 Namaste. Myself, Dr. Mrs. Preeti Sunil Joshi working as assistant professor in Valchan Institute of Technology, Solapur. We are going to start this module of crystallography with classification of solids. Learning outcomes are, by the end of this session, students will be able to state different types of solids and their properties. The contents include introduction, classification of solids. Before starting the session, a first question comes is why the study of crystals is important? Why we have to study the crystals? Crystals are the pillars on which the base of modern technology is founded. In fact, there is no field of technology that does not utilize crystal. As we know, a solid consists of atoms or clusters of atoms arranged in close proximity. The physical structure of a solid and its properties are closely related to the scheme that is the arrangement of atoms. How the atoms are arranged within the solid? Elements have different properties which are dependent upon how they are stacked together or what other elements they are stacked together with. We wouldn't put salt that is composed of sodium and chlorine on our food if it tested like the chlorine that we put into our swimming pools, isn't it? The great example of the importance of crystal structure is the difference between two minerals that is graphite and diamond. Graphite is soft dark colored material that is found in pencil lead. While diamonds are very hard, often transparent, colorless and very expensive gemstones. Both graphite and diamond are made up of only one element that is carbon. The reason that graphite and diamond are so different from each other is because the carbon atoms are stacked together into two different crystal structures. We can see in the figure here. You can see the structure of graphite which is composed of carbon that forms loosely bonded sheets in their crystal structure. These sheets rub off easily to mark the paper when you write on it while diamond. Diamond is composed of carbon atoms which are stacked tightly together in a cubic crystal structure making it a very strong material. This shows that it is not only important to know what elements are associated with the crystal but it is also required to know how those elements are stacked together. The crystal structures are analyzed using X-ray diffraction technique which was invented by Max von Lau and extensively employed by Bragg and Bragg. So, the study of crystal geometry helps us understand the diverse behavior of solids in their mechanical, metallurgical, electrical, magnetic and optical properties. Students, we know that all the objects and substances that we see in the world are made of matter. You know the matter it is classified into three states or it exists into three states that is solid, liquid and gases. Solid matter is composed of tightly packed particles. A solid will retain its shape and the particles are not free to move around. While for liquid what happens? The liquid matter is made of more loosely packed particles. It will take the shape of its container. Particles can move about within the liquid but they are packed densely enough that volume is maintained. And for gases matter what happens? It is composed of particles that are packed so loosely that it has neither a definite shape nor a definite volume. A gas can be compressed. Further these solids are classified into three categories that is based on the atomic arrangement within the solids as a single crystal, polycrystalline solids and amorphous solids. Let us start with the single crystal. So here single crystals are defined as the polyhedrons that have a distinctive shape for each material and are bounded by smooth shiny faces and straight edges. Here we can see that when a crystal is broken it cleaves along certain preferred direction. We can see in the figure here in single crystals as the periodic arrangement of item differs in different directions the physical properties vary with direction so they are called as anisotropic crystals. The examples of solids that occur as large size as single crystals are quartz, alum, diamond, rock salt etc. Polycrystalline solids are defined as the solids that are composed of many crystallites or grains of varying size and orientation. These grains are separated by well defined boundaries but oriented in different directions. Each grain is a single crystal of an irregular shape. Polycrystalline material is isotropic as their physical properties do not change with direction. Metals usually lie under this category and also we can see some examples of polycrystalline solid on the screen. Now the third category is amorphous solid. Amorphous solid is defined as the non-crystalline solid in which the atoms and molecules are not organized in a definite lattice pattern. Physical properties are independent of the direction of measurement and therefore they are called isotropic substances. They possess only a short range and maintain a fixed volume and shape and resemble solids in their external features. But internally they do not have the ordered crystalline arrangement. Glass, rubber and many polymers are the examples of amorphous solids. Students now pause the video and try to answer these questions. I think you are ready with the answers. Let us check your answers. The first one amorphous solids are isotropic in nature as their properties are independent of direction. And the second one is silica is not crystalline. So students summarizing this simply keep in mind that there are mainly two types that is crystalline and amorphous. So if we ask ourselves what is unique about the crystals which distinguishes them from other type of materials. So the answer comes is that the so-called microscopic crystal structure is characterized by groups of ions, atoms or molecules which are arranged in terms of some periodic repetition model. And this concept is easy to understand if we look at the picture of this military parade. If we look carefully at this picture you can see or you can discover that there is always a fraction of them that is repeated. And this repetition in three dimensions is known as the crystal. For example, each soldier in this picture could be a reticular point. But however, matter is not entirely ordered or disordered. And so we can find a continuous degradation of the order which goes from the perfectly ordered which we can say crystalline to the completely disordered that we can say amorphous. Now look at this image. This gradual loss of order which is present in materials is equivalent to what we see in the small details of this photograph of gymnastic training which is somewhat ordered. But there are some people wearing pants, some wearing skirts, some are in different positions or slightly out of line. So that is about crystalline and amorphous. So this is my small try to give an idea about the crystallinity and amorphous nature of the material. Students to understand crystal and crystal structure it is also necessary to know few basic definitions or concepts. So see you in the next session for these points. Thank you.