Molecular Structure of Solid, Liquid and Gas

Solid, Liquid and Gas

All matter whether metals, nonmetals or inert substances are principally composed of atoms, molecules and/or ions but their distribution in the matter depends upon the state it is representing, that is, either solid, liquid or gaseous. Depending on the state, the molecular structure of solid, liquid and gases are geometrically and structurally different. This difference in structure is primarily due to the variation of arrangement of molecules in liquid, solid and in gases. The particles in the gases are far away from each other and thus are well separated and do not have a definite shape.  Because of the large distance between the molecules of gases they move quite easily and very fast that cause vibration, therefore, possess a high kinetic energy. On the other hand liquid molecules are close together but are not tightly packed; they do not sow any definite molecular arrangements and have no definite shape of their own. The liquid vibrates and slides across each other with lesser speed as compared to gases and therefore shows less kinetic energy. In solid matters, the molecules are tightly  packed with each other in a definite arrangement and thus have a defined structure, shape and size. Solid vibrates but its molecules do not move from place to place. Molecular structure of solid, liquid and gas is represented by the following diagram.

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Some of the Characteristics of Solid, Liquid and Gases are:-

Characteristics of Solid, Liquid and Gases


Gas

Liquid

Solid

Takes the shape and volume of the container as particles can move past one another

Does not have any shape of its own but has a definite volume as particles are closer to each other and slide past one another.

Particles are closely packed and retained at their own place, therefore, it has fixed shape and volume.

High compressibility as there is lot of free space between particles

Not easily compressible as little space between particles

Not easily compressible as very little space between particles

Flow easily as particles can move past each other

Flows easily as particles glide as well as move past each other.

Does not flow as particles are fixed and cannot move or glide past each other.


Properties Based on Molecular Structure of Solid, Liquid and Gas

  1. Molecular Structure of Solid : There are two types of solid structure based on the molecular arrangements of solid, namely, crystalline solid and amorphous solid.  In crystalline solids the molecules that make up the solids are in definite and regular arrangements which gives them a well defined structure. Crystalline solids are made up of cell units that are the smallest repeating pattern of solid and are the building blocks of crystalline solid. They are identical in nature and repeating. Whereas amorphous solids do not have a definite structural pattern and order of themselves. Though the molecules are closely and tightly packed and have very little mobility, the arrangements of the molecules are not regular and in asymmetrical order. Few common examples of these types of solids are plastic and gases. Further on the basis of molecular structure of crystalline solid it is divided into four categories, namely, ionic solid, molecular solid, atomic solid also known as covalent network and metallic solid.

  1. Ionic Solid : These solids are made up of positive as well as negative ions that are held together closely by electrostatic force. They have high melting point, high brittle crystals and are bad conductors in their solid forms, for example, salt and sodium chloride (NaCl).

  2. Molecular Solid : The atoms or molecules of these solids are held closely by strong bonding forces like London Dispersion force, dipole-dipole force as well as hydrogen bonding. They have usually low melting points with very low conductivity, for example, molecular solid in sucrose.

  3. Atomic Solid : This type of solid is usually up with atoms that are held together by covalent bond and share a covalent force between them as well. They are usually hard and have high melting points but poor conductors of heat and electricity. Common examples of this type of solid are graphite and diamond. Now graphite has a 2D hexagonal structure unlike diamond and is therefore held together by weak London dispersion Force. Thus graphite is not as strong as diamonds. 

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  1. Metallic Solids : They are made up of metal atoms and are held together strongly by metallic bonds. Thus they range from soft and malleable to hard solids with high melting point as well as high electrical conductivity.

As particles of solid are arranged in unit cells that have different patterns. Now the molecules, atoms or ions that are together known as particles that always have some voids in whichever way the particles are arranged. Thus the quantitative aspect of the solid state can be expressed numerically as packing efficiency that is expressed as percentage. Thus the equation of packing efficiency can be expressed as, 

Number of atoms *  volume obtained by 1 share / total volume of unit cell *100

Alternatively, packing efficiency (%) = (volume of atoms in unit cell / volume of unit cell) *100

  1. Molecular Structure of Liquid : Liquid molecules, much like solid and unlike gases, are closer to each other and can glide across each other easily. But in solids as the particles are held strongly by the intermolecular forces, liquids possess too much thermal energy and thus cannot be held at a certain position by these forces and thus move around easily within liquid mass. Although liquid has a much greater cohesion force as compared to that of gas, this force cannot prevent a few molecules of the liquid from bonding with each other. On the other hand molecules at the surface of the liquid experience a cohesive force with the molecules that are within the surface and near to liquid mass. Because of this cohesive force the liquid molecules on the surface of the liquid acts as the stretching membrane. Thus the liquid molecules of the liquid surface tend to draw inwards towards the centre of mass and hence forms a sphere. This is called surface tension of liquid. The liquid wets a solid surface only when the intermolecular force of attraction between the molecules of liquid on the liquid surface to the molecules on the solid surface is greater. For example wetting of clothes where water is retained on the surface of fabric. But when the intermolecular force is not that high the liquid is not retained on the solid surface, for example, when mercury is run through a glass capillary. Also the molecules in liquid state exhibit properties of diffusion. That is, when two miscible liquids are poured into a container the molecules in liquid states due to greater intermolecular force stars diffusing into each other unit they form an uniform mixture. Liquid also has a very unique property called buoyant force in which the substance immersed into the liquid experiences an upward force that is equal to the weight of the liquid displaced by the substance. This behaviour of liquid matter at a molecular level is characterised by two factors, intermolecular pair potential function (u) and radial distribution function (g). The information about the energy due to the interaction of the pair of molecules is given by the pair potential function which is a function of the distance ® between the centres of the molecules in interaction. Information about the structure of the molecule or the distance between the centres of the molecules is given by the radial distribution function (g). Thus if the values of these two functions are known then the microscopic property of the substance can be derived.

