molecules and ions structureeeeee Flashcards
What is the shape of the compound/ion dictated by?
- the number of electron pairs around the central atom
- the nature of these pairs
- the bonding pair
- the lone pair
What is bonding pairs?
- pair(s) of electrons that are involved in bonding
- these pair(s) repel each other equally
What are lone pairs?
- pair(s) of electrons that are not involved in bonding,
- lone pairs repel other pairs more than bonding pairs
- more electron dense
Each lone pair reduces the bond angle by about 2.5^degrees
What two types of pairs are involved in electron pair repulsion?
Bonding, lone
What is the key for 3D molecule’s shapes?
The dotted wedge is going into the paper, away from you
The normally drawn line is in the plane of the paper.
The bold wedge is coming out of the paper, towards you.
What is the octet rule?
Through chemical bonding, elements usually achieve a noble gas configuration ( full outer shell)
Octet rule: 8 electrons in pairs, 4 electrons from covalent bonds
This is not always possible as there may be:
- too few electrons to form an octet
- more than enough electrons to form an octet
How do you shrink an octet using the example of beryllium and boron?
- in the outer shells neither elements have enough e- to pair and form an octet. The unpaired electrons will pair up and the element will not achieve an octet
Expanding the octet
Some elements can expand their octet as a result the bonding atom will have >8 e- in the outer shell. This can occur in group 15-17 from period 3 downwards :
- group 15 non metals-> 3/5 covalent bonds
- group 16 non metals-> 2/4/6 covalent bonds
- group 17 non metals -> 1/3/5/7 covalent bonds
E.g. sulfur hexafluoride
What is a linear pair?
X-X
One bonding pair an example of this is hydrogen
What is special about a 2 pair: linear?
Bond angle is 180 and there are 2 bonding pairs, an example of this is carbon dioxide
What is a trigonal planar?
- 3 bonding pairs
- bond angle 120
- example BF3
What is so special tetrahedral
4 bonding pairs, bond angle of 109.5 , example CH4
What is special about pyramidal
3 bonding pairs
One lone pair
Example ammonia
Bond angle 107
What is non linear ( 4 pairs)
2 bonding pairs
2 lone pairs
Bond angle 104.5
Example, water
Trigonal bipyramidal
5 bonding pairs
Bond angle 90 and 120
Example, phosphorus pentachloride
Octahedral
6 bonding pairs
Bond angle 90
Example, sulfur hexafluoride
What is the bond angle in H2O?
104.5
Hydrazine, N2H4 is a component of rocket fuel, using the electron pair repulsion theory predict the H-N-H bond angle.
107
Explain how the electron pair repulsion theory can help us to predict the size of the bond angle in PF3
P had 5 electrons in the outside shell. With the 3 electrons from the bonded chlorine atoms the outside shell now has 8 electrons, there are 3 bonding pairs and 1 non bonding pair. According to the electron pair repulsion theory, electr9on pairs repel each other as far as possible. The lone pair repel each other more than bonding pairs. As a result, PF3 has a tetrahedral shape and a bond angle of 107
Boron and nitrogen can form fluorides, BF3 and NF3.
Name the shape of BF3 and give the FBF bond angle.
Shape: trigonal planar
Angle:120
How do you work out the number of electron pairs?
Find the central atom, work out the number of e on the outer shell, add one e for every atom that the c atom is bonded to, if for an ion (+ you minus 1, - you add one) . Add up all e and divide 2 to find e pairs. Compare e pairs to number of bonds to find LPE
By how much do LPE affect the angle?
Roughly 2.5 degrees
How would one go about explaining the shape of molecules?
- state number of BPE and LPE
- state that e pairs repel and try to get as far apart as possible
- if no LPE , state that e pairs repel equally
- if LPE then state that LPE repel more than BPE
- state actual shape and bond angle
What is valence shell electron repulsion theory?
The shape of a molecule depends on the number of pairs of electrons in the outer shell of a central atom. E pairs will repel each other as far as possible, the type of electron pair will affect how much it repels the other electron pairs LPELPE>LPEBPE>BPEBPE.
Greatest angle is btwn LPE.
BPE is reduced by 2.5 degrees per LPE
What are the possible shapes for molecules?
Linear, non linear, trigonal planar, pyramidal, tetrahedral, trigonal bipyramidal, octahedral
What are some examples of linear ? And bond angle
BeCl2, CO2
Bond angle 180
Trigonal planar examples and bond angle
AX3
BF3
Bond angle 120
What are some examples of tetrahedral shapes
AX4
CH4
Bond angle 109.5
What are some examples of trigonal bipyramidal
AX5
PCl5
Bond angles, 120 and 180
What are some examples of octahedral and bond angle
AX6
SF6
Bond angle =90
What are some examples about non linear and. Bond angle
AX2
H2O
Bond angle = 104.5
What are some examples of pyramidal and bond angle?
AX3
NH3
Bond angle = 107
From a total number of 2 electron pairs, what shapes can be formed?
BPE,2, linear
From a total number of electron pairs being 3, what shapes can be formed?
BPE 3, LPE 0, trigonal planar (BA 120)
BPE 2, LPE 1, bent (linear) (BAA 118)
FROM A TOTAL NUMBER OF E pairs being 4, what are the possible shapes that can be formed?
BPE 4, LPE 0, tetrahedral (BA 109.5)
BPE 3, LPE 1, trigonal pyramid (BA 107)
BPE 2, LPE 2, linear bent (BA 104.5)
From a total electron pair number of 5, what are the possible shapes that can be formed?
BPE 5, LPE 0, trigonal bipyramid (BA 120)
BPE 4, LPE 1, a shape that resembles trigonal bipyramidal (BA89 and 119)
BPE3, LPE 2, trigonal planar/ T shape, (BA 120 or 89)
From a total electron pair number of 6, what are the possible shapes that can be formed?
BPE 6, LPE 0, octahedral (BA 90)
BPE 5, LPE 1, square pyramid (BA 89)
BPE 4, LPE 2, square planar(BA 90)
