Chapter 6 - Shapes Of Molecules & Intermolcular Forces Flashcards
What is the VSEPR theory?
Valence Shell Electron Pair Repulsion Theory
You can predict the shape of a molecular based on electrons
Electron pairs repel each other so they are as far apart as possible
It’s based on the central atom
The arrangement of electron pairs minimise the repulsion and holds the bonded atoms in a definite shape
How do you draw a bond going towards and away from you?
Wedge/ triangle shape = toward
Dashed line = away
Which pair of electrons repel more and why?
Lone pair electrons
This is because they are closer to the central atom and occupy more space than a bonded pair so they repel more strongly than bonded pairs
How do you find the number for bonded pair of electrons and lone pairs in a compound?
Draw the compound covalently bonded and find the number of bonded and lone pairs of electrons
What is the name of the shape and bond angle for a compound with 4 bonds and 0 lone pairs of electrons? Draw it:
Tetrahedral shape
109.5 degrees
E.g. methane
Check drawing
What is the name of the shape and bond angle for a compound with 3 bonds and 1 lone pairs of electrons? Draw it:
Pyramidal shape
107 degrees
E.g. ammonia
Check drawing
What is the name of the shape and bond angle for a compound with 2 bonds and 2 lone pairs of electrons?
Non - linear shape
104.5 degrees
E.g. water
Check drawing
Why do compounds with more lone pairs have smaller bond angles?
As lone pairs repel more strongly than bonded pairs. Therefore lone pairs repel the bonded pairs slightly closer together, decreasing the bond angle
What is the name of the shape and bond angle for a compound with 2 bonds and 0 lone pairs of electrons? Draw it:
Linear shape
180 degrees
E.g. carbon dioxide
Check drawing
What is the name of the shape and bond angle for a compound with 3 bonds and 0 lone pairs of electrons? Draw it:
Trigonal planar shape
120 degrees
E.g. boron trifluoride
Check drawing
What is the name of the shape and bond angle for a compound with 6 bonds and 0 lone pairs of electrons? Draw it:
Octahedral shape
90 degrees
E.g. sulfur hexafluoride
Check drawing
How are multiple bonds regarded?
Each multiple bond is treated as a bonding region
Double bond = 1 “bonded pair”
What is electronegativity?
Power of an atom to attract the electron density in a covalent bond towards itself
How is electronegativity measured?
Pauling system
0-4, 4 being the highest
What factors affect electronegativity?
Atomic radius (larger radius = lower)
Nuclear charge (higher nuclear charge = greater)
Electron shielding (more shielding = lower)
Same reasons as affecting ionisation energy
What is the trend of electronegativity down the group?
Decrease of electronegativity:
- greater electron shielding
- atomic radius increases
What is the trend of electronegativity across a period?
Increase of electronegativity:
- nuclear charge increases
- smaller atomic radius
So nuclear attraction to shared pair of electrons increases
How can electronegativity be used to estimate type of bonding?
No difference in electronegativity between elements = covalent (Cl2)
0 - 1.8 difference in electronegativity = polar covalent (HCl)
1.8< = ionic (NaCl)
What are pure covalent bonds?
Covalent bonds where the electrons are shared evenly as the bonded atoms are either the same or have the same electronegativity
What are polar covalent bonds?
Shared pair of electrons where the electron pair is not shared equally between the bonded atoms
How are polar covalent bonds shown?
Partial charges (lower case delta)
What is a dipole?
The separation of opposite charges
Which molecules contain intermolecular forces and what properties do they have?
Simple molecules
- LOW MP/BP
- don’t conduct electricity
What are the 3 types of intermolecular forces?
1) induced dipole-dipole (London forces) - occurs in ALL atoms and molecules
2) permanent dipole-dipole forces - only for polar molecules
3) hydrogen bonding - certain molecules
