Chapter 6 + 7 - Order & Chaos; Crystalline structures Flashcards
What are the 3 most common atoms found in the earth’s crust?
Oxygen (47%); Silicon (28%); Aluminium (8%)
How does silicon appear in the earth’s crust? What do we call this? How much does silica make up from the earth’s crust?
SiO2; Silica or silicon dioxide; 59%
Give 4 applications of Si.
Hard ceremics (SiC), soepele rubbers (siliconen), glas/zand, vochtopnemers (silica-gel), …
What is a metallic bond? Why is it not stereospecific?
A bond between an atom and all of its neighbours. It’s a non-directional bond
How can we represent a metallic bond?
Hard spheres of equal size that want to be as close to each other as possible
Why does in an ionic bond the anion get bigger than its original atom and why does the cation get smaller?
The anion gets an electron (or more) and thus the electron cloud around it gets bigger. The opposite is true for the cation.
How can we represent an ionic bond?
Every cation wants to have as many anions around itself as possible and the opposite is true for anions. However, there’s still repulsion forces from the same type of ions. This makes that the ion diameter is a limit for how many it can have around itself. Depending on the type of salt it will put itself in an ordered lattice.
Why are covalent bonds stereospecific?
The bonds are directional, they are oriented in a specific way.
What does the stereospecificity result in?
Covalent bonds will have specific bonding angles depending on the type of bond
What is a crystalline material?
In a crystalline material, atoms or molecules (or ions) are ordered in a specific way so that there is periodicity (repetition of the structure).
What is an amorphous material?
In an amorphous material, the molecules have no periodicity. This results in the average bond length being greater than a crystalline material.
What is crystallisation?
When for example, the temperature drops below the melting point, the building blocks of the material will bond in a certain way. If this way has some structure and periodicity, a crystalline substance will form.
How does the extraction of salt from water happen?
When seawater evaporates, it will become saturated since there is too much salt in the water. This excess salt will crystallise and precipitate.
What does crystallisation have to do with melting and solidifying?
The temperature at which a liquid solidifies is also when (if possible) a crystalline structure will be formed.
Why is the density of a crystal greater than that of a liquid?
In a liquid, molecules are further apart than in a solid.
When it crystallises and thus becomes solid, the bonds will be closer together (in most cases that isn’t water). This changes the density because the number of molecules didn’t change, but the volume did.
What factors influence the ease at which the liquid solidifies?
- Size and complexity (smaller and less complex = easier to crystallise),
- viscosity (the more “liquidy” or “runny” the liquid is, the easier the building blocks can move closer to each other meaning easier crystallisation ),
- type of bond (stereospecificity)
Why do metals crystallise easily?
They have small and simple atoms as building blocks, are very runny in the liquid form and are non-stereospecific.
Why do salts crystallise fairly easy?
They have simple and small building blocks, but they do have opposite charges and a non-stereospecific bond. This means that they crystallise fairly easy.
What do we need to watch out for when solidifying something?
The lower density of the solid material can cause shrinkage cavities (holes). Since solidifying happens from outside to inside which can cause these holes because of the higher density
What is a solution for these shrinkage cavities?
having a mould that has some extra space to pour excess liquid so that there will always be enough liquid to make a solid block without cavities (if you still don’t understand how cavities form look at page 7)
Why do ceramic materials not crystallise easily?
They are built of fairly complex, stereospecific molecules and will only crystallise when cooled down really slowly. This is what happens in the earth’s crust creating crystalline stones and minerals.
