Intermolecular Forces, Electronegativity, and States of Matter: Study Notes for General Chemistry

Chapter 9: Electronegativity and Bond Types

Electronegativity

Electronegativity is the ability of an atom to attract electrons to itself in a chemical bond. It is a fundamental property that influences the type of bonding and the polarity of molecules.

• Definition: Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.

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

• Example: Fluorine (F) is the most electronegative element.

Relative Electronegativities of the Elements

The periodic table can be color-coded to show electronegativity values, which helps predict bond types and molecular polarity.

• High electronegativity: Nonmetals, especially in the upper right of the periodic table (e.g., F, O, N).

• Low electronegativity: Metals, especially in the lower left (e.g., Cs, Fr).

Bond Type and Electronegativity Difference

The difference in electronegativity between two atoms determines the type of bond formed.

Electronegativity Difference	Bond Type	Example
Small (0-0.4)	Pure Covalent	Cl2
Intermediate (0.5-2.0)	Polar Covalent	HCl
Large (2.0+)	Ionic	NaCl

• Pure covalent bond: Electrons shared equally.

• Polar covalent bond: Electrons shared unequally, creating partial charges ($ext{δ}^+$ and $ext{δ}^-$).

• Ionic bond: Electrons are transferred from one atom to another.

Polar Molecules

To determine if a molecule is polar:

• Draw the Lewis structure.

• Check for polar bonds (difference in electronegativity).

• Assess the geometry: If the geometry does not cancel out the dipole moment, the molecule is polar.

Chapter 12: States of Matter and Intermolecular Forces

States of Matter at Room Temperature

The physical state of a substance at room temperature depends on the strength of intermolecular forces.

• CO2: Gas

• H2O: Liquid

• I2: Solid

• Key Concept: Intermolecular forces are forces that hold molecules together and determine physical properties such as boiling and melting points.

Types of Intermolecular Forces

There are four main types of intermolecular forces:

• Dispersion Forces (London Forces, van der Waals)

• Dipole-Dipole Forces

• Hydrogen Bonding

• Ion-Dipole Forces

Dispersion Forces

• Present in all atoms and molecules.

• Caused by fluctuations in electron distribution, leading to temporary dipoles.

• Strength increases with molecular size and surface area.

• Example: I2 has strong dispersion forces due to its large electron cloud.

Dipole-Dipole Forces

• Present in all polar molecules.

• Result from permanent dipoles interacting.

• Lead to higher melting and boiling points compared to nonpolar molecules.

• Example: Formaldehyde (polar) vs. ethane (nonpolar).

• Polar and nonpolar molecules do not mix well.

Hydrogen Bonding

• Occurs in polar molecules where hydrogen is bonded to highly electronegative atoms (F, O, N).

• Strongest type of dipole-dipole interaction.

• Examples: HF, NH3, H2O.

Ion-Dipole Forces

• Occur when an ionic compound is mixed with a polar compound.

• Example: NaCl dissolved in water.

Intermolecular Forces Strength

Type	Present in	Strength
Dispersion	All molecules and atoms	Weakest
Dipole-dipole	Polar molecules	Intermediate
Hydrogen bonding	Molecules with H bonded to F, O, or N	Strong
Ion-dipole	Mixtures of ionic and polar compounds	Strongest

Examples of Intermolecular Forces

• PH3: Dispersion

• HBr: Dispersion and dipole-dipole

• CH3OH: Dispersion, dipole-dipole, hydrogen bonding

• I2: Dispersion

Comparing Boiling Points

Boiling point increases with stronger intermolecular forces.

• I2 vs H2O: I2 is a solid (256 g/mol), H2O is a liquid (18 g/mol). I2 has stronger dispersion forces due to its larger size.

• NH3 (polar, hydrogen, dispersion) vs CH4 (dispersion): NH3 has a higher boiling point.

• CS2 (dispersion) vs CO2 (dispersion): CS2 has a higher boiling point due to larger molecular size.

• CO2 vs NO2: NO2 has a higher boiling point due to its polarity.

Additional info: These notes cover key concepts from General Chemistry chapters on solutions, electronegativity, molecular structure, and intermolecular forces, providing foundational knowledge for understanding chemical and physical properties of substances.