Ch. 1 - A Review of General ChemistrySee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Johnny Betancourt

Polar molecules have an imbalance of electron density due to differences in the electronegativity of their component atoms; nonpolar molecules generally have equal distribution of electron density.

What’s the difference between polar and nonpolar molecules? 

To understand what makes molecules polar or not, we need to understand electronegativity and dipoles. The more electronegative (see the Pauling scale) an atom is, the more it’ll pull electron density toward itself and away from other atoms in the molecule. If there's no difference in electronegativity, the electrons will be shared equally and the bond between them will be nonpolar. 

Methanol dipoleMethanol dipole

In this C—O bond, for example, the oxygen pulls electron density away from the carbon. This uneven sharing of electron density is called a molecular dipole, dipole moment, or dipole for short. Neither atom has a formal charge, but they each have a partial charge—this is distinct from an ionic charge, where an atom has full possession of an extra electron. 

Water molecules are also polar because of the arrangement and electronegativity values of oxygen and the two hydrogen atoms.

Water dipolesWater dipoles

How does symmetry come into play? 

Polar molecules have polar bonds in them, but there’s a catch! Let’s talk about a couple of examples.

Cyclic ethersCyclic ethers

These molecules are very similar, but they have very different polarity. 

Cyclic ethers dipolesCyclic ethers dipoles

When determining if a molecule is polar, it’s important to consider symmetry. I’ve drawn all the partial charges and component vectors for each dipole here for visibility, but I combine them below. 

Planes of symmetryPlanes of symmetry

The red dotted lines are planes of symmetry, and the blue arrows are dipole vectors (combined component dipole vectors). The left molecule only has one dipole, so it’s polar! The molecule on the left has two dipoles… so it’s extra polar, right? NOPE! The dipoles are completely equal and opposite in magnitude and direction, so this molecule is nonpolar. 

Hopefully this quick guide helps clarify the difference between polar and nonpolar molecules. Remember that polar solvents will dissolve polar molecules and that nonpolar solvents will dissolve nonpolar molecules. For full coverage of this topic, including how molecular geometry affects polarity, check out my videos here

Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.