University:
Emory University
Course:
CHEM 141 | General Chemistry I
Academic year:

2020
Views:

149

Pages:

4
Author:

Lance Gutierrez

How to find the geometry of the molecule using VSEPR theory? Step 1: Draw the Lewis structure of the molecule Step : 2: Count the number of electron pair(lone pair and bond pair) in the structure. Consider multiple bond as single bond Step : 3 : Arrange electron pairs to minimise repulsion Step : 4: Position the atoms to minimise the lone pair - lone pair repulsion if > 1 lone pair Step 5: Name the geometry according to the lone pair and bond pairs Feeling Lengthy! I have a shortcut for you Step 1: ADD the number of electrons in the Valence shell of the atoms present in the molecule Example: H_2O = (1*2 + 6*1) = 8 Step 2: if the number > 8 Divide the number by 8 if the number <8 Divide the number by 2 Example: H_2O = (1*2 + 6*1) = 8 /2 = 4 Step 3: Number of atoms bonded with the central atom is equal to bond pair Example: H_2O = (1*2 + 6*1) = 8 /2 = 4 Bond pair = 2 Step 4: By Subtracting bond pair from the number we got from step 2, you will get lone pair Example: H_2O = (1*2 + 6*1) = 8 /2 = 4 Bond pair = 2 Lone pair = 4-2 =2 Step 5: Find the geometry of the molecule from the shape as given below Example: H_2O = (1*2 + 6*1) = 8 /2 = 4 Bond pair = 2 Lone pair = 4-2 =2 Steric number = 2 + 2 = 4 Shape - Tetrahedral Geometry - Bent Steric Number Number of bond pairs Number of Lone pairs Geometry of the molecule Approximate bond Angle (deg) 1 1 0 Linear - 2 2 0 Linear 180 3 3 0 Trigonal planar 120 𝑜 2 1 Bent or V shape 120 𝑜 4 0 Tetrahedral 109 28' 3 1 Trigonal Pyramidal 𝑎𝑟𝑜𝑢𝑛𝑑 109 28' 2 2 Bent 𝑎𝑟𝑜𝑢𝑛𝑑 109 28' 5 0 Trigonal bipyramidal 120 & 90 4 1 See saw 3 2 T-shape 90 2 3 Linear 180 6 0 Octahedral 90 5 1 Square pyramidal 90 4 2 Square planar 90 7 0 Pentagonal bipyramidal 72 & 90 4 5 6 7 0 0 0 0 𝑜 𝑜 𝑜 0 𝑜 𝑜 𝑜 0 𝑜 6 1 Pentagonal pyramidal Mnemonics to remember the above table: Bond pair Lon e pair Geometry 2 1 Bent Big 2 2 Bent Bossy 2 3 Linear Lina 3 1 Trigonal pyramidal Tried 3 2 T-shape To 4 1 Seesaw See 4 2 Square planar Special 5 1 Square pyramidal Super 6 1 Pentagonal pyramidal Pen Pyramids 0 𝑎𝑟𝑜𝑢𝑛𝑑 72 & 90 Big Bossy Lina Tried To See Special Super Pen Pyramids Examples: 1) ClF_3 Number of electrons in the valence shell electrons in the atoms in the molecule = (7*1 + 7*3) = 28 /8 = 3+2 = 5 (After dividing 28/8, divide the remainder by 2 which is less than 8. So you will get 2 ) Bond pair = 3 Lone pair = 5-3 =2 Steric number = 3 + 2 = 5 Shape - Trigonal bipyramidal Geometry - T-shape 𝑜 2) I_3^Number of electrons in the valence shell electrons in the atoms in the molecule = (7*3) +1 (for negative charge) = 22 /8 = 3+3 = 5 (After dividing 22/8, divide the remainder by 2 which is less than 8. So you will get 3 ) Bond pair = 2 Lone pair = 5-2 =3 Steric number = 2 + 3 = 5 Shape - Trigonal bipyramidal Geometry - Linear