topic 5🅱️ Flashcards
ionic solids
cations and anions held together by electrostatic interactions between opposing charged ions
what does the ionic model assume
ions are hard spheres that have no covalent character with fixed ionic radii
how do we determine the ionic radii
by determining the distance between the nuclei of the 2 ions very accurately using x ray crystallography.
what is distance ‘d’
the sum of the cat ionic and anionic radii
radius of cation + radium of anion.
what is difficult about ‘d’
it’s hard to be sure where the boundary between the 2 ions occurs.
what are the value of ionic radio dependent on
the compound used to determine them
what does the radius of an ion depend on
- coordination geometry
- ion charge
- electron configuration
coordination number and ionic radii
increase in coordination number = increase in ionic radii
charge of the ion and ionic radii
increase the positive charge = decrease in radius
diff oxidation states = diff radii
electron configuration and ionic radii
electron configuration and ionic radii
ionic radii trends
- change in e- number = change in radii
- increase electrons = larger radii (anion forms,, Zeff decreases, e- e- repulsion increases)
- decrease electrons = smaller radii (cation forms,, Zeff increase,, e- e- repulsion decreases)
ionic radii trends down a group
increase in ionic radii
larger principle quantum number, ‘n’.
ionic radii trends: ionic radii across a period
if they have the same charge,, the radius will decrease across a period
electrons in the same shell but an increase in protons. poor shielding from electrons in the same shell.
feels more Zeff so radius decreases
what are isoelectronic ions
same number of electrons but different number of protons.
ions have different charges that allows them all to have the same number of e-.
ionic trends: isoelectronic ions
greater z = smaller radius: larger Zeff
predicting ionic structures
- most stable structure is formed
what is the most stable structure that is most likely to form
- the structure that maximises all cation-anion contacts
- the structure with the highest coordination number will still allowing cations and anions to remain in contact.
- 4 below,, 1 above.
burger: bottom bun is always massive,, top bun disappeares.
what is the radius ratio
the ratio of the smallest ion radius to the largest ion radius.
which ion usually has the smaller ratio
the cation,, lacking in e-,, greater Zeff,, less e- e- repulsions
radius ratio equation
radius of cation // ratio of anion
what is the limiting radius ratio
smallest possible value of the radius ratio that still allows the cations and anions to be in contact
limiting radius ratio
- defined for a particular geometry around the cation
table needs to be memorised <3
what is the radius ratio rule
predicting a compounds structure from the radius ratio of its ions
limiting radius ratio rule steps
always gonna be 2 right handed triangles between the 2 anion and 1 cation radii.
use sin rule
r+/r- = (1/sinX) -1
where X is the angle that doesnt equal 90
radius ratio values
less than 0.1555
0.155 - 0.225
0.225 - 0.414
0.414 - 0.732
0.732 - 1.000
larger than 1.000
coordination number: radius ratio values
2
3
4
6
8
12
geometry of cation: radius ratio
2 = linear <0.155
3 = trigonal 0.155 - 0.225
4 = tetrahedral 0.225 - 0.414
6 = octahedral 0.414 - 0.732
8 = cubic 0.732 - 1.000
12 = cuboctahedral >1.000
does the radius ratio rule always work
nope
why doesnt the radius ratio rule not apply in all cases
relies on the ionic model,, the ionic model is based on a lot of assumptions
what are some assumptions of the ionic model + their correction
- ions in crystals arent hard spheres
- we dont know the exact radii of individual ions (radii lengths are dependent on the compound theyre in)
- not all compounds are 100% ionic,, there is a degree of covalency dpending on the electronegativity difference between the 2 atom types.
compounds that deviate from ionic model + radius ratio
LiI
LiCl
both said to be octahedral coordination + adopt rock salt structure (NaCl)
but LiCl is octahedral
and LiI is tetrahedral coordination
what causes discrepancies between real and predicted structure types
ionic + covalent character
electronegativity differences.
why does LiI go against the radius ratio predicted structure
difference in electronegativity,, allows a degree of polarizability.
in LiCl there is also some covalent character.
larger ions are more polarisable than smaller ions
smaller,, highly charged cations are strongly polarizing
the small Li+ cation can polarise the diffuse e- cloud on the larger I- ion leading to a higher degree of covalent character.
the bonding in LiI isnt well described by the ionic model as there is a strong degree of covalent character.
