Module 2: Chapter 6 (Shapes of molecules and intermolecular forces)
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What are the 3D symbols used when drawing molecules?
solid line = a bond in the plane of paper solid wedge = comes out of the plane dotted wedge = going into the plane
Draw the example of two electron pairs: BeCl2, state bond angle and the shape
bond angle = 180 linear
Draw the example of 3 electron pairs: BF3, state bond angle and the shape
bond angle = 120 trigonal planar
Draw the example of 4 electron pairs: CH4, state bond angle and the shape
bond angle = 109.5 tetrahedral
Draw the example of 6 electron pairs: SF6, state bond angle and the shape
bond angle = 90 octahedral
What is the effect of lone pairs?
• lone pairs are slightly closer to the central atom, occupies more space • results in lone pair repelling stronger than the bonded pair • they repel the bonding pairs slightly closer together, decreasing the bond angle 2.5 per lone pair
Draw the example of 4 electron pairs with one being a lone pair: NH3, state bond angle and the shape
bond angle = 107 pyramidal
Draw the example of 4 electron pairs with 2 being lone pairs: H20, state bond angle and the shape
bond angle = 104.5 non linear
How do you deal with multiple bonds or polyatomic ions?
multiple bonds treated as a bonding pairs, same with polyatomic ions
Draw the example of CO2, state bond angle and shape
bond angle = 180 linear
Draw the example of carbonate, state bond angle and shape
CO3^2- bond angle = 120 trigonal planar
Draw the example of ammonium, state bond angle and shape
NH4+ bond angle = 109.5 tetrahedral
Draw the example of sulphate, state bond angle and shape
SO4^2- bond angle = 109.5 tetrahedral
What is electronegativity?
Electronegativity is a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond
How is electronegativity measured?
pauling scale
Describe electronegativity of elements
• non metals: nitrogen, oxygen, fluorine and chlorine are the most electronegative (tend to form δ-), all above 3.0 • group 1 metals including lithium, sodium and potassium are the least electronegative (δ+), all below 1.0
What is the list of comparative elements for electronegativity?
δ+ = C,H δ- (in descending order of electronegativity) = F O Cl, N (similar value) Br I
Describe bond polarity
In a non-polar bond, bonded electrons are shared equally between bonded atoms. This happens when: • bonded atoms are the same • have a similar/equal electronegativity (for example a C-H bond is ALWAYS NON-POLAR) In a polar bond, bonded electrons are shared unequally. This happens when: • atoms have different electronegativity values -> more electronegative atom has a small partial negative charge and the other a small partial positive charge -> separation of opposite charges is called a dipole
What is the exception for polar molecules?
• dipoles can cancel each other out if the shape is symmetrical, results in the molecules itself being non-polar with no overall dipole • for example: CO2 (linear) and CCl4 (tetrahedral) • if there were lone pairs it would be polar
What are intermolecular forces?
• weak interactions between neighbouring molecules • broken when a molecular substance melts, boils or dissolves • significantly weaker than the covalent bonds • three main types: london forces, permanent dipole-dipole and hydrogen bonding
Describe london forces
• exist between all molecules, polar or non-polar • movement of electrons produces a change in dipole in a molecule and at any point an instantaneous dipole will exist, constantly changing • the instantaneous dipole induces a dipole on a neighbouring molecule, which induces further molecules and attract one another • they are only temporary, in the next instant could disappear
What factors increase the strength of london forces?
The more electrons in each molecule • larger the instantaneous and induced dipoles • greater the interactions • stronger attractive forces For example as you go down the noble gases group, boiling point increases as there are more electrons and therefore stronger induced dipoles
Describe permanent dipole-dipole interactions
• only act between permanent dipoles in polar molecules - more energy than London forces needed to overcome forces -> gives substances higher boiling points
What is a hydrogen bond?
A hydrogen bond is an attraction between a lone pair of electrons on a highly electronegative atom (O, N or F) in one molecule and an electron-deficient hydrogen atom (H attached to a highly electronegative O, N or F atom) in another molecule