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• CBSE Class 11 Physics Notes

# Molecular Nature of Matter – Definition, States, Types, Examples

The distinct forms that different phases of matter take on is called the state of matter. The most common state matter that is easily observable in daily life is – Solid, liquid, gas and plasma. There are many other states known to us like – Bose-Einstein condensate and neutron degenerate matter, but these states exist only in extreme weather conditions like – ultracold or ultra-dense matter. Other theoretical states are also present such as quark-gluon plasmas. Since early stages, the difference between states of matter is made on qualitative differences in properties.

• Solid-state always maintains a fixed volume and shape with the constituting particles, which are atoms, molecules and ions, staying close together and fixed in place.
• Whereas the liquid state always maintains a fixed volume but takes the shape of the container. Its constituting particles stay closely but move more freely as compared to solids.
• In the gaseous state, both the shape and the volume are variable, adapting to the container. Its constituting particles are most mobile and never stay fixed to the same place.
• In the plasma state, the matter has variable shape and volume, but it also has neutral atoms. It contains a huge number of electrons and ions, both of them can move around freely. Plasma is also the most common form of visible matter in the universe.

### Fundamental States of Matter

Solids

In the solid, the constituent particles (ions, atoms or molecules) are very closely packed together. The forces between the particles are so strong that the particles cannot move freely but can only vibrate. As a result of which, a solid has a stable, definite shape, and a definite volume. Solids can only change their shape by an external force, as when they are broken or cut.

In crystalline solids, the constituent particles are packed in an organized order and repeating pattern. The position of the next particle can be easily predicted. There are multiple crystal structures, and one substance can have multiple structures. For example, iron at temperatures below 912 °C has a body-centred cubic structure and a face-centred cubic structure between 912 and 1394 °C. Ice itself has fifteen known crystal structures, or fifteen solid phases, which exist at various temperatures and pressures. Glass is a non-crystalline and amorphous solid. Glass and various other non-crystalline and amorphous solids do not have long-range order and do not have thermal equilibrium ground states; that is why they are described as nonclassical states of matter. Solids can be changed into liquids by melting and can also be changed directly into gasses using the sublimation process.

Liquid

A liquid is also called fluid and it has a nearly incompressible nature which means it adapts to the shape of its container but also retains the constant volume and is independent of pressure. The volume of a liquid is definitely provided that pressure and temperature are constant. When the pressure is above the triple point of a substance and the substance is heated above its melting point, it becomes a liquid. Intermolecular forces still act but now the molecules have enough energy to move more than that in solids and the structure is mobile. This is the reason why the volume of liquid is not definite and it attains the volume of the container. The critical temperature of a liquid is the highest temperature at which it can exist.

Gas

Gas also comes under the category of fluid and it is compressible. Gas not only takes the shape of the container but also expands to fill the entire container. In the case of a gas, the constituent molecules have enough kinetic energy so that the effects of intermolecular forces is small, and the intermolecular distances are much larger than the molecule’s size. In the case of an ideal gas, intermolecular forces are zero. A gas does not have a definite shape or volume, but they occupy the entire container in which it is stored. If we heat a liquid to its boiling point at a constant pressure it converts into gas.

Gas is also called vapour at a temperature that is below its critical temperature. By using the compression-only a gas can be liquefied even without cooling it. Vapour and liquid can exist in equilibrium and in that case vapour pressure of the liquid are equal to gas pressure. A gas whose temperature and pressure are above the critical temperature and critical pressure respectively is called a supercritical fluid (SCF). It has all the physical properties of a gas but its high density is the reason for solvent properties in some cases, which leads to many useful real-world applications.

Plasma

Just like gas, plasma also does not have any definite shape or volume. But unlike gasses, plasma is electrically conductive, it can produce magnetic fields and electric currents and also respond strongly to electromagnetic forces. There is a nucleus that is positively charged and it swims in a sea of disassociated and free electrons. This sea of electrons is the reason plasma is able to conduct electricity.

