b) the electron-pairs (bonding pairs and lone pairs) arrange themselves to be as far apart as possible to minimise the force of repulsion, and
(c) the force of repulsion decreases in the order:
lone pair-lone pair repulsion (strongest repulsion) > lone pair-bond pair repulsion (medium repulsion) > bond pair-bond pair repulsion (weakest repulsion).
For molecules and ions that contain lone pairs of electrons, the shape of the molecules or ions is different from the geometry of electron-pairs.
1) The VSEPR model turn decreases the molecule’s energy and increases its stability, which used to predict the geometry of molecules.
2) The electron pairs around an atom are assumed to arrange themselves to reduce electron repulsion.
3) The molecular geometry is determined by the position of the bonding electron pairs .
There are eleven molecular shapes. The shapes of covalent molecules can be explained by using
(a) the concept of overlapping and hybridisation of orbitals
(b) the repulsion of the valence shell electron-pair.
(c) If the Lewis structure (or dot-and-cross structure) can be written for a molecule or a polyatomic ion, the shape of this molecule or ion can be predicted using the electron-pair repulsion theory.
VSEPR theory predicts:
1. one possible shape for molecules with two bonding electron pairs:
2. two possible shapes for molecules with three electron pairs:
bent and trigonal planar.
3. three possible shapes for molecules with four electron pairs:
tetrahedral, trigonal pyramidal, and bent.
4. four possible shapes for molecules with five electron pairs:
trigonal bipyramidal, sawhorse, T-shape, and linear.
5. three possible shapes for molecules with six electron pairs:
octahedral, square pyramidal, and square planar.
F2O, v-shaped or bent shape.
AsH3, trigonal pyrimidal.
H3O+, trigonal pyramidal.
CH3+, trigonal planar.
CH3–, trigonal pyramidal.