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 of the way to predict a molecule’s shape?
Electron pair repulsion theory
What shape name is given to a molecule with no lone pairs?
Tetrahedral
What is the bond angle in a tetrahedral molecule?
109.5
What shape name is given to a molecule with one lone pair?
Trigonal pyramidal
What is the bond angle in a trigonal pyramidal molecule?
107
What shape name is given to a molecule with two lone pairs?
Non-linear
What is the bond angle in a non-linear molecule?
104.5
What shape name is given to a molecule with 2 electron pairs around the central atom?
Linear
What is the bond angle in a linear molecule?
180
What shape name is given to a molecule with 3 electron pairs around the central atom?
Trigonal planar
What is the bond angle in a trigonal planar molecule?
120
What shape name is given to a molecule with 5 electron pairs around the central atom?
Trigonal bipyramidal
What is the bond angle in a trigonal bipyramidal molecule?
There are actually two, 120 in one plane and 90 in the other plane.
What shape name is given to a molecule with 6 electron pairs around the central atom?
Octahedral
What is the bond angle in an octahedral molecule?
90
What is electronegativity?
An atom’s ability to attract the electron pair in a covalent bond.
How is electronegativity measured?
On the Pauling scale.
What does a higher Pauling value mean?
A higher electronegativity and thus a greater attraction for an electron pair in a covalent bond.
What makes a bond polar?
Two atoms with electronegativities differing by at least 0.5, the bonding electrons are pulled towards the more electronegative atom, making it polar.
What does a polar bond form?
A permanent dipole
How is a dipole formed?
A difference in charge between the two atoms, caused by a shift in electron density in the bond.
Are diatomic gases polar or non-polar, why?
Non-polar
Because the atoms have equal electronegativities and so the electrons are equally attracted to both nuclei.
What happens if the polar bonds are arranged symmetrically in a molecule?
The dipoles cancel each other out, so the molecule has no overall dipole and is non-polar.
What happens if the polar bonds are arranged unsymmetrically in a molecule?
Uneven distribution of charge and the molecule will have an overall dipole.
What can electronegativity be used to predict about bonding?
Whether it will be covalent or ionic bonding.
What is the general strength of intermolecular forces?
Very weak
What is the order of the intermolecular forces from strongest to weakest?
Hydrogen bonding
Permanent dipole-dipole interactions
Induced dipole-dipole
What are induced dipole-dipole forces?
Fluctuation in the electron density around a molecule creates an instantaneous dipole in a molecule. The instantaneous dipole induces a dipole in a neighbouring molecule.
What is the effect of stronger induced dipole-dipole forces?
Higher boiling points
Why are higher boiling points brought about?
Larger molecules have larger electron clouds so stronger induced dipole-dipole forces.
Boiling point is overcoming the intermolecular forces so stronger ones means a higher boiling point.
How are permanent dipole-dipole interactions labelled?
With charges of delta+ and delta- on the atoms.
When can hydrogen bonding occur?
When hydrogen is covalently bonded to fluorine, nitrogen or oxygen.
Why are hydrogen bonds brought about?
Hydrogen has a high charge densiy 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 on a neighbouring molecule’s F, N or O.
State and explain two anomalous properties of water.
- Ice is less dense than liquid water because hydrogen bonds hold molecules apart in open lattice structure
- Higher melting and boiling points than expected because a lot of energy is needed to overcome hydrogen bonds
Explain why simple molecular compounds have low melting and boiling points.
Weak intermolecular forces are broken by the energy present at low temperatures
Explain why simple molecular compounds do not usually dissolve readily in water.
There is little interaction between the molecules in the lattice and the polar solvent molecules
How do intermolecular forces explain simple covalent compounds not conducting electricity?
Overall covalent molecules are uncharged, permanent dipoles are not strong enough.
What happens to nuclear charge and atomic radius across the periodic table?
- The nuclear charge increases
- The atomic radius decreases
