Shapes of molecules Flashcards
How to work out shapes of molecules with single bonds only
- Find central atom and work out group number
- Add no. atoms to group number
- If ion is positively charged subtract that charge, if negatively charged then add the charge
- Add up all electrons and divide by 2 to get number of electron pairs
- Lone pairs = electron pairs - number of bonded groups
What does covalent bonds are directional in space mean?
Molecules have a set shape depending on the number of electron pairs around the central atom. The outer electrons of an atom in a molecule are arranged in pairs.
What can electron pairs be thought as?
Clouds of electron density which repel each other so that they are as far apart as possible. An electron pair may be a shared pair of electrons or a lone pair of electrons
Electron pair repulsion theory
1) Electron pairs repel as far away as possible
2) Lone pair / lone pair repulsion is greater than any lone pair / bonding pair repulsion which is greater than bonding pair / bonding pair repulsion
Where a molecule has no lone pair of electrons, number of bonding pairs repel equally
Linear e.g. BeCl2
Bond angle = 180 degrees Electron pairs = 2 Bonding pairs = 2 Lone pairs = 0 Symmetric
Trigonal planar e.g. BeCl3
Bond angle = 120 degrees Ep = 3 Bp = 3 Lp = 0 Symmetric
Tetrahedral e.g. NH4+ or CH4
Bond angle = 109.5 degrees Ep = 4 Bp = 4 Lp = 0 Symmetric
Pyramidal e.g. NH3 or PH3
Bond angle = 107 degrees Ep = 4 Bp = 3 Lp = 1 Asymmetric
V or bend shape e.g. H2O
Bond angle = 104.5 degrees Ep = 4 Bp = 2 Lp = 2 Asymmetric
Trigonal bypyramidal e.g. PF5
Bond angle = 120 and 90 degrees
Ep = 5
Bp = 5
Lp = 0
Modified trigonal bypyramidal e.g. SF4
Bond angle = 188 and 89 degrees
Ep = 5
Bp = 4
Lp = 1
T shaped e.g. ClF3
Bond angle = 86 degrees
Ep = 5
Bp = 3
Lp = 2
Linear e.g. I3-
Bond angle = 180 degrees
Ep = 5
Bp = 2
Lp = 3
Octahedral e.g. SF6
Bond angle = 90 degrees Ep = 6 Bp = 6 Lp = 0 Symmetric
Square planar e.g. ICl4
Bond angle = 90 degrees
Ep = 6
Bp = 4
Lp = 2
How would you know that a molecule has a linear shape? Give the bond angle.
Molecules with TWO ELECTRON PAIRS
Bond angle: 180 degrees
The pairs of bonding electrons want to be far away from each other as possible.
e.g. BeCl2
How would you know that a molecule has a trigonal planar shape? Give the bond angle.
Molecules with THREE ELECTRON PAIRS - and there are THREE BONDING PAIRS - repulsion of charge clouds is the same between each pair.
Bond angle: 120 degrees
e.g. BF3
How would know that a molecule has a tetrahedral shape? Give the bond angle.
FOUR ELECTRON PAIRS and FOUR BONDING PAIRS. no lone pairs, so that the charge clouds all repel each other equally.
Bond angle: 109.5 degrees
e.g. NH4+
How would you know that a molecule has a trigonal pyramidal shape? Give the bond angle.
FOUR electron pairs and 3 BONDING PAIRS and 1 LONE PAIRS
the lone pair/bonding-pair repulsion will be greater than the bonding-pair/bonding-pair repulsion so angles of the atoms changes.
Bond angle is 107 degrees.
larger bond angle between lone and bonding pairs and smaller bond angle between bonding pairs.
How would you know that a molecule has a bent / v-shaped / non-linear shape? Give the bond angle.
FOUR electron pairs and 2 BONDING PAIRS and 2 LONE PAIRS.
The lone-pair/lone-pair repulsion will squish the bond angle even further.
Bond angle: 104.5 degrees
e.g. H2O
How would you know that a molecule has a trigonal bipyramidal shape? Give the bond angle.
FIVE electron pairs and 5 BONDING PAIRS, no lone pairs.
Repulsion between bonding pairs means that three of the atoms will form a trigonal planar shape with bond angles of 120 degrees and the other two will be at 90 degrees.
Bond angles: 120 and 90 degrees
e.g. PCl5
How would you know that a molecule has a seesaw shape? Give the bond angle.
FIVE electron pairs: FOUR BONDING PAIRS and 1 LONE PAIR.
The lone pair is always positioned where one of the trigonal planar atoms would be in a trigonal bipyramidal molecule.
e.g. SF4 - bond angle: 102 and 86.5 degrees
How would you know that a molecule has a T-shape? Give the bond angle.
FIVE electron pairs: 3 BONDING PAIRS and 2 LONE PAIRS.
e.g. ClF3 - central Cl has 3 bonding and 2 lone pairs.
Bond angle: 87.5 degrees
How would you know that a molecule has an Octahedral shape? Give the bond angle.
SIX electron pairs: 6 BONDING PAIRS and no lone pairs.
e.g. SF6 - central atom has six bonding pairs and no lone pairs.
Bond angle: All bonds 90 degrees
How would you know that a molecule has a square pyramidal shape? Give the bond angle.
SIX electron pairs: 5 BONDING PAIRS and 1 LONE PAIR
e.g. ClF5
Bond angle: 90 degrees
How would you know that a molecule has a square planar shape? Give the bond angle.
SIX electron pairs: 4 BONDING PAIRS and 2 LONE PAIRS
e.g. XeF4
Bond angle: 90 degrees
What shape is carbon dioxide? (CO2)
Linear
It has four bonding pairs of electrons (found in two sulfur-oxygen double bonds) and no lone pairs.
Double bonds can be treated as one bond - two bonds no lone pairs.
Bond angle: 180 degrees
What shape is sulfur dioxide? (SO2)
Bent / v-shaped / non-linear
It has four bonding pairs of electrons and one lone pair It has two double bonds that include the four bonding pairs.
Bond angle: 120 degrees
