Polarity and Intermolecular forces

Molecular polarity

Atoms in a covalent molecule will often have varying electronegativities; higher electronegativity of an atom = stronger pull on electrons

→ atom with the electrons closer to it will develop a slight negative charge

→ atom having the electrons pulled away will have a slight positive charge

Molecules in which this occurs have a bond dipole

Polar bonds occur when the difference in polarity is between 0.5 and 2

Some molecule may have polar bonds between atoms, but due to symmetrical shape and charge distribution they lack net dipoles and are non-polar overall

Determining polarity

1. identify the atoms involved in the molecule

2. using electronegativity values (pt), determine difference in electronegativities

3. use VSEPR rules to draw shape

4. if polar, add dipole symbols (δ^- at most electronegative and δ⁺ at the most electronegative)

5. if the charges are asymmetric, the molecule is polar

Hydrogen bonding

Strongest of intermolecular forces

only occurs when oxygen is bonded to fluorine, oxygen, or nitrogen, due to electronegativity difference

Dipole-dipole interactions

between polar molecules

positive dipoles of polar molecules are attracted to negative dipoles of other polar molecules due to difference in electronegativity

Dispersion forces

weakest of intermolecular forces

caused by instantaneous dipoles; the electrons may cluster at random on a particular side of the atom, causing a slight negative charge on that side

→ atoms or molecules next to the negative pole of another molecule will have their electrons repelled, exposing positive nuclei and causing a weak attraction

→ more electrons = stronger dispersion forces

→ shapes of molecules dictate how close they can get to each other; closer allows for stronger attraction

→ all covalent molecular substances have dispersion forces

→ boiling and melting points increase as molecules become larger