chemistry

VSEPR, Valance shell electron pair repulsion theory

VSEPR, Valance shell electron pair repulsion theory:

VSEPR, Valance shell electron pair repulsion theory is a simple and useful way in chemistry, which is used to predict the shapes and the geometry of individual molecules from the number of electron pairs surrounding their central atoms.

♦VSEPR Postulates

1. The influence of the pauli exclusion principle on electron-electron repulsion is dominant in establishing the geometry of main group molecules i.e there is correlation
2. A non-binding pair has a greater steric repulsion effect for neighbouring pairs. ( repulsion order lp-lp>lp-bp>bp-bp )
3. A non-bonding pair has about the same effect as the two pairs of a double bond.
4. A Molecules adopts the structure in which valance shell electron pair repulsion is minimum

♦Limitations:-

In general the scheme does not work for-
1. Transition metal molecules
2. Ionic crystals

♦Bond lengths and bond angle

◊VSEPR assumes that:
Bond angles are established by the repulsion between electron pairs. Bond lengths are modified by the repulsion between electron pairs during the binding in molecules.

♦VSEPR Geometries

♦VSEPR Procedure

1. At the beginning only the structure ( pattern of bonding ) in the molecule is known. (Lewis pair bond)
2. The shape may be obtained by the following steps.
a. Established the central reference atom keeping in mind-
i. The least electronegative atom as the central atom
ii. Avoid chain structures if possible
iii. H and F are never at the centre. Other halogen atoms do not occupy the centre unless combined with oxygen or more electro negative halogen atom
iv. O is not at the centre unless it is combined with the other atoms
b. Draw the best Lewis structure including double bond or triple bond if necessary.
c. Connect the number of-
i. Bind pairs, bp and
ii. Lone pairs, lp, Around the central atom
Note:
• Double bonds and triple bonds Count as a single bond pair
• Single electron ( unpaired ) may count as a single bond or be counted in a bond pair with three electron
d. Find occupancy using following relation- Occupancy = total number of bond pairs, bp + total number of lone pairs, lp
e. Inter shape from VSEPR occupancy table

♦Modification of Structures:

◊Repulsion

• Lp larger than bp
• Very electronegative atoms yield smaller bp
• Double bonds and triple bonds occupy more space than single bonds
• lp-lp>lp-bp>bp-bp
• For CN 6 larger groups occupy axial position
• For CN 5 larger groups occupy equatorial position
• For CN 7 larger groups occupy equatorial position (rule is uncertain)

READ ABOUT HYBRIDIZATION AND HYBRID ORBITAL

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