Shapes of Molecules and Ions Flashcards
according to electron pair repulsion theory, what is the shape of a molecule or ion caused by
- the repulsion between the pairs of electrons
- (both bond pairs and non-bonded pairs)
- that surround the central atom
how do electrons arrange themselves in a molecule or ion
- they arrange themselves around the central atom
- so that the repulsion between them is a minimum
how does the strength of the repulsion between the electrons differ according to whether it is an interaction between a lone pair or bonded pair
- lone pair to lone pair repulsion is the strongest
- followed by lone pair to bond pair repulsion
- then bond pair to bond pair repulsion is the weakest
what does VSEPR stand for
valence shell electron pair repulsion (theory)
what are the three rules usually used for
- rule 1 and 2 are used to obtain the basic shape of a molecule or ion
- rule 3 is used to estimate the values for the bond angles
what is the first thing you need to work out if you want to determine the shape of a molecule
the number of lone pairs and bonded pairs of electrons around the central atom
how would this be done easily
by drawing a dot and cross diagram
if a molecule had multiple bonds between two atoms, like a double bond, how should you treat that multiple bond
like it is only one pair of electrons
using that knowledge, how many pairs of electrons are around the C in O=C=O
2
if a double bond is treated like an electron pair, what is the shape of a CO2 molecule and why
- its a linear molecule
- because carbon doesnt have any lone pairs of electrons due to the double bonds
what is the shape, number of bond pairs and lone pairs for BeCl2
- linear
- 2 bond pairs
- 0 lone pairs
what is the shape, number of bond pairs and lone pairs for BCl3
- trigonal planar
- 3 bond pairs
- 0 lone pairs
what is the shape, number of bond pairs and lone pairs for CH4
- tetrahedral
- 4 bond pairs
- 0 lone pairs
what is the shape, number of bond pairs and lone pairs for PCl5
- trigonal bipyramidal
- 5 bond pairs
- 0 lone pairs
what is the shape, number of bond pairs and lone pairs for NH3
- trigonal pyramidal
- 3 bond pairs
- 1 lone pair
what is the shape, number of bond pairs and lone pairs for H2O
- v-shaped
- 2 bond pairs
- 2 lone pairs
what is the shape of sulfur trioxide and why
- sulfur has the oxidation number of +6
- an oxygen atom has the oxidation number -2
- meaning that for a neutral molecule its formula would be SO3
- this also means that there are double bonds between the sulfur and each oxygen
- however, double bond can be treated as one bonded pair of electrons
- and as there are no lone pairs of electrons, its shape would be trigonal planar (like BCl3)
what is the bond angle for a linear molecule
180
what is the bond angle for a trigonal planar molecule
120
what is the bond angle for a tetrahedral molecule
109.5
using the bond angle for tetrahedral molecules as the base angle, what does a lone pair of electrons in a molecule do to the bond angles
it reduces them by 2.5 degrees
what is the bond angle for the trigonal pyramidal molecules like NH3 and why
- 107
- N has 5 electrons in its outer shell whereas H has 1
- this means that N would have 2 electrons left after bonding, aka a lone pair
- they reduce the base angle of 109.5 by 2.5 making it 107
what is the bond angle for v-shaped molecules like H2O and why
- 104.5
- oxygen has 6 electrons in its outer shell and H has 1
- after bonding O would have 4 electrons aka 2 lone pairs
- joint together they reduce the angle by 5 from 109.5
- making it 104.5
what are the bond angles for trigonal bipyramidal molecules and why
- 90 and 120
- they would have 3 atoms bonded to the centre atom in one plane (horizontal)
- meaning they would be dispersed evenly at 360 / 3 = 120 each
- then the last two are bonded at the top and bottom on the same plane (vertical)
- making the angle between one of the ones in the vertical and the horizontal 90
what are the bond angles for hexagonal molecules and why
- 180 and 90
- 4 atoms are bonded to the centre atom in the same horizontal plane
- so the angle between all of them would be 90
- then there are two atoms on the top and bottom of the centre atom in the vertical plane
- the angle between them is 180 (dont ask me why it doesnt apply to the previous one)
