CHE 100: Electronegativity, Bond Polarity, and Types of Chemical Bonds

Electronegativity & Intermolecular Forces

Electronegativity

Electronegativity is a fundamental concept in chemistry that describes the ability of an atom to attract shared electrons in a chemical bond. It is crucial for understanding bond polarity and the behavior of molecules.

• Definition: Electronegativity is the tendency of an atom to attract electrons towards itself in a chemical bond.

• Periodic Trends:

  • Electronegativity increases from left to right across a period.

  • Electronegativity decreases from top to bottom down a group.

  • Nonmetals generally have higher electronegativity values than metals.

  • Fluorine is the most electronegative element.

• Applications: Electronegativity differences help predict bond type and molecular polarity.

Bond Types and Polarity

The type of chemical bond formed between atoms depends on the difference in their electronegativity values. This difference also determines whether a bond is polar or nonpolar.

• Nonpolar Covalent Bonds:

  • Formed between identical nonmetals or atoms with very similar electronegativity.

  • Electrons are shared equally.

  • Example: $\text{H}_2$ (hydrogen molecule), $\text{Cl}_2$ (chlorine molecule).

• Polar Covalent Bonds:

  • Formed between different nonmetals with moderate electronegativity difference.

  • Electrons are shared unequally, creating partial charges (dipoles).

  • Example: $\text{HCl}$ (hydrogen chloride), $\text{H}_2\text{O}$ (water).

• Ionic Bonds:

  • Formed between metals and nonmetals with large electronegativity difference.

  • Electrons are transferred from one atom to another, forming ions.

  • Example: $\text{NaCl}$ (sodium chloride).

Electronegativity Difference and Bond Type

The difference in electronegativity values between two atoms can be used to predict the type of bond formed:

Electronegativity Difference	Type of Bond	Electron Sharing
0.0 – 0.4	Nonpolar Covalent	Electrons shared equally
0.5 – 1.8	Polar Covalent	Electrons shared unequally
1.9 – 3.3	Ionic	Electrons transferred

Example Calculation:

• For $\text{C-H}$ bond: $2.5 - 2.1 = 0.4$ (Nonpolar covalent)

• For $\text{C-O}$ bond: $3.5 - 2.5 = 1.0$ (Polar covalent)

• For $\text{Na-Cl}$ bond: $3.0 - 0.9 = 2.1$ (Ionic)

Bond Polarity and Dipoles

Bond polarity arises when electrons are shared unequally, resulting in a dipole moment. The molecule has a partial positive end ($\delta^+$) and a partial negative end ($\delta^-$).

• Dipole Moment: Arrow points from positive to negative end.

• Notation: Greek letter delta ($\delta$) with $+$ or $-$ indicates partial charges.

• Example: $\text{C}^\delta^+-\text{O}^\delta^-$ in carbon monoxide.

Summary Table: Bond Type from Electronegativity Difference

Type of Electron Sharing	Electronegativity Difference	Type of Bond
Shared equally	2.1 – 2.1 = 0.0	Nonpolar covalent
Shared about equally	3.0 – 2.8 = 0.2	Nonpolar covalent
Shared unequally	2.8 – 2.1 = 0.7	Polar covalent
Shared unequally	3.0 – 2.1 = 0.9	Polar covalent
Electron transfer	3.0 – 0.9 = 2.1	Ionic
Electron transfer	3.5 – 1.2 = 2.3	Ionic

Key Point: The larger the electronegativity difference, the more polar the bond.

Application to Large Molecules

Electronegativity differences are used to analyze the polarity of bonds within large molecules, which affects their physical and chemical properties.

• Example: In an amino acid, the $\text{C-H}$ bond is nonpolar, $\text{C-O}$ bond is polar, and $\text{C-C}$ bond is nonpolar.

• Implication: The overall polarity of a molecule depends on the sum of its bond polarities and molecular geometry.

Additional info: These notes are foundational for understanding intermolecular forces, solubility, and reactivity in GOB Chemistry. Students should be able to use electronegativity values to predict bond types and molecular polarity for exam preparation.