Question

How are the particles in a crystalline solid arranged?

Answer 1

Use the concept of crystal lattice and unit cell, where the unit cell which is the smallest portion if rotated in all three dimensions gives the big crystal lattice. Due to the difference in types of unit cell every solid crystal is of different arrangement of particles. Generally as we know there are three types SCC, BCC and FCC.

Complete step-by-step answer:
We know that atoms are considered as hard spheres which do not compress, they are arranged in different patterns and thus form crystal lattices. For your convenience let’s say we have a small cube like a playing rhombus cube, in its positions considering that there are different atoms at corners this type of arrangement is of simple cubic cells which on rotating in three dimensional give SCC (Simple cubic crystal).

Now take the arrangement as it is and add an atom just at the center. This type of arrangement is of body centered unit cell which on rotation gives BCC (Body centered crystal). If we eliminate the atom which is present at the body centered and add atom on center of each face, the unit cell which will form is called a face centered unit cell and crystal called FCC (Face centered crystal). $NaCl$ table salt which we used for eating is of FCC type of arrangement. While $CsCl$ forms a BCC type of arrangement.

Now the concept is as simple as it but a little bit of complexity comes when we take the crystal forming for ions. In ionic crystals there are two types of ions- cation and anion. So as we know that anion is bigger in size and cation is the smaller one. So, in most of the cases, anion used to form the basic skeleton of unit cell and cation as it is small used to fit in the voids or we can call them simply as gaps.

Now the question arises which type of unit cell will form in the ionic crystal. So, in ionic crystals, a rule called radius ratio rule decides the types of arrangement and we can say a cation also decides in which type of unit cell it wants to be lived in.

Note: When you are dealing with the number of atoms and lattice points, don’t get confused both terms are used differently in solid state chemistry. Lattice points are the atoms which take an individual position in a unit cell while the number of atoms is calculated by contribution of each part of that atom in a whole crystal lattice.