Determining Polarity of a Molecule Print PDF

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CHEM2210

Dr. T Determining Polarity of a Molecule

To determine the polarity of a molecule, follow these steps: 1) Draw the Lewis structure 2) Determine the electron and then the molecular geometry of the molecule 3) Look at each individual bond and determine the polarity of the bond using the difference in electronegativity of the 2 atoms connected through the bond. 4) Draw vectors representing the dipoles of each bond. 5) Use vector addition to determine if the vectors add up to zero (nonpolar molecule), or different than zero (polar molecule). Vector addition methods (math review): Method 1: the tail-to-head method using H2O as an example: 1) Draw the Lewis electron dot diagram for water and determine its molecular shape. Water has 4 electron domain, 2 lone pairs - it has bent molecular geometry.

2) Determine if the individual bonds have distribution of charges: ∆𝐸𝑁 = 3.5(𝑂) − 2.1(𝐻) = 1.4. Therefore, yes the two bonds are polar.

3) Draw in the dipole arrows for the polar covalent bonds with arrows

2

1

pointing to the more electronegative atom. The magnitude of the two vectors is the same. 2

4) Connect the dipole arrows tail-to-head by shifting one of the vectors 1

(vector 2) by translation only. Remember that the magnitude and direction of neither vector changes.

5) Draw a new line connecting the tail of the first vector (vector 1) with the net head of vector 2. This is the net molecular dipole.

6) Finally, superimpose the net dipole on the molecule. This is a polar molecule.

dipole

2 1

CHEM2210 Method 2: the vector component method using H2O as an example: The first 3 steps are the same as in Method 1. 4) Separate the vectors into their horizontal and vertical components. For vector 1: its horizontal componet is a and vertical b For vector 2: its horizontal componet is c and vertical d

5) Add the horizontal components: 𝑎1 + 𝑐1= 0 (same magnitude, opposite direction). 6) Add the vertical components 𝑏61+ 𝑑1 = 2 𝑏61(same magnitude and direction of the vectors. 7) Show the net dipole on the molecule.

Dr. T

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2 b d a

c...