8/9/10 Flashcards
why do atoms share e- in covalent bonds
to form more energetically favourable structures
filling of valence shells allows atoms to obey the
octet rule
ns2 np6 obeys the octet rule
non bonding e- are
lone pairs
this is when s valence e- dont have the correct symmetry to react with px py valence electrons
( s and pz can) so px and py are non bonding
2nd period Li to Ne never have what
never have more than 8 e- in valence shell
they can be e- poor tho
elements that break the octet rule and have more than 8e- in their outer shell are called
(expanded octets)
hypervalent!!!!
period 3 and onwards <3
the molecule it forms and the element itself is called hypervalent.
formal charge equation
diff between valence e- in a free atom and e- it has in a lewis structure
valence - number of bonds - number of unshared electrons
most likely structure arrangement of atoms in a covalent bond is the one withhh
the lowest formal charge
valence shell electron repulsion theory isss
how valence electrons help us predict the shapes of a molecule
what are assumptions of the valence shell electron pair repulsion theory
- bonding pairs (e- pairs that hold atoms together)
- lone pairs ( e- pairs that arent involved in a bond)
- e- pairs are - and so repel eachother. e- pairs are as spaced out as possible.
finding the shape of a structure
identify central atom (use formal charge if smt)
calculate the number of e- pairs around it (draw the lewis structure)
see if these are all bonding e- pairs.
why is the repulsion from a lone pair greater than a bonding pair
its closer to the central atom which allows it to take up more space
if u get a shape with a charge outside a [box] what does this mean
- sign: add that number of e- to the central atom
+ sign: take away that number from the central atom
what is special about the trigonal bipyramidal structure
the central atom has 2 axial atoms and 2 equitorial atoms
these are in 2 different environments
5 e- pairs gives what shape
trigonal bipyramidal
even if one of them is a lone pair
where does the lone pair go on a trigonal bipyramidal
It goes on the equitoral position as there are less groups 90* to it.
lone pairs repel more
disphenoidal geometry // sawhorse geometry
it will repel/bend the axial and equitorial slightly from their ‘ideal’ positions.
180* and 120* –> 173. 102
what repulsion do we want to stay away from most in trigonal bipyramidal geometry
the 90*
we try put the lone pairs on equitorial positions in order to minimise the 90* repulsions.
2 x 90
3 x 120
with 2 lone pairs this is called the t position, T turned 90* anticlock
what does a double bond consist of
1 sigma
1 double
in POCl there are 3 single bonds and 1 double bond, what is the shape of this and are there any alterations
4 electron domains
tetrahedral shape
double bond = more electrons than usual so it repels the other bonds more.
tetrahedral but a bit more distorted.
if there is a compound with a charge but is bonded with double/triple bonds, the charges are usually where?
on the outer atoms
when does the VSEPR not work
- d block elements (shape is based on the nergy and occupation of the d orbitals)
VSEPR theory allows us to do what
predict the shapes of a molecule
molecules with no lone pairs are
non polar
dipoles cancel out
0 dipole moments
molecules with 1 lone pair
polar
dipoles cannot cancel out completely
molecules with 2 lone pairs
if its polar or not depends on where the lone pairs are
if theyre 180*, the molecule is non polar
if theyre less than 180* apart, theyre polar
