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The structure of atoms has something to do with the stability of its configuration. Materials generally can be divided into two classes based on their structure: Crystalline and Non-Crystalline. Crystallines are those whose atomic structure arrangement is very nicely ordered in every directions while non-crystalline solids or Amorphous do not possess such characteristics. For example, salt (NaCl) is a stable crystalline. What we have in salt is Sodium which has positive charge (a positive ion) and Chlorine, which has negative charge (negative ion). These two opposite charged elements attract each other and form the salt spontaneously when mixed together. This strong electrostatic (ionic) attraction has enough force to form the salt crystals. Every ion of 1 charge is surrounded by six neighbors of the opposite charge and keeps the electrostatic attraction strong.
Also, it is good to know that this story of crystallization of atoms is somehow related to the amount of atomic energy in the compound.
["Amorphous": A (without) + Greek "morphe" (shape, form). Similarly, morphosis means a change in from and shape)
Next thing to know in Material Science is about Atomic Bonding.
In an ionic compound, the Coordination Number is the number of touching atoms, or the number of Anions surrounding the Cation. The coordinate directions in lattices create planes in the cell. Miller Index is used to specify a plane in a unit cell which are shown by three integers h,k, and l inside parenthesis.
Packing Factor is the ratio of the volume of atoms by volume of cell. The lower this value (e.g. for simple cubic or hexagonal structures) means there is huge empty spaces in the cells, which makes it hard to form naturally.
The size of the cell depends on the type of lattice and the size of the atoms connected to each other. The distances between these atoms in 3D are a, b, and c which are called Lattice Constants. Cells are connected to each other, and this means some of the atoms are shared between adjacent cells. That is, for example, each of the corner atoms in a BCC lattice is shared between 8 cells. Some of the atoms also may be completely inside the cell like the center atom in the BCC lattice, and therefore, the number of atoms would be equal to the center atom and 8 times 1/8 atoms which is equal to (average of) 2 atoms per cell.
Bravais Lattices are the 14 different 3D crystalline structures, named after Aguste Baravias a french physicist who is known in crystallography. The smallest unit of a Bravais lattice that is repeated in the lattice is called a Cell or a Unit Cell. There are 7 different Crystalline Systems: Cubic, Tetragonal, Orthorhombic, Monoclinic, Triclinic, Hexagonal, and Rhombohedral.
Most of the common metallic crystals for in one of the three cell systems of Body-Centered Cubic (BCC), Face-Centered Cubic (FCC), and Hexagonal Close-Packed (HCP).
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