The plasma state is very much common on Earth, unlike it is generally thought to be rare. Most people do observe it in day to day life and they do not even realize this. Lightning, sparks, fluorescent lights, plasma TVs are the typical examples of matter in the plasma state.

### Types of Intermolecular Forces

Mainly there are three types of intermolecular forces. They are London dispersion forces (LDF), hydrogen bonding and dipole-dipole interactions. Molecules can also have the mixture of all three forces at once, but they have LDF for sure.

London Dispersion Forces (LDFs): London dispersion forces always exist in every kind of substance. It doesn’t matter whether the substance is composed of polar or nonpolar molecules. The formation of temporary and instantaneous polarities across a molecule gives rise to LDF. These polarities form due to circulations of electrons.

If there is an instantaneous polarity in a molecule, then that molecule can induce the polarity of opposite sign in an adjacent molecule. This results in a series of attractive forces among nearby molecules. It’s a very well known fact that molecules that have higher molecular weights have more electrons. Being more in number the electron cloud becomes more deformable due to nearby charges, and this characteristic is known as polarizability. This proves that molecules that have higher molecular weights have high LDF and this makes their melting point, boiling point and enthalpies of vaporization go higher.

Dipole-Dipole Interactions: Molecules that have permanent dipole moments because of uneven sharing of electrons, experience dipole-dipole interactions. Due to the uneven sharing of electrons, one side of the molecule becomes partially positive and the other side becomes partially negative. Substances that have dipole-dipole interactions usually have a higher melting point and boiling point as compared to molecules that only have LDF.

Hydrogen Bonds: A very strong dipole gets formed when a hydrogen atom gets covalently bonded with nitrogen, oxygen or fluorine. These dipoles give rise to dipole-dipole interactions, and these interactions are termed hydrogen bonding. The hydrogen bond is a special case of dipole-dipole interaction.

### What is Kinetic Theory?

The theory that completely describes the gases behaviour. It also assumes that gas is made up of fast-moving atoms or molecules. As solids are hard there is no intermolecular space in them, they are highly densely packed. These intermolecular gaps are larger in liquids as compared to solids. In gases, they are very loosely packed because of very high and random intermolecular spaces.

The kinetic theory completely explains the random movements of these molecules of a gas. The kinetic theory explains the following things: Pressure and temperature interpretation at the molecular level is described. It goes along with Avogadro’s theory and various gas laws. It also explains the specific heat capacity of a variety of gases.

### Molecular Nature of Matter

Many hypotheses about the atomic behaviour of matter have been proposed by various Scientists. According to these hypotheses, everything in the universe is made up of atoms. Atoms are nothing but just tiny particles that always move around in some order. They attract each other when the distance between them reduces. But when they are forced to be in very close proximity to each other, they repent because of having the same charges.

One such theory was proposed by Dalton, and it came to be known as the molecular theory of matter. This theory results in that matter is made up of molecules and those molecules are made up of atoms.

• According to Gay Lussac’s law, when two or more gases combine chemically to form new gases, their volumes are in small integer ratios.
• According to Avogadro’s law, all gases at the same temperature and pressure have the same number of molecules in equal volumes.
• All these principles are proof of the molecular nature of gases. Kinetic theory is based on Dalton’s molecular theory.
• Dalton’s theory was a success because he said that matter is made up of molecules and they are made up of atoms, and now we can observe atomic structure using an electronic microscope.

### Sample Questions

Question 1: Define Boyle’s law.

According to Boyle’s Law, the pressure of a gas is inversely proportional with the volume, given that the temperature and number of moles of the gas are fixed.

Question 2: Why do helium and hydrogen not liquefy at room temperature even after applying very high pressure?

Hydrogen and helium have a critical temperature that is lower than the room temperature. For the liquefaction of gas, the temperature must be lower than the critical temperature.

Question 3: What are matter particles?

The matter is made up of elementary particles at the most fundamental level, such as quarks and leptons. This class of elementary particles also includes electrons. Quarks get fused into protons and neutrons and form atoms of the elements of the periodic table, such as hydrogen, oxygen, and iron, along with electrons.

Question 4: What is the Significance of Avogadro’s Law?