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  1. Molecular Structure of Gases : as the molecules of the gases are at very high kinetic energy and are separated by large distances they can easily slide and cross each other and don't possess any particular shape, size or volume. Based on its molecular arrangements few of the properties of gases are:

  1. Particles are in constant random motion

  2. Particles often collide with each other and with the wall of the container but due to its high elasticity there is no net loss of energy from the particles of gas.

  3. There is no force of attraction or repulsion between the molecules of the gas.

  4. Since the molecules of the gases are very small thus the volume occupied by the gas molecules is negligible as compared to the total volume of the container.

  5. The kinetic energy of the gas molecules depends on the absolute temperature of the gas and all the gas molecules with the same absolute temperature will have the same kinetic energy.

Now since the molecules or the atoms of the gases are very tiny and therefore have a lot of empty space between them, the particles are constantly in motion. Now as the volume of the gaseous is very small, hence, its density is also minimal due to which a gas can be compressed or expanded according to the condition. As the particles are in constant motion, therefore, the two gases in one container will mix with each other as their molecules will move constantly and will collide with each other. Thus the no. of particles colliding with the surface of the container and the force with which they are colliding on the wall is determined as the pressure of that particular gas. Thus the kinetic energy of an individual atom in the gas is measured by the given mathematical expression.

Ek = ½ mu2, where m is the mass of the atom and u is the speed of the atom. As different gases have different particle motion and hence they collide with various speeds giving rise to different kinetic energy distribution of various gases. The graph represents the same.

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Molecular Model of Solid Liquid and Gas

The molecular model of the solid liquid and gas helps in determining the kinetic molecular theory of matter as these two factors are closely related to one another. Various molecular arrangements and their motion kinetics give rise to the force with which one molecule will collide with another in the  same substance or intermolecular interaction for different substances giving rise to the kinetic molecular theory. The kinetic molecular theory explains the interaction between atoms and their microscopic properties that in turn give rise to the macroscopic properties of the matter such as pressure, temperature and volume. This theory was developed to explain the existence of various states of matter and the way by which matter converse from one state to the another. Thus the key points of kinetic molecular theory that can clarify the molecular model of solid, liquid and gas are:- 

  1. Matters are made up of particles that are in constant motion, that is, they are continuously moving.

  2. The temperature of matter is measured by the average kinetic energy of the moving particles.

  3. The change in phase occurs when the change in the kinetic energy of the molecules of matter changes.

  4. All particles of the matter have energy and the energy of the particles varies with the temperature of the matter. This in turn determines if the matter should be in solid, liquid or gaseous state. The matter in solid state has least kinetic energy followed by liquid and gaseous particles have highest kinetic energy.

  5. Even if the particles are closely packed with each other, there is still some space between the particles which are called voids. These voids start becoming larger and larger as the matter changes from solid to liquid and finally to gaseous state.

  6. There are always attractive forces between particles that are called intermolecular attractive forces and it starts getting bigger and bigger as the particles come closer to one another.

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In case of  an ideal gas, as there is absolutely no force of attraction between the molecules, the radial distribution factor (g) becomes unity which indicates that the chance of interaction of the molecule that is moving away from the center molecule does not depend upon the position. But in solids, g takes various values that are corresponding to the distance and the positioning of the molecules in the crystal structure of solid. Thus the structure of the liquid falls in between these two extremes. The molecules in the liquid are free to move as they are far apart from each other but there are some close molecules that experience very weak intermolecular force of attraction. The graph shows the function of the radial distribution factor for dense liquid. Here g represents the possibility of finding the centre atom who is at distance r from the center of another atom. If the value of r is less than the diameter of the molecule then g becomes zero.

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FAQs (Frequently Asked Questions)

1. What Is the Molecular Structure of Liquid?

Ans. Liquid does not have any definite molecular structure and takes the shape of the container in which it is poured. Though liquid has a definite volume.

2. How Particles in Solid, Liquid and Gases Move?

Ans. solid liquid and gases are made of tiny particles that move with a definite kinetic energy. In solids the molecules are tightly packed with each other with strong force of attraction thus they are not free to  move but vibrates at their own position. In liquid, since the particles are little far away from each other and possess more kinetic energy than solid, thus they can move freely and glide across each other. In gaseous state molecules are most apart from one another and move with high velocity and collide with each other with highest kinetic energy as well as collide with the wall of the container.