Structural - lecture 3 Flashcards
What is the equation for the volume of a sphere?
4/3 πr3
Which type of arrangement do atoms mostly have?
Close packed arrangement
How many tetrahedral and octahedral holes are there per close-packed sphere?
2 Tetrahedral
1 octahedral
What is the maximum size for small spheres to fit into the holes without disrupting the close packing of the larger spheres for octahedral?
r oct = 0.414 Rᶜᵖ
What is the maximum size for small spheres to fit into the holes without disrupting the close packing of the larger spheres for tetrahedral?
r tet = 0.225 Rᶜᵖ
What happens when the ratio of R+/R- increases?
More anions will be able to fit around one cation
What are the characteristics for a molecule with cubic geometry?
Co-ordination number is 8
R+/R- = 1.000 to 0.732
What are the characteristicsfor a molecule with octahedral geometry?
Co-ordination number is 6
R+/R- = 0.732 to 0.414
What are the characteristics for a molecule with tetrahedral geometry?
Coordination number is 4
R+/R- = 0.414 to 0.225
What are the characteristics for a molecule with trigonal planar geometry?
Co-ordination number is 3
R+/R- = 0.225 to 0.155
What are the characteristics of a molecule with linear geometry?
Co-ordination number is 2
R+/R- is 0.155 downwards
What is R+/R-?
The ratio of anions to cations
What are the most common co-ordination shapes?
Tetrahedral and octahedral
Why will 2 tetrahedra never share three corners?
Because that brings te two cations too close to each other
Can octahedra share faces?
Unlike tetrahedra, yes
What happens when polyhedra are linked together by sharing corners?
Larger units which may be discrete are formed or they may be polymeric structures
What happens to many metals that are close packed under normal conditions when they are under higher temperatures?
When heated, they will lean towards the less efficiently packed body centred cubic structure
What are the three different types of model?
Ball and spoke
Close-packed spheres
Linked polyhedra
