Intermolecular forces + Electronegativity

Define electronegativity

A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

Define polar covalent bond

A bond with a  permanent dipole, having d+ and d- partial charges on the bonded atoms

Define dipole

A separation in electrical charge so that one atom of a polar covalent bond, or one end of a polar molecule, has a small positive charge d+ and the other has a small negative charge d-

Define polar

d+ and d- charges at different ends of the molecule

electronegativity difference for a pure covalent bond

0

Electronegativity difference range for a polar bond

0 to 1.8

Electronegativity difference for an ionic bond

Greater than 1.8

Which factors affect electronegativity (3)

-          Shielding

-          Number of protons in the radius

-          Atomic radius

what is the trend of increasing electronegativity?

up the group, to the right of the period

what is the most electronegative element?

fluorine

what is the trend in electronegativity down a group and why?

Decreases - shell number increases, shielding increases, electrons less strongly attracted

what is the trend in electronegativity across a period and why?

Increases - shell number remains constant, same shielding, proton number increases, electrons are attracted more strongly

What is the trend in boiling point across period 3 and why?

Decreases - electronegativity difference decreases - shell number is constant, shielding is constant, proton number increases, electrons more strongly attracted

can a molecule contain polar bonds but be non-polar?

yes, the dipoles cancel

What molecules have London forces?

any with electrons

define London forces

temporary dipoles due to the uneven electron distribution caused from the constant movement of electrons

As molecule size increases what happens to the strength of the London forces?

increases - more electrons

London forces: trend in boiling point down the noble gases

Increases - electron number increases, stronger London forces

how do London forces occur? (3)

• Instantaneous dipole induced from uneven electron distribution

• induces a temporary dipole in a neighbouring molecule

• induces a temporary dipole-dipole interaction

What type of molecule have permanent dipole-dipole attraction?

polar

Why do polar molecules have higher boiling points?

permanent dipole-dipole attraction and London forces present - stronger forces, more energy to overcome

Requirements for hydrogen bond formation (3)

• need lone pair

• H atom

• O, F or N atom (very electronegative)

define hydrogen bonding

permanent dipole-dipole attraction where a H atom is bonded to a very electronegative atom

Why do hydrogen bonds form?

The H atom has only one electron, strong attraction from the lone pair on the N,O or F for the H atom.

order of decreasing strength of intermolecular forces

H-bonds > Permanent dipole-dipole > London forces

What scale is used to compare the electronegativity of atoms of different elements?

Pauling electronegativity scale

What are polar solutes soluble in?

polar solvents

what are non-polar solutes soluble in?

non-polar solvents

what do simple molecular substances form when solid?

simple molecular lattice

Why are non-polar simple molecular substances soluble in non-polar solvents?

New london forces form between the molecules and the solvent

The interactions weaken the London forces in the simple molecular lattice

The London forces break and the compound dissolves

Why are non-polar simple molecular substances insoluble in polar solvents?

There is little interaction between the molecules in the lattice and the solvent molecules

The intermolecular bonding within the polar solvent is too strong to be broken

Why are polar simple molecular substances soluble in polar solvents?

The molecules attract each other due to the permanent dipoles

Define hydrophilic

Tendency to mix with water

Define hydrophobic

Tendency to repel water