1.5 Orgo Electronegativity, Polar Bonds, and Electrostatic Potential Maps

Electronegativity Trends and Classification

• Electronegativity measures the ability of an atom to attract electrons.

• Periodic trend: Electronegativity increases moving from left to right across a row and from bottom to top up a column toward Fluorine ($F$), which is the most electronegative element.

• Electronegativity values of common elements:

  • Nonmetals: $F = 4.0$, $O = 3.5$, $N = 3.0$, $Cl = 3.0$, $Br = 2.8$, $C = 2.5$, $S = 2.5$, $H = 2.1$, $P = 2.1$

  • Metalloids: $B = 2.0$, $Si = 1.8$

  • Metals: $Al = 1.5$, $Be = 1.5$, $Mg = 1.2$, $Li = 1.0$, $Na = 0.9$, $K = 0.8$

Types of Chemical Bonds

• Chemical bonds exist along a continuous spectrum determined by the difference in electronegativity between bonded atoms:

  • Nonpolar Covalent Bonds: Occur when the electronegativity difference is very small; electrons are shared equally or nearly equally (e.g., $C-C$, $C-H$).

  • Polar Covalent Bonds: Occur when the electronegativity difference is between $0.5$ and $1.7$; electrons are shared unequally, producing a dipole with partial positive ($\delta+$) and partial negative ($\delta-$) charges (e.g., $C-O$, $O-H$, $N-H$, $C-Cl$).

  • Ionic Bonds: Generally occur when a nonmetal bonds with a Group 1A or Group 2A metal and the electronegativity difference exceeds $1.7$; electrons are transferred to form distinct cations and anions held together by electrostatic attraction (e.g., $NaCl$, $KOH$, $LiBr$).

• The $1.7$ difference threshold is a general guideline rather than a strict cutoff (e.g., a $C-Li$ bond has an electronegativity difference of $1.5$ and can be represented as either polar covalent or ionic).

Electrostatic Potential Maps

• Three-dimensional visual maps that display electron density distributions and locations of partial charges.

• Map values are calculated by positioning an imaginary point positive charge around a molecule and determining the potential energy of attraction with surrounding electrons.

• Standard color interpretation:

  • Red: Electron-rich regions ($\delta-$) with strong positive charge attraction.

  • Blue: Electron-deficient regions ($\delta+$) with weak positive charge attraction.

• Direct comparisons between electrostatic potential maps are valid only if both maps use an identical color scale.