Molecule Polarity BHS

A nonpolar molecule ​has no net dipole​

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This can be achieved by the molecule having

• only nonpolar bonds (no pulling!)

• polar bonds arranged symmetrically so the bond dipoles cancel out (no pulling!)

A polar molecule ​has a net dipole​

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​This is achieved by the molecule being made up of polar bonds arranged unsymmetrically so that the dipoles do not cancel out.

To determine if a molecule is polar or nonpolar, you need to:

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1. use the electronegativity of the atoms in each bond to decide if each bond is polar or nonpolar

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2. if bond(s) are polar, determine the shape of the molecule using VSEPR Theory to decide if the bonds are arranged symmetrically (nonpolar molecule) or not (polar molecule)

​Symmetrical shapes of molecules include:

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• linear (bond angles 180°), Ex: CO₂ and H₂

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• trigonal planar (bond angles 120°), Ex: AlCl₃

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• tetrahedral (bond angles 109.5°), Ex: CH₄

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• octahedral (bond angles 90°), Ex: SF₆

​A molecule will be polar if it has a net dipole; that is, it will be polar IF and only if

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1. it is made up of elements with different electronegativities (example HCl)​

** AND **

2. the bonds are arranged unsymmetrically so that the dipoles do NOT cancel out

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Examples of unsymmetrical shapes of molecules are:

• bent (Ex: H₂O)

• trigonal pyramidal (Ex: NH₃)

• substitutions to symmetrical shapes (Ex: CH₄ nonpolar --> CH₃Cl polar)