SSI - shapes Flashcards
(25 cards)
lewis structure definition
simplified molecular structures that show the bonds and lone pairs in a molecule (electrons represented as dots
when is resonance relevant?
when there is more than 1 valid choice of where to place the double or triple bond in the Lewis structure
what is VSEPR?
valence shell electron pair repulsion theory
- the idea that shapes of molecules are determined by bonds and lone pairs as atoms within molecules arrange themselves to maximise the angle between bonds and minimise electron repulsion
what shape does an AX2 molecules with no lone pairs adopt?
linear shape
bond angle - 180
what shape does an AX3 molecule with no lone pairs adopt?
trigonal planar
bond angle - 120
what shape does an AX4 molecule with no lone pairs adopt?
tetrahedral
bond angle - 109.5
what shape does an AX5 molecule with no lone pairs adopt?
trigonal bipyramidal
bond angle - 120 between equatorial atoms, 90 between axial and equatorial atoms
what shape does an AX6 molecule with no lone pairs adopt?
octahedral
bond angle - 90
what shape does an AX7 molecule with no lone pairs adopt?
pentagonal bipyramidal
bond angle - 72 between equatorial atoms, 90 between axial and equatorial atoms
pseudostructure definition
refers to structures of molecules with lone pairs that closely resemble/are derived from structures of molecules without lone pairs
how does the presence of lone pairs change the shape of the molecule?
lone pairs are more repulsive than electrons in bonds so bonds are pushed together away from the lone pairs, changing bond angles
what shape does an AX3 molecule with 1lone pair adopt?
trigonal pyramidal - a tetrahedral pseudostructure
bond angle ~ 107
what shape does an AX3 molecule with 2 lone pairs adopt?
angled - a tetrahedral pseudostructure
bond angle ~ 104.5
how does size of central atom affect the shape of the molecule?
the size of the central atom can affect bond angles - the smaller the atom, the smaller the distance between bonding electron pairs, so repulsion is stronger leading to wider bond angles
how does the size of the outer atoms affect the shape of the molecule?
the size of outer atoms can affect bond angles - larger atoms take up more space so bond angle widens to accommodate this
do lone pairs take up axial or equatorial positions?
as lone pairs are more repulsive they occupy the more spacious site, usually the equatorial site (if all sites aren’t equivalent)
what shape does an AX4 molecule with 1 lone pair adopt?
seesaw - a trigonal bipyramidal pseudostructure
bond angle < 90, < 120
what shape does an AX3 molecule with 2 lone pairs adopt?
T-shaped - a trigonal bipyramidal pseudostructure
bond angle < 90, < 120
what shape does an AX2 molecule with 3 lone pairs adopt?
linear
bond angle = 180 (between actual bonds)
what shape does an AX5 molecule with 1 lone pair adopt?
square pyramidal - an octahedral pseudostructure
bond angle < 90
what shape does an AX4 molecule with 2 lone pairs adopt?
square planar - an octahedral pseudostructure
bond angle = 90 - lone pairs occupy axial positions
how does the presence of double bonds affect the shape of a molecule?
double bonds contain 4 electrons, not just 2, so they are more repulsive than single bonds, which can affect bond angles
do double bonds take up axial or equatorial positions?
as double bonds are more repulsive, they prefer to occupy the more spacious site, usually equatorial (depends on shape) - similar to lone pairs
how can you determine which molecules will occupy which sites if 2 different elements are present in the molecules?
if not all sites are equivalent, the larger atom will usually occupy the more spacious site
