Electronegativity

Electronegativity is a chemical property that describes the ability of an atom to attract electrons toward itself in a chemical bond. It plays a key role in determining the nature of the bond (ionic, polar covalent, or nonpolar covalent) between atoms in a molecule.

Key Features of Electronegativity

• Unitless: Electronegativity is a dimensionless quantity.
• Relative Scale: It is measured on a relative scale, the most common being the Pauling scale, developed by Linus Pauling.
  • Fluorine (F) has the highest value of 4.0 on the Pauling scale.
  • Cesium (Cs) has one of the lowest values, about 0.7.

Factors Affecting Electronegativity

• Atomic Size: Smaller atoms have a higher electronegativity because the nucleus is closer to the bonding electrons.
• Nuclear Charge: Atoms with a greater number of protons exert a stronger pull on electrons, increasing electronegativity.
• Shielding Effect: Inner electron shells reduce the effective nuclear attraction, lowering electronegativity in larger atoms.

Variation in Electronegativity

• Across a Period (Left to Right): Electronegativity increases because atoms have a greater nuclear charge and smaller size.
• Down a Group (Top to Bottom): Electronegativity decreases as atomic size increases and the shielding effect becomes more pronounced.

Role in Bonding

• Ionic Bonds: Form when the difference in electronegativity between two atoms is very large (typically > 1.7), leading to electron transfer.
• Polar Covalent Bonds: Occur when the electronegativity difference is moderate (between 0.4 and 1.7), causing unequal sharing of electrons.
• Nonpolar Covalent Bonds: Occur when the electronegativity difference is very small or zero (< 0.4), leading to equal sharing of electrons.

Applications of Electronegativity

• Predicting Bond Type: Determines whether a bond is ionic, polar covalent, or nonpolar covalent.
• Molecular Polarity: Helps determine whether a molecule is polar (has a dipole moment) or nonpolar.
• Chemical Reactivity:
  • Elements with high electronegativity (e.g., fluorine, oxygen) are highly reactive nonmetals.
  • Elements with low electronegativity (e.g., alkali metals) are highly reactive metals.
• Acid-Base Behavior: Helps predict the strength of acids and bases, as electronegativity affects the stability of ions.

Examples of Electronegativity Values (Pauling Scale)

Understanding electronegativity allows chemists to predict chemical behavior, bond characteristics, and molecular properties effectively.