Module 2, Chapter 6: Shapes of Molecules and Intermolecular Forces

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Electron Pair Repulsion Theory
is a model used in chemistry for xplaining and predicting the shapes of molecules and polyatomic ions
What determines the shape of the molecule or ion?
the electron pairs surrounding a central atom
Why are electrons repelled?
so that they are arranged as far apart as possible
How do atoms hold a definitive shape?
the arrangement of electron pairs minimise repulsion and this holds the bonded atoms in a definite shape
4 Bond Angles
tetrahedral
What pairs repel more strongly?
lone pairs repel more strongly than bonded pairs
Bond Angle
the angle between the bonded pairs of electrons
What degree is the bond angle reduced by for each lone pair?
2.5 degrees
Lone pairs, Shape and Bond Angle of a Molecule with 4 Bonded Pairs
- 0 Lone Pairs

- Tetrahedral

- 109.5
Lone pairs, Shape and Bond Angle of a Molecule with 3 Bonded Pairs
- 1 Lone Pairs

- Pyramidal

- 107
Lone pairs, Shape and Bond Angle of a Molecule with 2 Bonded Pairs
- 2 Lone Pairs

- Non Linear / Bent

- 104.5
What gives a molecular a linear shape?
the 2 bonded regions repel one another as far apart as possible
What gives an octahedral shape?
6 bonded pairs of electrons
Name of Shape with 2 Electron Pairs
linear
Name of Shape with 3 Electron Pairs
trigonal planar
Name of Shape with 4 Electron Pairs
tetrahedral
What can be used to explain and predict the shapes of ions?
can also be used to explain and predict the shapes of ions
What changes when the bonded atoms are different elements?
nuclear charges are different

the atoms may be different sizes

the shared pair of electrons may be closer to one nucleus than the other
Electronegativity
the attraction of a bonded atom for the pair of electrons in a covalent bond
How is electronegativity measured?
Pauling scale
What's the Pauling Scale?
sed to compare the electronegativity of the atoms of different elements
What does Pauling's Electronegativity Value depend on?
on an element's position in periodic table
For Pauling's Electronegativity Value, what happens across the periodic table?
- The nuclear charge increases

- The atomic radius decreases
What does a large Pauling Value indicate?
that the atoms of the element are very electronegative
What is the most electronegative atom?
Fluorine
What has the least electronegative atoms?
group 1
What happens if the the electronegativity difference is large in an Ionic Covalent?
one bonded atom will have a much greater attraction for the shared pair than the other bonded atoms
What happens to the more electronegative atom in and Ionic Covalent?
will have gained control of the electrons and the bond will now be ionic rather than covalent
Electronegativity Difference in Covalent, Covalent Covalent and Ionic
- Covalent: 0

- Polar Covalent: 0 to 1.8

- Ionic: > 1.8
What happens in Non Polar Bonds?
the bonded electron pair is shared equally between the bonded atoms
A Bond will be Non Polar when:
- the bonded atoms are the same or,

- the bonded atoms have the same or similar electronegativity
Pure Covalent Bond
neutral atoms held together by equally shared electrons
What bonds do hydrogen and carbon atoms have?
have very similar electronegativities and form nonpolar bonds
What bonds do Hydrocarbon Lipids have?
hydrocarbon liquids such as hexane are non polar solvents and don't mix with water
Polar Bonds
the bonded electrons pair is shared unequally between the bonded atoms
When will a bond be polar?
when the bonded atoms are different and have different electronegativity values, resulting in a Polar Covalent Bond
Dipole
created by equal but opposite charges that are separated by a short distance
A Dipole in a Polar Covalent
does not change and is called a permanent dipole to distinguish it from an induced dipole
Depending on the Shape of the Molecule, dipoles may...
reinforce on another to produce a larger dipole over the whole molecule, or cancel out if the dipoles act in opposite direction
How do 2 dipoles act?
2 dipoles act in opposite directions and exactly oppose one anothe
Strength of Intermolecular Forces
weak
Strongest Intermolecular Force
hydrogen bonds
What are London Forces?
cause all atoms and molecules to be attracted to each other
London Forces
electrons in charge clouds are always moving really quickly

at any particular moment, the electrons in an atom are likely to be more to one side than the other

at this moment, the atom would have a temporary dipole

this dipole can cause another temporary (induced) dipole in the opposite direction on a neighbouring atom

the 2 dipoles are then attracted to each other

the second dipole can cause yet another dipole in a 3rd atom

because the electrons are constantly moving, the dipoles are being created and destroyed all the time
Stronger Induced Dipole Dipole Forces
= higher boiling points

= larger molecules have larger electrons cloud

= molecules with greater SA
Boiling Points of Liquids with stronger induced dipole - dipole forces?
high
What Forces can hold molecules in a lattice?
Induced Dipole Dipole Forces
How are Iodine atoms are held together?
- in pairs by strong covalent bonds to form molecules of I2

- but the molecules are then held together in a molecular lattice arrangement by weak induced dipole dipole attractions
Permanent Dipole Dipole Interactions
delta positive and delta negative charges on polar molecules cause weak electrostatic forces of attraction between molecules
When did Permanent Dipole Dipole interactions happen?
in addition to induced dipole dipole interactions
Hydrogen bonding can only happen....
can only happen when hydrogen is covalently bonded to fluorine, nitrogen or oxygen
Hydrogen Charge Density
- High: because it's so small and F, N and O are very electronegative

- the bond is so polarised that a weak bond forms between the hydrogen of one molecule and a lone pair of electrons on the F, N or O in another molecule
Effects Hydrogen bonding has on the properties of substances
- soluble in water

- higher boiling and freezing points = more energy needed to break bonds

- ice = less dense than water
Ice Melting
- an ice, molecules of water are held together in a lattice by hydrogen bonds
when ice melts, hydrogen bonds are broken, so ice has more hydrogen bonds than liquid water

- ice = less dense than water
The Main factor that determines the boiling point of a substance
s the strength of the induced dipole - dipole forces (unless the molecule form hydrogen bonds)
What happens when you have 2 molecules with similar number of electrons?
the strength of their induced dipole dipole interactions will be similar
Boiling Points of Simple Covalent Compounds
low
Why don't Simple Covalent Compounds Conduct Electricity?
overall covalent molecules are uncharged
Polar Molecules are Soluble in Water
- water is a polar molecule, so it only tends to dissolve other polar substances

- compounds with hydrogen bonds such as ammonia or ethanoic acid, can form hydrogen bonds with water molecules, so will be soluble

- molecules that only have induced dipole-dipole forces such as methane, will be insoluble
Covalent Bonds
- they don't break during melting / boiling EXCEPT for giant molecular substances, like diamond
to melt or boil a simple covalent compound you only have to overcome the intermolecular forces that hold the molecules together

you don't need to break the much stronger covalent bonds that hold the atoms together in the molecules