6. Shapes of molecules and intermolecular forces Flashcards
What do electron pairs do because of their negative charge?
They repel each other
What does the number of bonded and lone pairs determine?
The shape of a molecule
Which two pairs of electrons are the most repelling?
2 lone pairs
Which two pairs of electrons are the least repelling?
2 bonded pairs
Why do lone pairs repel more strongly than bonded pairs?
Because lone pairs are slightly closer to the central atom and occupies (佔用) more space.
What is the name of shape that has 4 bonded pairs and 0 lone pairs?
Tetrahedral
What is the bond angle of a tetrahedral compound?
109.5°
How to draw a tetrahedral compound (in terms of lines and number of lone pairs) ?
2 solid lines, 1 solid wedge, 1 dotted wedge and 0 lone pairs (crosses).
What is an example of a tetrahedral compound?
CH4 (methane)
What is the name of shape that has 3 bonded pairs and 1 lone pair?
Pyramidal
What is the bond angle of a pyramidal compound?
107°
How to draw a pyramidal compound (in terms of lines and number of lone pairs) ?
1 solid line, 1 solid wedge, 1 dotted wedge and 1 lone pair (crosses).
What is an example of a pyramidal compound?
NH3 (ammonia)
What is the name of shape that has 2 bonded pairs and 2 lone pair?
Non-linear
What is the bond angle of a non-linear compound?
104.5°
How to draw a non-linear compound (in terms of lines and number of lone pairs) ?
1 solid line, 1 solid wedge, 0 dotted wedge and 2 lone pairs (crosses).
What is an example of a non-linear compound?
H2O (water)
What is the name of shape that has 2 bonding regions?
Linear
What is the bond angle of a linear compound?
180°
How to draw a linear compound (in terms of lines and number of lone pairs) ?
All bonds are drawn in solid lines and 0 lone pairs (crosses) needed.
What is an example of a linear compound?
CO2 (carbon dioxide)
What is the name of shape that has 3 bonding regions?
Trigonal planar
What is the the bond angle of a trigonal planar compound?
120°
How to draw a trigonal planar compound (in terms of lines and number of lone pairs) ?
All bonds are drawn in solid lines and 0 lone pairs (crosses) needed.
What is an example of a trigonal planar compound?
BF3 (boron trifluoride)
What is the name of shape that has 4 bonding regions?
Tetrahedral
What is the name of shape that has 6 bonding regions?
Octahedral
What is the bond angle of an octahedral compound?
90°
How to draw an octahedral compound (in terms of lines and number of lone pairs) ?
2 solid lines, 2 solid wedges, 2 dotted wedges and 0 lone pairs (crosses).
What is electronegativity?
The ability of an atom to attract the bonding electrons in a covalent bond.
What is the Pauling scale used to?
Compare the electronegativity of atoms of different elements.
Does the nuclear charge of elements increase or decrease across the periodic table?
Increase
Does the atomic radius of elements increase or decrease across the periodic table?
Decrease
Does electronegativity increase or decrease across the periodic table?
Increase
Does electronegativity increase or decrease down the periodic table?
Decrease
Which atom is the most electronegative?
Fluorine
Which atoms are the least electronegative?
Group 1 metals
What is the electronegativity difference of covalent bonds?
0
What is the electronegativity difference of polar covalent bonds?
0 to 1.8
What is the electronegativity difference of ionic bonds?
Greater than 1.8
What is a polar molecule?
A molecule with an overall dipole, considering any dipoles across bonds and the shape of the molecule.
How is the bonded electron pair shared in a non-polar bond?
It is shared equally
What is a non-polar molecule?
A molecule with no charge separation across its bonds.
How can a bond be non-polar?
- when the bonded atoms are the same element
- when the bonded atoms have the same or similar electronegativity
How is the bonded electron pair shared in a polar bond?
It is shared unequally
What is dipole?
Separation of opposite charges across a bond with one atom having a δ+ charge and one atom having a δ- charge.
What is permanent dipole?
A dipole in a polar covalent bond that doesn’t change.
What are intermolecular forces?
A weak attractive force between molecules
What are the 3 main types of intermolecular forces?
- induced dipole-dipole interactions (London forces)
- permanent dipole-dipole interactions
- hydrogen bonds
What is the origin of induced dipole-dipole interactions (London forces) ?
- the position of dipole is constantly changing
- the instantaneous dipole induces a dipole on a neighbouring molecule (eg. the δ+ side of the molecule attracts the δ- side of another molecule)
- the induced dipole induces further dipoles on neighbouring molecules, which then attract one another
Are induced dipole-dipole interactions (London forces) permanent or temporary?
Temporary
What will happen to the dipole and force when there are more electrons in each molecule?
- the larger the instantaneous and induced dipoles
- the greater the induced dipole-dipole interactions
- the stronger the attractive forces between molecules
What will happen to the boiling point when there are larger induced dipoles? Why?
Increase, because more energy will be needed to overcome the intermolecular forces.
What is a simple molecular lattice?
A three-dimensional structure of molecules, bonded together by weak intermolecular forces.
What bonds/forces hold the molecules in a simple molecular lattice? Are they strong or weak?
Intermolecular forces, which are weak
What bonds/forces hold the atom within each molecule in a simple molecular lattice? Are they strong or weak?
Covalent bonds, which are strong
How are simple molecular substances’ melting point and boiling point? Why?
Simple molecular substances have low melting point and boiling point, because they have weak intermolecular forces between molecules, which are easy to overcome.
What bonds/forces will break when a simple molecular lattice is broken apart?
Weak intermolecular forces
Are non-polar simple molecular substances soluble in non-polar solvents? Why?
They are soluble, because intermolecular forces form between the molecule and the solvent. The interactions weaken the intermolecular forces in the simple molecular lattice, causing them to break and the compound dissolves.
Are non-polar simple molecular substances soluble in polar solvents? Why?
They are insoluble, because there is only little interaction between the molecule and the solvent. The intermolecular bonding within the polar solvent is too strong to be broken.
Are polar simple molecular substances soluble in polar solvents? Why?
They are soluble, because the molecule and the solvent can attract each other. However, the solubility depends on the strength of the dipole and can be hard to predict.
Are polar simple molecular substances soluble in non-polar solvents? Why?
They are insoluble, because there is only little interaction between the molecule and the solvent. The intermolecular bonding within the polar molecule is too strong to be broken.
How are simple molecular structures’ electrical conductivity? Why?
Simple molecular structures can’t conduct electricity, because they don’t have mobile charged particles.
What bond is the strongest type of intermolecular attractions?
Hydrogen bonds
What is hydrogen bond?
A strong dipole-dipole attraction between a hydrogen atom and a lone pair of electrons on a highly electronegative atom containing N, O or F on a different molecule.
What are the conditions needed to form hydrogen bonds?
- an electronegative atom containing N, O or F with a lone pair
- a hydrogen atom from a different molecule attached to an electronegative atom
- the ability for these to be aligned (對齊) at 180°
What are the unusual properties of H2O?
- the solid state (ice) is less dense than liquid state (water)
- high melting and boiling point
- high surface tension
- high heat capacity
- high viscosity
Why is the solid state of water (ice) is less dense than the liquid state (water) ?
- hydrogen bonds hold water molecules apart in an open tetrahedral lattice structure
- water molecules in ice are further apart than in water
- therefore solid ice floats instead of sinking
Why don’t water under ice freeze?
Because ice has air/gap, so it acts as insulator to prevent water below it to freeze.
Why does water have a high melting and boiling point?
- water has London forces between molecules
- it requires a lot energy to break hydrogen bonds
