CHAPTER 18: ELECTRONEGATIVITY AND POLARITY
CLASSNOTES
NON-POLAR VERSUS POLAR MOLECULES
Atoms can share electrons evenly or unevenly. Atoms that share electrons evenly form covalent
molecules. Atoms that share electrons unevenly form polar molecules.
POLAR MOLECULES
Because electrons in polar molecules are distributed unevenly, polar molecules have partial positive (+)
regions and partial negative (-) regions. Polar molecules are sometimes called dipoles because, like
magnets, they form two opposite poles.
THE WATER MOLECULE
Water is a typical example of a polar molecule. The central oxygen atom holds valence electrons closer
to itself than the hydrogen atoms. This creates a partial negative charge around the oxygen and partial
positive charge around the hydrogen atoms within the molecule.
THE WATER MOLECULE
+ = partial positive charge
- = partial negative charge
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ELECTRONEGATIVITY
Some elements attract and hold on to electrons better than others. Electronegativity measures the
affinity that particular atoms have for electrons.
ELECTRONEGATIVITY AND THE PERIODIC TABLE
Electronegativity values range from 0.7 (cesium) to 4.0 (fluorine). In general, electronegativity
increases from the lower left-hand corner of the periodic table (large alkali metals) to the upper righthand corner of the periodic table (small halogen gases).
ELECTRONEGATIVITY TRENDS
CHEMICAL BONDS
In general, there are three types of chemical bonds: ionic, covalent, and polar. Ionic bonds form when
electrons completely transfer from metal atoms to non-metal atoms. Covalent bonds form when atoms
share their electrons evenly. Polar bonds form when atoms share their electrons unevenly.
ELECTRONEGATIVITY TABLES
Electronegativity tables predict the types of bonds formed between atoms.
Example 7. A fluorine and a hydrogen atom combine and form hydrogen fluoride. According to
electronegativity values, what type of compound is hydrogen fluoride?
(A) covalent
(B) polar
(C) ionic
7A. (B) polar. Fluorine has a 4.0 electronegativity and hydrogen has a 2.1 electronegativity. The
difference between these electronegativity values is: 4.0 – 2.1 = 1.9. This means hydrogen fluoride is a
polar molecule.