Chapter 6 - Shapes of Molecules and Intermolecular Forces Flashcards
Shapes of molecules and ions, Electronegativity and polarity, Intermolecular forces and Hydrogen bonding.
What does molecular shape depend on?
Electron pairs around the central atom
In what way do electron pairs interact?
They repel each other
Which type of pair repels the most?
Lone pairs
What is the order of pair repulsion, from strongest to weakest?
• Lone pair + Lone pair
• Lone pair + Bonding pair
• Bonding pair + Bonding pair
What is the name given to the way of predicting a molecule’s shape?
Valence shell electron pair repulsion theory (VSEPR)
What do dotted wedges, solid wedges, and lines stand for?
• Dotted wedge - into the paper’s plane
• Solid wedge - out of the paper’s plane
• Line - in the paper’s plane
What name is given to the shape of a molecule with one lone pair?
Pyramidal
What is the bond angle in a pyramidal molecule?
107°
What name is given to the shape of a molecule with two lone pairs?
Non-linear
What is the bond angle in a non-linear molecule?
104.5°
What is electronegativity?
An atom’s ability to attract the electron pair in a covalent bond
How is electronegativity measured?
The Pauling scale is most commonly used
What does a higher Pauling value mean?
• A greater electronegativity
• Thus a greater attraction for the electron pair in a covalent bond
What is needed to make a bond polar?
Two atoms with electronegativities differing by at least 0.5
How is bond polarity shown?
δ+ (partial positive)
δ- (partial negative)
Are diatomic gases polar or non-polar, and why?
• Non-polar
• Both atoms have equal electronegativities so the bonding pair of electrons isn’t attracted to one or the other
What happens when polar bonds are arranged symmetrically in a molecule?
• The dipoles cancel eachother out
• So there is no overall dipole
What happens when polar bonds are arranged unsymmetrically in a molecule?
• The dipoles won’t cancel eachother out
• So there is an overall dipole
What is the order of the intermolecular forces from weakest to strongest?
• London forces (induced dipole-dipole interactions)
• Permanent dipole-dipole interactions
• Hydrogen bonding
How do London forces come about?
• Electron density can instantaneously become uneven
• Therefore one side of the atom/molecule has more electrons than the other side
• Leading to δ+ and δ- charges (a temporary dipole)
• This temporary dipole induces dipoles in neighbouring atoms/molecules
• Opposing partial charges or neighbouring molecules attract each other, creating London forces
What is the effect of stronger London forces?
Higher boiling points
Why are higher boiling points brought about?
• Larger molecules have larger electron clouds
• With more electrons, temporary dipoles are stronger and thus London forces are stronger
• So a higher boiling point is needed to provide the energy to overcome the stronger London forces
What are the conditions for hydrogen bonding to occur?
• A hydrogen must be covalently bonded to either a fluorine, nitrogen, or oxygen atom
• The more electronegative atom (F, N, O) must have a lone pair of electrons
How are hydrogen bonds brought about?
• When bonded to either an F, N, or O atom, a permanent dipole occurs where the hydrogen has a δ+ charge
• This δ+ hydrogen atom interacts with a lone pair of electrons on a neighbouring molecule (either from F, N, or O)
• The interaction between the δ+ hydrogen and lone pair of electrons creates the hydrogen bond
What effect(s) does hydrogen bonding have on a molecule?
• More soluble in water
• Higher boiling and freezing points compared to molecules of a similar size that don’t form hydrogen bonds
What are the anomalous properties of water, caused by hydrogen bonding?
• Higher melting and boiling points than expected
• Ice is less dense than water
• High surface tension
How do intermolecular forces explain simple covalent compounds usually having low melting and boiling points?
• IMFs are relatively weak and so don’t require as much energy to overcome
• Thus they have low melting and boiling points than
How do intermolecular forces explain simple covalent compounds sometimes being soluble in water?
• Water is a polar molecule so, if applicable, solute molecules can form hydrogen bonds with water molecules and dissolve
• Polar solute molecules also show dipole-dipole interactions with water and can dissolve
Why can’t covalent molecules conduct electricity?
• No free electrons
• Overall neutral entities
Molecules’ shapes table:
Note: 5 bonding pairs, 0 lone pairs = “Trigonal Bipyrimidal” (both 90° and 120° bond angles)
How many degrees does a lone pair reduce bond angles by?
-2.5° for each lone pair
