General Chemistry – CHM110: Water, Electronegativity, Polarity, and Hydrogen Bonding

Water Everywhere: A Most Precious Resource

Overview

This section introduces the unique chemical and physical properties of water, emphasizing its importance as a resource and its role in chemical systems. Key topics include electronegativity, molecular polarity, hydrogen bonding, and the states of water.

Electronegativity

Definition and Trends

• Electronegativity (EN) is a measure of the attraction of an atom for electrons in a chemical bond.

• Atoms with higher electronegativity attract electrons more strongly.

• Electronegativity increases across a period (left to right) and decreases down a group (top to bottom) in the periodic table.

• Fluorine (F) is the most electronegative element.

Example: In the H–F bond, fluorine is more electronegative than hydrogen, resulting in a polar bond.

Order of Increasing Electronegativity: Ba < Ca < Ga < Se

Electronegativity and Bond Polarity

• Electrons in an O–H bond are pulled closer to the more electronegative oxygen atom.

• This creates a polar covalent bond with a partial negative charge (δ–) on oxygen and a partial positive charge (δ+) on hydrogen.

• The difference in electronegativity ($\Delta EN$) between two bonded atoms determines bond polarity.

Equation:

$\Delta EN = EN_{\text{more electronegative atom}} - EN_{\text{less electronegative atom}}$

Example: For O–H, $EN_O = 3.5$, $EN_H = 2.1$, so $\Delta EN = 1.4$

Predicting Molecular Polarity

Criteria for Polarity

• A molecule with only nonpolar bonds is nonpolar (e.g., H2, Cl2).

• All diatomic molecules of identical atoms are nonpolar.

• A molecule with polar bonds may be polar or nonpolar, depending on its shape and symmetry.

• If the central atom has lone pairs and/or the terminal atoms are different, the molecule is likely polar.

Examples:

• H2O is polar (bent shape, polar bonds).

• CO2 is nonpolar (linear shape, polar bonds cancel).

• SO3, CCl4 are nonpolar (symmetrical, polar bonds cancel).

Hydrogen Bonding

Intermolecular Forces (IMFs)

• Intermolecular forces are attractions between neutral atoms and molecules.

• Main types: dispersion forces, dipole-dipole attractions, and hydrogen bonding.

Hydrogen Bonding Explained

• A hydrogen bond is an electrostatic attraction between a hydrogen atom bonded to a highly electronegative atom (O, N, or F) and a neighboring O, N, or F atom.

• Hydrogen bonds are stronger than other IMFs but weaker than covalent bonds.

Example: Water (H2O) molecules form hydrogen bonds with each other, leading to high boiling and melting points.

Physical Properties of Water

• Water exists as solid (s), liquid (l), and gas (g) under different conditions.

• Melting point of water: 0°C

• Boiling point of water: 100°C

• Hydrogen bonding is responsible for water's high boiling point compared to similar-sized molecules.

Substance	Molar Mass (g/mol)	Boiling Point (°C)	Hydrogen Bond?
H2O	18	100	Yes
CH4	16	-164	No
N2	28	-196	No
O2	32	-183	No

Additional info: Water's anomalously high boiling point is due to extensive hydrogen bonding between molecules.