topic 5π ±οΈ 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 βdβ
the sum of the cat ionic and anionic radii
radius of cation + radium of anion.
what is difficult about βdβ
itβs 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, βnβ.
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