Ionic Bonding
This is the bond when one of the atoms is negative (has an extra electron) and another is positive (has lost an electron). Then there is a strong, direct Coulomb attraction. An example is NaCl. In the molecule, there are more electrons around Cl, forming Cl– and less around Na, forming Na+. Ionic bonds are the strongest bonds.
Covalent Bonding
In covalent bonding, electrons are shared between the molecules, to saturate the valency. The simplest example is the H2 molecule, where the electrons spend more time in between the nuclei than outside, thus producing bonding.
Metallic Bonding
In the metallic bond encountered in pure metals and metallic alloys, the atoms contribute their outer-shell electrons to a generally shared electron cloud for the whole block of metal.
· Secondary Bonding (Van der Waals)
· Fluctuating Induced Dipole Bonds Polar
· Molecule-Induced Dipole Bonds
· Permanent Dipole Bonds
CRYSTAL STRUCTURES
Atoms self-organize in crystals, most of the time. The crystalline lattice is a periodic array of the atoms. When the solid is not crystalline, it is called amorphous. Examples of crystalline solids are metals, diamond and other precious stones, ice, graphite. Examples of amorphous solids are glass, amorphous carbon (a-C), amorphous Si, most plastics
1.Unit Cells
The unit cell is the smallest structure that repeats itself by translation through the crystal. The most common types of unit cells are the faced centered cubic (FCC), the body-centered cubic (FCC) and the hexagonal close-packed (HCP). Other types exist, particularly among minerals.
2.Polymorphism and Allotropy
Some materials may exist in more than one crystal structure, this is called polymorphism.
If the material is an elemental solid, it is called allotropy. An example of allotropy is carbon, which can exist as diamond, graphite, and amorphous carbon.
3.Polycrystalline Materials
A solid can be composed of many crystalline grains, not aligned with each other. It is called polycrystalline. The grains can be more or less aligned with respect to each other. Where they meet is called a grain boundary.
4 IMPERFECTION IN SOLIDS
Materials are not stronger when they have defects.
The study of defects is divided according to their dimension:
0D (zero dimension) – point defects: vacancies and interstitials Impurities. 1D – linear defects: dislocations (edge, screw, mixed)
2D – grain boundaries, surfaces. 3D – extended defects: pores, cracks
