3.6- THE SHAPE OF MOLECULES AND IONS Flashcards
What kind of dimension do molecules have?
three-dimensional
What sorts of shapes do molecules come in?
may different shapes
How do electrons in molecules exist?
in pairs in volumes of space called orbitals
What ideas can you use to predict the shape of a simple covalent molecule e.g. one consisting of a central atom surrounded by a number of other atoms? (2)
each pair of electrons around an atom will repel all other electron pairs
pairs of electrons will take up positions as far apart as possible to minimise repulsion
What is being able to predict the shape of a simple covalent molecule using ideas: each pair of electrons around an atom will repel all other electron pairs + pairs of electrons will take up positions as far apart as possible to minimise repulsion called?
electron pair repulsion theory
How can electron pairs be?
be a shared pair or a lone pair
What does the shape of a simple molecule depend on?
number of pairs of electrons that surround central atom
What do you need to do first when trying to work out the shape of any molecule?
draw a dot-and-cross diagram to find the number of pairs of electrons
What will the shape of the molecule be if there are two pairs of electrons around the central atom?
molecule will be linear
What is the furthest away from each other that the two pairs can get? (two pairs of electrons)
180°
What is an example of molecule that has two pairs of electrons that surround the atom? (two pairs of electrons)
beryllium chloride
When is beryllium chloride covalently bonded molecule? (two pairs of electrons)
in the gas phase despite being a metal- non-metal compound
How far apart would they be if there are three pairs of electrons around the central atom?
120° apart
What would the shape of the molecule be like if there are three pairs of electrons around the central atom?
molecule is planar and is called trigonal planar
What is an example of a molecule that has three pairs of electrons around the central atom?
boron trifluoride
If there are four pairs of electrons, when are they the furthest apart?
when they are arranged so that they point to the four corners of a tetrahedron
Where is one of the atoms positioned? (four pairs of electrons)
at the centre
what is the shape of the molecule with four pairs of electrons?
tetrahedral
What is an example of a molecule with four pairs of electrons?
methane
Where is the carbon atom of methane situated? (four pairs of electrons)
at the centre of the tetrahedron
Where are the hydrogen atoms of methane situated? (four pairs of electrons)
at the vertices
What are the angles like? (four pairs of electrons)
109.5°
What is the shape of the molecule like with four pairs of electrons?
three-dimensional, not planar
As the molecule is not three-dimensional, how does this affect the sum of the angles? (four pairs of electrons)
sum of angles can be more than 360°
What ion is tetrahedral?
ammonium ion
Why is the ammonium ion tetrahedral?
four groups of electrons surrounding the nitrogen atom
Does the ammonium ion having an overall charge affect the shape?
no
What is the shape usually when there are five pairs of electrons?
trigonal bipyramid
What is an example of a molecule with five pairs of electrons?
phosphorus pentachloride, PCl5
What shape is adopted if there are six pairs of electrons?
octahedral
What are the bond angles when there are six pairs of electrons?
90°
What is an example of a molecule with six pairs of electrons?
sulphur hexafluoride, SF6
what do some molecules have? (hint- electrons)
unshared (lone) pairs of electrons
What are unshared (lone) pairs of electrons?
electrons that are not part of a covalent bond
Examples of molecules where lone pairs affect shape? (2)
ammonia
water
How many pairs of shared and unshared electrons does ammonia have?
four pairs of electrons and one of the groups is a lone pair
With its four pairs of electrons around the nitrogen atom, what is the shape of the ammonia molecule based on?
a tetrahedron
As there are only three ‘arms’ in ammonia what is the actual shape of ammonia?
triangular pyramid
Where are the bonding pairs of electrons attracted towards in ammonia?
towards nitrogen nucleus + hydrogen nucleus
What is the lone pair of electrons in ammonia attracted by and what happens?
attracted only by nitrogen nucleus and so is pulled closer to it than shared pairs
As the lone pair on ammonia is pulled closer to the nitrogen than the shared pairs, what is the repulsion like?
repulsion between lone pair of electrons + bonding pair of electrons greater than that between two bonding pairs
What does the effect of increased repulsion between the lone pair and bonding pairs of electrons in ammonia have?
squeezes hydrogen atoms together, reducing all H-N-H angles
What is the approximate rule of thumb per lone pair? (hint-an angle)
2°
What is the bond angles in ammonia approximately?
107°
How many pairs of electrons are there around the oxygen atom in water?
four pairs of electrons
What is the shape of water based on?
tetrahedron
How many ‘arms’ of the tetrahedron are lone pairs that are not part of a bond? (water)
two
As two of the ‘arms’ of the tetrahedron are lone pairs that are not part of a bond what shape does this result in? (water)
V-shaped or angular molecule
What shape do the electrons form in water? (same as ammonia)
tetrahedron
What shape do the bonds form in water?
V-shaped
What is the H-O-H angle? (water)
104.5°
What shape does the electrons form in ammonia?
tetrahedron
What shape do the bonds form in ammonia?
triangular pyramid
How many bonding pairs and lone pairs are there in chlorine tetrafluoride ion, CLF4-?
four bonding pairs
two lone pairs
What do one of the lone pairs in chlorine tetrafluoride ion, CLF4- contain?
electron that has been donated to it, so charge on ion is negative (-1)
How many pairs of electrons are there around the chlorine atom in chlorine tetrafluoride ion, CLF4-?
six pairs of electrons
What is the shape of chlorine tetrafluoride ion, CLF4- based on?
octahedron in which two arms are not part of a bond
As lone pairs repel what position do they adopt? (chlorine tetrafluoride ion, CLF4-)
adopt a position furthest apart
What shape does it leave as the lone pairs adopt a position furthest apart? (chlorine tetrafluoride ion, CLF4-)
flat square-shaped ion described as square planar
Bonding repulsion from lowest to highest:
bonding pair- bonding pair
lone pair- bonding pair
lone pair- lone pair
