230709 - Structure Of Matter (P1-C1) Flashcards
Generally, the physical, mechanical and chemical properties of any material depend on
- Atomic structure
- Inter-atomic bonding
- Arrangement of atoms in space
Atom consists of
- Positive central nucleus
- Negative peripheral electrons
Electrical state of atom
Neutral
What is atomic number?
Number of electrons
What is valence electrons?
- electrons in the outer most shell
- Determines the chemical reactivity of the element
What is the atomic weight?
Protons + neutrons
How many electrons in the outer shell of an atom should have to achieve a stable state?
8 electrons
How can we obtain a stable state atom?
- receive electrons
- Release electrons
- Share electrons
Three types of primary bonds
- Ionic bond
- Covalent bond
- Metallic bond
What is a ionic bond?
- involving electron transfer
- Attraction of positive and negative ions occurred
Sodium chloride is an example of which primary bond?
Ionic bond
Glasses in ceramics containing what type of primary bond?
Ionic bond
Characteristics of ionic solids
- High thermal resistance
- High strength and hardness
- Insoluble in organic solvents
- Soluble in ionized solvents (water)
What is covalent bonds?
Sharing of electrons
Diamonds and polymers containing which type of primary bond?
Covalent bond
Characteristics of covalent bonds
- Insoluble in water
- Very strong and hard
- Can withstand high temperature (insulator)
- Directional (electrons spend most of their time in the region, where the orbit overlap)
What is metallic bond?
- Attraction between positive cores and free electrons or electron cloud
- in metal
Characteristics of the metallic bonds
- High thermal and electrical conductivity
- High strength and hardness
- Opacity (absorption of light by the free electrons)
- Lustrous (free electrons reemit the light)
- Malleable (can be hammered into thin sheets without fracture)
- Ductile (can we drive into thin wires)
Two types of formation of electrical dipoles in secondary bonds
- Fluctuating dipole
- Permanent dipole
What is fluctuating dipole?
- more electrons on one side of the nucleus than the other
- Asymmetry electrons distribution
- Causing weak attractive forces between the fluctuating dipole in joining atoms
What is permanent dipole?
- Example: hydrogen bond
- Electrons around oxygen nucleus are more than those around the hydrogen nucleus
- Positive hydrogen and negative oxygen will cause weak attractive force
Characteristics of secondary bonds
- Low strength and hardness
- Low thermal resistance
Factors of spacing of solid atoms
- Forces of attraction
- Forces of repulsion
Factors affecting inter-atomic distance
- Number of adjacent atoms (-ve)
- Effect of temperature (+ve)
- Type of bond (primary>secondary)
- External force application (increase tensile stress)
Classification of solids according to the type of bond between atoms
- Atomic solid
- Molecular solid
Difference between atomic solid and molecular solids
Atomic solid
1. Primary bond exists between the atoms and molecules
2. High strength and hardness
Molecular solid
1. Primary bond between atoms, but secondary bond between the molecules
2. No strength and hardness
Classification of solids, according to arrangement of atoms
- Crystalline solids
- Amorphous solid
What is space lattice?
Regular arrangement of atoms in space
How many types of space lattice exist
About 14 but only few of dental interest
What is unit cell?
The smallest repeating unit in the space lattice
Unit cells are classified according to
- Length of axes (a,b,c)
- Interfacial angle (alpha beta gamma)
Types of cubic system
- Simple cubic system (S.C.)
- Body centered cubic (B.C.C)
- Face centered cubic (F.C.C.)
Formula of atomic packing factors
Atomic packing factors = volume of atoms inside the unit cell / volume of unit cell
^ APF ^ density ^ strength properties
What is the meaning of amorphous?
Without shape
Difference between crystalline and amorphous solids
Crystalline solids
1. Definite melting point
2. Regular unit cell with repetition
3. Low internal energy
Amorphous solids
1. Melting range
2. No regular unit cell
3. High internal energy
What is polymorphism?
Polymorphic materials that can exist with more than one crystal structure by changing the surrounding physical condition, such as silica (SiO2)
Forms of silica structure
- Quartz
- Tridymite
- Cristobalite
- Fused quartz
Types of transformations
- Reconstructive transformation
- Displacive transformation
Difference between reconstructive transformation and displasive transformation
Reconstructive transformation
1. Breakdown atomic bond followed by reconstruction of new space lattice
2. No expansion
3. Slow transformation
4. High temperature needed
Displacive transformation
1. No breakdown of atomic bond
2. Expansion occurred
3. Rapid transformation
4. Low temperature
Types of crystalline imperfections
- Point defects
- Line defects (dislocation: displacement of a row of atoms from their normal position in the lattice)
- Plane defects
