What makes same chemical compound to have different physical properties in solid state; a very important topics for IIT JEE, Mains & Advanced, NEET level.
Next time you go for grocery shopping, notice there are two kinds of sugar available in the market. One which is fine cube, generally comes in packet (not necessarily though) , and one kind of dusty. You will see, the one with fine crystalline shape takes time to dissolve in your tea, but the dusty one gets mixed easily. You can also buy a machine which makes ice cubes with pure water, which does not melt up to (-) 40 degree Celsius (while water freezes at zero degree). Isn’t that amazing? As per latest report in popular Science journal, there are nearly 300 kinds of ice crystals are there in nature, and all have different properties. Same thing happens in case of diamond and charcoal. Despite being formed by same chemical element, carbon, they have totally different sets of properties. What makes them different? What factors are responsible for that.? What is their structures like? Let us find this answer.
Solid and it’s types:
You must have noticed that, here we are discussing about solid, one of the four common state of matter (others are, liquid, gas and plasma). There are a particular set of characteristics for solid state. They are compact with least intermolecular distance but have highest molecular force of attraction among all the states of matter.
They have definite mass, volume and shape. Due to highest intermolecular forces of attraction, they are hard, non-compressible. Sometimes some solid compound shows special properties like electrical conductivity, magnetic properties etc. The origin of such peculiar properties lies in the arrangement of molecules or atoms inside the solid framework.
Based on the arrangements of molecules in a solid, it can be classified into two broad category: Crystalline and Amorphous. In crystalline solid, the arrangement of constituent particles is a regular orderly arrangement. Iron, copper, diamond, graphite falls under this category. On the other hand, amorphous solid lacks such regular arrangements, some common example being, Glass, plastics, rubber etc. That is why, sometimes amorphous solids are called pseudo-solid.
This also makes the properties of crystalline solid very much distinct from amorphous. While the former shows regularity in properties like melting point, heat of fusion and other physical properties, amorphous solid, which do not have any regular geometry does often possesses irregularity in physical behaviour.
How to determine the structure of solid?
Now you may ask, how do we know that such an arrangement is present in crystalline solid, since atoms or molecules are not visible, even under microscope? For such study, we take help of some indirect method. Since, atom is made up of charged particle like electron and protons, and as we have studied in atomic structure chapter in our textbooks, we know that electrons have both particle as well as wave nature, they interacts with electromagnetic radiations like X-ray, radio waves etc. Same science is used in medical equipment like MRI, CT scan, X-ray and other diagnosis process. In this structural determination of solid, the father-son duo, Sir W. H. Bragg and Sir W. L Bragg and their X-ray diffraction study become quite useful. This is famously known as Bragg’s Law, which establish a relationship between wavelength of incident X-ray, the distance between layers and angle of diffraction.
Lattice and Unit Cell :
Now we know that solids have different sets of parameter in three dimension. The regular ordered arrangement of constituent particles in three dimensions is called crystal lattice. The fixed positions on which the constituent particles are present are called lattice points or lattice sites. A group of lattice points which when repeated over and over again in 3 dimensions give the complete crystal lattice. The smallest repeating unit in space lattice which when repeated over and over again generates the complete crystal lattice is called unit cells, which can be of infinite number.
You may wonder, why lattice is formed actually? The argument may sound philosophical, but nature loves symmetry. You will find symmetry in mostly all the natural things like plants, flower, fruits, animals etc. Similar trend is observed in arrangement of particles inside solid.
Now this particle can be anything, like atoms, molecules, ions etc and according to the interactions present between them, they are called, metallic, ionic, covalent and molecular crystals.
Types of unit cells:
Such unit cells also can be categorised according to the geometry of unit cell. As mentioned before, a unit cell can have six parameter, 3 distance in 3 axis, and 3 angles between three planes. Primitive or simple unit cells have constituent particles only at its corners. Centred unit cells are those unit cells in which one or more constituent particles are present at positions in addition to those present at the corners. These can be called FCC (face centred cubic), BCC (body centred cubic, end-centre cubic, edge centred cubic. There are seven type of crystal systems eg. Orthorhombic, tetragonal, monoclinic, rhombohedral, triclinic and hexagonal primitive and total and 14 Bravais lattices.
Reason for different properties
Due to this different arrangement of particles in unit cell, there are different number of atoms per unit cell. These particles follow these patterns according to their size. It is their natural tendency is to make the closest approach. In other word they tend to pack as compact way possible. This brings the difference in density as well as coordination number. And this brings the different properties in solid. This is why some substances are electrical conductor, some are insulator, some have magnetic properties like iron, cobalt nickel, but others not.
Radius Ratio Rule
In case of ionic solid, the situations are little bit different. There, unlike metallic crystals, the size of particles are not same. Cation is generally smaller than the anion. This bring a situation, where the arrangement of salt should be in such a way that, there will be closest packing of ions, and at the same time, they have to maintain their empirical formula. For example, as we know, the formula for our daily used table salt is NaCl. But in reality, there are no separate molecules of NaCl inside its crystal. In a unit cell there are 4 sodium and 4 chloride atoms per unit cell arranged in different positions, where coordination number of both sodium and chloride are 6. But in case of Cesium chloride, this coordination number will be 8 for each cation and anion. But how we can calculate or predict this coordination number for a salt? This when we need to know about radius ratio rule. In the adjacent video, you will find out what this rule is and how to derive it.
Lets come back to original problem.
But real question remains, why they adopt different structural arrangement sometimes? Like in our case, coal and diamond. The main anchors here are temperature and pressure. When pressure are more, particles are forced to come close together and so their coordination number increase and become more dense. In coal mines, coal are generally found near the surface, while diamonds are available in deep down where pressure is more. This excessive pressure is the reason diamonds are at different structural arrangement than graphite. The former has a coordination number of 4, while the later as 3 and their properties are very distinct.
Now this solid state chemistry has found its expanding role in several fields. Research on the development of novel materials with optimized properties are on high rise. Due to this importance it has been included in basic science chapter and is a very important topic for NEET, IIT JEE, Mains & Advance, Board syllabus.