6.2, 3: Polarity, Negativity, Intermolecular Forces
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What is electronegativity?
The ability of an atom to attract a pair of electrons in its own covalent bonds.
Why can atoms attract electrons in a bond by different strengths?
Because atoms have different numbers of protons and different atomic radii.
What is the Pauling Scale?
A scale used to represent the relative electronegativities of an atom. The higher the value, the higher the electronegativity.
What scale is used to represent the relative electronegativities of an atom?
The Pauling Scale
What makes a bond polar?
If the distribution of charge across the bond is asymmetrical (i.e one atom has a higher electronegativity than the other)
What does the δ (Delta) sign mean?
Partially / a little bit
What is the electronegativity difference in a covalent bond?
0
What is the electronegativity difference in a Polar Covalent bond?
≥0 ≤1.8
What is the electronegativity difference in an ionic bond?
> 1.8
What can electronegativity values be used to estimate?
The type of bonding.
When is a molecule polar?
When there is an asymmetrical distribution of charge across the entire molecule, so the dipoles don’t cancel out.
What is a dipole?
Charge separation across a bond with one atom having a δ+ charge and another having a δ- charge.
How can you obtain a permanent dipole?
The molecule has to be polar.
What is a permanent dipole-dipole interaction?
Where permanent dipoles attract one another / Electrostatic forces of attraction between two permanently polar molecules
What is a polar covalent bond?
A shared pair of electrons, where the electron pair is not shared equally between the two bonded atoms.
Can ionic compounds be dissolved by polar molecules?
Yes.
Describe how water molecules break down an NaCl ionic lattice.
• Water molecules attract Na+ AND Cl- ions • The ionic lattice breaks down as it dissolves • In the resulting solution, water molecules surround the Na+ and Cl- ions.
What are the three main categories that intermolecular forces fall into?
• Induced dipole-dipole interactions (LONDON DISPERSION FORCES) • Permanent dipole-dipole interactions • Hydrogen bonding.
What is an alternative name for London Dispersion Forces?
Induced dipole-dipole interactions. (Sometimes called Van der Waals forces)
What types of molecules show London Dispersion Forces?
All molecules, but only in non-polar molecules is it the strongest force.
Explain London Dispersion Forces work.
• Random electron movement in a molecule creates an instantaneous dipole, where charge is momentarily/instantaneously asymmetrically distributed. • This charge affects the electrons in a neighbouring molecule, inducing a dipole. This affects all other molecules in the substance.
The more electrons in each molecule…
• The larger the induced dipoles • The greater the induced dipole-dipole interactions • The stronger the attractive forces between molecules
How does a larger number of electrons increase the boiling point?
• Larger number of electrons mean larger induced dipoles • More energy is needed to overcome the intermolecular forces.
What does immiscible mean?
When two liquids don’t mix, but instead form 2 separate layers