Shapes Of Molecules And IMF Flashcards
What is a valence shell?
The outer shell
What are lone electron pairs?
Two electrons that are not involved in a Bond
What is the valence shell electron pair repulsion theory?
Pairs of electrons in valence shell repel each other as far as possible
Multiple bonds have same effect as single bonds
Lone pair of electrons have larger repelling effect than bonded electrons (more lone electrons= reduces bond angle)
How do you determine the shape of a molecule and bond angles?
Draw dot and cross diagram
Take into account repelling effects of bonds
What is the shape and bond angle of a molecule with 2 bonded pair of electrons/ regions?
Linear
180
What is the shape and bond angle of a molecule with 3 bonded pair of electrons/ regions?
Trigonal planar
120
What is the shape and bond angle of a molecule with 4 bonded pair of electrons/ regions?
Tetrahedral
109.5
What is the shape and bond angle of a molecule with 6 bonded pair of electrons/ regions?
Octahedral
90
What is a dative bond?
A covalent bond where nth electrons come from the same atom
What does the triangle in a dot and cross diagram signify?
+ when is it used?
Used in poly atomic ions
To signify electrons gained
What is a covalent bond?
The attraction between the positive nucleus of two atoms and the negative electrons they share .
What is electronegetivity?
The ability of an atom to attract the bonded pair of electrons in a covalent
What are the electronegativity values called?
Pauling electronegativity values
What is the most electronegative atom?
Fluorine
What is a polar bond ?
A covalent bond where each atom has different electronegativities with + and - partially charges on the atoms
Will the more electronegative atom has a + or - partial charge?
Partial negative charge
What is a dipole?
The separation (short distance) of partial charges in a molecule
What is a permanent dipole?
The separation of partial charges across a polar bond due to different electronegativities
When is molecule polar?
When a molecule has more than one polar bond and is not symmetrical
Why are some molecules non-polar even though they have 2+ polar bonds?
The molecule is symmetrical so the dipole acts in opposite directions and cancels out
What are intermolecular forces?
Weak attraction between molecules
What are the types of intermolecular forces?
Induced dipole-dipole attraction (London forces)
Permanent dipole-dipole interactions
Hydrogen bonding
What are temporary dipoles and why do they form?
Dipoles caused by uneven distribution of electrons
Form due to random nature of electron movement
What is an induced dipole and how is it created?
A dipole caused by the presence of a temporary dipole in a molecule
Chain reaction
What is a London force?
The force of attraction between induced dipoles
Where do London forces occur?
In all molecules
What does the head of an arrow indicate in terms of dipoles?
Partial negative charge
What causes a stronger London force ?
More electrons
Why do elements with more electrons have higher MP and BP?
More electrons= stronger London forces (occurs in all molecules and is a type of intermolecular force)
More energy needed to break the bonds
What is a permanent dipole-dipole interaction?
Type of intermolecular force.
Between two polar molecules with permanent dipoles
Attraction is between + and - partial charges
What occurs to permanent dipole-dipole interactions in a liquid?
They are constantly breaking and reforming (molecules move)
What occurs to permanent dipole-dipole interactions in a solid?
The interactions are constant so hold molecules in a fixed position
What is a hydrogen bond also known as?
Very strong permanent dipole-dipole interaction
What is a hydrogen bond?
Attraction between
hydrogen atom bonded to electronegative atom in one molecule
+
Lone pair of electrons of an electronegative element in a different molecule
Give examples of molecules with hydrogen bonding
H2O
NH3
HF
Why is ice less dense than water?
Water molecules are held in open lattice structure in ice (very regular structure) making it less dense.
When it melts the lattice collapses
How are molecules in a simple molecular lattice held in place?
By weak intermolecular forces
Are the MP and BP of simple molecular lattices high or low ?
Why?
Low
Weak intermolecular forces are easy to overcome
Why are simple molecular lattices poor conductors of electricity?
They aren’t charged
Are highly polar molecules soluble or insoluble in water?
Why?
Soluble
They can interact with the dipoles in water
What are simple molecules soluble and insoluble in?
Why?
Soluble = non-polar substances
Insoluble= water
Due to weak dipoles
What is an instantaneous dipole?
A dipole only present for an instant (caused by the accumulation of electrons)
What is the name for the electrons involved in a covalent bond?
Bonding pair
What is the rule for drawing ions?
Same as uncharged molecules (based on types and number of bonding regions)
What effects electronegativity?
Electron shielding
Nuclear charge
Atomic radius
How does electronic shielding affect electronegativity?
More shells= less electronegative
Inner shell electrons repel outer shell electrons making attraction to nucleus weaker
What letter is used to depict a partial charge?
Lower case delta
What value determines a covalent, polar covalent and ionic bond?
Covalent = 0
Polar covalent = 0 >_ 0.18
Ionic = > 1.8
In a simple molecular lattice what forces hold the molecules in place?
London forces
What may occur due to permanent dipole-dipole interaction?
The partial charges may increase
Electrons are reopened by partial charge on neighbouring molecule making them move further away from partial positive charge
How many hydrogen bonds are there per water molecule in ice?
4
What are van der Waals forces?
Induced dipole-dipole interactions (London forces)
And
Permanent dipole-dipole interactions
What does a lone pair reduce the bond angle by?
2.5 per lone pair
What increases the strength of London forces ?
More electrons
Larger SA = bigger exposed electron cloud = greater attraction
What are the anomalous properties of water?
Higher M+Bp than expected
Ice is less dense than water
Explain why ice is less dense than water
There are more hydrogen bonds in ice than water (4)
When ice melts some hydrogen bonds break and molecules are closer so it’s more dense
Hydrogen bonds are relatively long and so hold water molecules in an open lattice structure
Place the IMF in order of increasing strength
Induced dipole-dipole
Permanent dipole-dipole
Hydrogen
What is the structure of simple molecules held by London forces
Molecular lattice
How many bonded and lone pairs does trigonal pyramidal have?
What is the bond angle?
Bonded= 3
Lone= 1
Angle =107
How many bonded and lone pairs does a molecule with bond angle 104.5 have?
What is it’s name?
2 bonding regions (double bonds)
Two lone.
Non-linear
What shape of molecule has two bond angles and what are they?
Trigonal bipyramidal
90+ 120
Are molecules with hydrogen bonding soluble in water?
Why?
Yes
Can form hydrogen bonds with water
Describe the solubility of polar and non-polar solutes
Like dissolves like
Will a polar solvent dissolve and non-polar solute?
Why?
No
Partial charges aren’t strong enough the separate atoms (break covalent bond)
Give pauling En values for pure covalent, polar covalent and ionic bonds
Pure = 0 Polar= up to 1.8 Ionic= 1.8+
Define bond in molecules with 0, 0.6 and 2.5 difference in electronegetivity values of atoms
0= pure covalent
- 6 =polar covalent
- 5 =ionic
What IMF does water have?
London forces
Permanent dipole-dipole
Hydrogen
Why does Br2 have a higher boiling point than f2?
Br has more electrons so stronger London forces