Electronegativity and intermolecular forces

Electronegativity

The ability of an atom to attract the pair of electrons in a covalent bond

Electron density

The amount of electrons surrounding a bond

Electronegativity depends on…

Nuclear charge - More protons in the nucleus create a stronger positive charge, which increases the attraction for the bonding pair of electrons.

Atomic radius - A smaller atomic radius means the bonding electrons are closer to the nucleus, resulting in a stronger pull

Shielding - More inner electron shells increase the distance and shield the outer bonding electrons from the nucleus, decreasing attraction

Periodic trends of electronegativity

Across a period (left to right): Electronegativity increases because nuclear charge increases and atomic radius decreases.

Down a group (top to bottom): Electronegativity decreases because atomic radius increases and inner-shell shielding increases, outweighing the increase in nuclear charge.

Polarity

The unequal distribution of electrons in a covalent bond

Differences in electronegativity :

0 Pure covalent bond

0-0.4 Non polar covalent bond

0.4-1.7 Polar covalent bond

<1.7 Ionic bond

Dipole moments

Measure overall polarity, sums up the effect of polarity on all bonds in a molecule. If polar bonds cancel out, there is no dipole moment in that molecule.

Summary

• If there is a big difference in electronegativity in the atoms forming a covalent bond, the distribution of the pair of electrons in the covalent bond is uneven. 

• There is a small positive charge (δ⁺ ) and a small negative charge (δ^- ) at each ends of the bond. The bond is said to be polar and there is a dipole.

• In symmetric molecules, the dipoles cancel out and the molecule is non-polar eg CCl₄ 

• In asymmetric molecules, the polar bonds, or bond dipoles, can produce a polar molecule with a permanent dipole. eg HCl, H₂O

Induced dipole-dipole force (Van der Waals)

-Weakest intermolecular forces

-Formed when the electron orbitals around a molecule are influenced by another charged particle

-The larger the Mr of the molecule, the larger the intermolecular forces

-Straight chain molecules that are packed close together has strong van der waals forces as they have a closer distance for the forces to act

-Instantaneous dipole : as the electron distribution of the original atom changes, it will induce new dipoles in the atoms around it, which will be attracted to the original one

Permanent dipole-dipole interaction

-Stronger than van der waals forces

-Occur between molecules that are polar

-The δ+ and δ− regions attract each other and hold the molecules together in a lattice-like structure

-In molecules with more than one polar bond, the effects of each bond may cancel, leaving a molecule with no dipole moment

-Polar molecules will flip into an arrangement in which they attract each other

Hydrogen bonding

-Strongest intermolecular forces

-Forms between hydrogen atom and other highly electronegative atom with a lone pair of electrons such as oxygen, which makes the hydrogen electron deficient in water

-The lone pair of electrons forms a bond with a hydrogen atom from another molecule

-High melting and boiling points