Polarity of Molecules

Electronegativity

The ability of an atom to attract and hold onto valence electrons in a chemical bond.

Valence Electrons

Electrons found in the outermost shell of an atom.

Octet Rule

States that atoms are most stable when they have 8 electrons in their valence (outermost) shell.

Electronegativity Trend

The higher the electronegativity, the more strongly an element attracts shared electrons.

Most Electronegative Element

Fluorine (F).

Least Electronegative (Most Electropositive) Element

Francium (Fr).

Bond Polarity

The polarity of a bond involving the sharing of electrons in a molecule or compound.

Electronegativity Difference (EN Difference)

The difference in electronegativity between bonded atoms used to predict whether a bond is nonpolar covalent, polar covalent, or ionic.

Nonpolar Covalent Bond

Occurs when bonding atoms have approximately equal electronegativity (EN difference: 0.0–0.4).

Polar Covalent Bond

Occurs when electron pairs are unequally shared between two atoms, creating a dipole with one end more negative and the other more positive (EN difference: 0.5–2.0).

Dipole

A molecule with two poles, where one end is more negative and the other is more positive due to unequal sharing of electrons.

Ionic Bond

Occurs when the electronegativity difference is so great that electrons are transferred to the atom with greater electronegativity (EN difference: 2.1 and above between a metal and a nonmetal).

0.0–0.4 (Electronegativity Difference Rule)

Nonpolar covalent bond

0.5–2.0 (Electronegativity Difference Rule)

Polar covalent bond

2.1 and above = metal + nonmetal (Electronegativity Difference Rule)

Ionic bond

Molecular Geometry

The three-dimensional arrangement of atoms in a molecule.

VSEPR (Valence Shell Electron Pair Repulsion) Theory

Explains molecular shape based on the repulsion between electron pairs around the central atom.

Lone Pair

A pair of valence electrons not shared in a bond.

Bond Pair

A pair of electrons shared between two atoms in a covalent bond.

Electron Pair Repulsion (Four Ideas of VSEPR Theory)

Electron pairs stay as far apart from each other as possible to minimize repulsion.

Molecular Shape (Four Ideas of VSEPR Theory)

Determined by the number of bond pairs and lone pairs around the central atom.

Multiple Bonds Rule (Four Ideas of VSEPR Theory)

Treat multiple bonds as if they were single bonds when predicting molecular shape.

Lone Pair vs. Bond Pair

Lone pairs occupy more space (volume) than bond pairs.

Note on Molecular Geometry

Molecular geometry is determined by the positions of the atomic nuclei; lone pairs are not directly observed.