Molecular Structure of Solid, Liquid, Gas and Its Effects

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 structures of solid, liquid, and gases are geometrically and structurally different. This difference in structure is primarily due to the variation of the arrangement of molecules in liquid, solid, and 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 causing vibration, therefore, possessing high kinetic energy.

On the other hand, liquid molecules are close together but are not tightly packed; they do not show 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. The molecular structure of solid, liquid, and gas is represented by the following diagram.

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Characteristics of Solid, Liquid, and Gases

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. A few common examples of these types of solids are plastic and gases.

Further on the basis of the 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.

• 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 points, high brittle crystals and are bad conductors in their solid forms, for example, salt and sodium chloride (NaCl).

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

• Atomic (or Covalent) Solid: This type of solid is usually up with atoms that are held together by a 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 (with exceptions). 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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• 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 points as well as high electrical conductivity.

Particles of solid are arranged in unit cells that have different patterns. Now the molecules, atoms, or ions that are together are 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 a percentage. Thus, the equation of packing efficiency can be expressed as, 

\[\text{Packing Efficiency} = \frac{\text{Number of Atoms} \times \text{Volume obtained by 1 share}}{\text{Total Volume of Unit Cell}} \times 100%\]

Alternatively, \[\text{Packing Efficiency}\] (%) = \[\frac{\text{Volume of Atoms in Unit Cell} }{\text{Total Volume of Unit Cell}} \times 100%\]

2. 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 the 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 act 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 the surface tension of the liquid.

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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 the 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 the 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 start diffusing into each other unit, they form a 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. 

3. 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.

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Based on its molecular arrangements, few of the properties of gases are listed below:

• Particles are in constant random motion.

• 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.

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

• 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.

• 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. 

As different gases have different particle motions, and hence, they collide with various speeds giving rise to the 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 another.

Thus, the key points of kinetic molecular theory that can clarify the molecular model of solid, liquid, and gas are given below: 

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

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

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

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 a solid, liquid, or gaseous state. The matter in the solid-state has the least kinetic energy followed by liquid and gaseous particles have the highest kinetic energy.

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 a gaseous state.

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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Conclusion

Here, our discussion on the molecular structure of solids, liquids, and gases completes. You should now be able to differentiate between the arrangements of molecules in different states of matter: solid, liquid, and gas. You should also be able to relate the arrangement of the molecules at a microscopic level and their inference on the macroscopic properties of the matter.