#6 Shapes Of Molecules And Intermolecular Forces Flashcards
Describe a linear shape?
2 bonded pairs
0 lone pairs
180• bond angel
Describe a trigonal planar shape?
3 bonded pairs
0 lone pairs
120• bond angle
Describe a tetrahedral shape?
4 bonded pairs
0 lone pairs
109.5• bond angle
Describe an octahedral shape?
You’d think 8 but no no no
6 bonded pairs
0 lone pairs
90• bond angle
Describe a non-linear shape?
2 bonded pairs
2 lone pairs
104.5•
Describe a pyramidal shape?
3 bonded pairs
1 lone pair
107• bond angle
Which repel more bones pairs or lone pairs?
Lone pairs of electrons repel more than bonded pairs
State and explain the bond angle in an ammonia molecule?
-Electron pairs repel, so get as far away as possible
-NH3 has 1 lone pair and 3 bonded pairs of electrons
-Lone pair repel more that bonded pairs so the bond angle is 107•, with a pyramidal shape
What to always include in an answer about shape and bond angle?
-shape
-bond angle
-electron pairs repel
-lone pairs repel more than bonded
-how many lone and bonded pairs
What bonds form due to varying electronegativities?
-Large diff in electronegativity between atoms= ionic bonds form
-no difference= covalent bond
-in between= polar covalent (result in dipole) (ie delta charges)
H and C have similar elcetronegativities
Most electronegative elements?
F
O
Cl
N
What do polar bonds have?
A permanent dipole
Fill in the blank: dipoles must not_____.
Oppose and cancel one another
What is a net dipole?
When one end of the molecule is delta - and the other delta + therefore there is a net dipole= polar molecule
What are the 3 intermolecular forces?
-Induced dipole-dipole interactions (London forces)
-permanent dipole-dipole interactions (polar molecules)
-hydrogen bonding (H-F, H-O, H-N bonds only)
Out of the 3 intermolecular forces which one is the strongest and weakest?
Strongest= Hydrogen bonding
Weakest= Induced dipole-dipole interactions (London forces)
Out of the 3 intermolecular forces which are van der waals forces?
-permanent dipole-dipole (polar molecules)
-Induced dipole-dipole (London forces)
How do London forces form?
Uneven distribution of electrons creates a temporary instantaneous dipole which causes induced dipoles on neighbouring molecules
What makes a London forces (induced dipole-dipole)
-Bigger molecule (higher Mr) as more electrons result in larger induced dipole-dipole interactions
-greater surface area between molecules ie:
What interactions does a polar molecule have?
-permanent dipole
-induced dipoles/ London forces
True or false: everything has london forces
True: everything has London forces some have additional forces
Why is F-F non polar?
Same charge
Symmetrical
Electrons attracted equally
What are the only elements that can form hydrogen bonds? Why?
Fluorine
Nitrogen
Oxygen
Because they are the most electronegative
What is hydrogen bonding?
Intermolecular bonding between H and N or O or F
Draw hydrogen bonding in water?
Draw hydrogen bonding in ammonia?
Why is ice less dense than water?
-Hydrogen bonds holding the water molecules apart in an open lattice structure
-water molecules in ice are further apart than in liquid water
Why does water have a relatively high melting/boiling 0
Point?
-Hydrogen bonds contribute extra forces that need to be overcome to melt of boil water
-More energy required to overcome these forces
Ice forms an ____ lattice.
Open
What is a simple molecular lattice?
Covalent bonded molecules attracted by intermolecular forces
Ionic bonds are very strong and form______.
A giant lattice
Covalent bond are very strong and form _____.
Small molecules/ giant lattice
Are the melting/boiling points of simple molecular lattices high or low? Why?
Low: weak intermolecular forces break, not much energy required to melt/boil
Do simple molecular lattices conduct electricity?
No: no mobile charge carriers (ions)
How to use electron pairs to predict the shapes of molecules?
1.work out the no. Electrons in outer shell of central atom (use periodic table)
2.use molecular formula to find how many atoms the central molecule is bonded to
3. Add up the electrons and divide by 2 to find no. Electron pairs on central atom. If it an ion account for its charge
4.compare no. Electron pairs with no. Bonds to find no. Lone pairs (if there a double bond count it as 2 bonds)
5.use the no. Electrons pairs and lone pairs of central atom to work out shape and angle
What is a dipole?
A dipole is a difference in charge between the two atoms caused by a shift in electron density in the bond
