Molecular Nature of Matter: Dalton’s Atomic Theory, Avogadro’s Law, Evolution

Molecular Nature of Matter: Since junior classes, we have studied matter. Anything that occupies space and has some mass is known as matter. This is the basic definition which you might have studied earlier. So, matter is the material substance that contributes to the observable universe and along with energy, forms all the phenomena. Matter generally exists in three states viz. sold, liquid and gas. 

Everything in the universe is made up of small particles called atoms, and atoms combine to form molecules. Thus, all the states of matter depend upon the placement of molecules. For example, in a solid state, molecules are closely arranged with respect to each other. They are tightly bound so that there is almost no gap between them. That’s why they have a definite shape and volume. Whereas in liquid, there is a small significant amount of gap between its molecules, and hence it has a definite volume but no definite shape. It moulds itself in the shape of the container in which it is poured. And finally, for gas, its constituent molecules are far away from each other, and hence, it doesn’t have any definite shape and volume. 

In this article, we will study in detail about the molecular nature of matter and its properties. 

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Dalton’s Atomic Theory

Dalton was the first one to theorize atoms in \(1803\). He gave a theory of chemical combinations. For a long time, his theory was considered to be concrete because, at that time, atoms were considered to be the smallest particles of any element. Until the discovery of subatomic particles like electrons, protons and neutrons. The important points of his theory are given as below:

1. All matter is made up of indivisible particles called atoms
2. Atoms of the same element are the same in shape and size, but they differ from atoms of different element
3. Atoms can neither be created nor destroyed
4. Atoms of different elements may combine with each other in a fixed simple ratio to form compounds
5. Atoms of the same element can also combine in different proportions to form compounds
6. Atoms are the smallest particle of any element that can take part in any chemical reaction

This theory successfully explained the laws of chemical combinations, and Dalton was the first person to differentiate between atoms and a compound or molecule. Although later, there were some drawbacks of this theory because of the discovery of some different particles. The drawbacks of Dalton’s theory are as follows:

1. Atoms are not indivisible. It can be further divided into subatomic particles like electrons, protons and neutrons.
2. According to this theory, all the atoms of the same element are similar in all respect. But there were a few atoms discovered of some elements which differed in mass or density. They had the same atomic number but different masses. These types of atoms were called isotopes. 
3. It was also stated that atoms of different elements are completely different. Later we discovered atoms of different elements with the same mass. They were known as isobars. 
4. According to this theory, to form compounds, atoms are mixed in simple proportions. But for some complex compounds like sugar or glucose, the proportions were complicated.
5. It does not explain allotropes like graphite, diamond etc.

Avogadro’s Law

Avogadro’s law deals with the amount of gas. It is an experimental gas law and is stated for an ideal gas. According to Avogadro’s law, at constant pressure, volume and temperature, the number of molecules for any gas is always constant no matter what the gas is. This amount is known as Avogadro’s number \({{\rm{N}}_{\rm{A}}} = 6.022 \times {10^{23}}\) molecules which is considered to be \(1\) mole. Thus, \(1\) mole of gas contains a number of molecules equal to the Avogadro number. This simple law has helped us to develop the complete mole concept, which gives the equation of the number of moles of a gas,

\({\rm{Number}}\,{\rm{of}}\,{\rm{moles}}\, = \,\frac{{{\rm{Given}}\,{\rm{number}}\,{\rm{of}}\,{\rm{particles}}\,{\rm{of}}\,{\rm{a}}\,{\rm{gas}}}}{{{\rm{Avogadro’s}}\,{\rm{number}}}} = \frac{{{\rm{Given}}\,{\rm{number}}\,{\rm{of}}\,{\rm{mass}}}}{{{\rm{Molecular/Atomic}}\,{\rm{mass}}}}\)

Gay-Lussac’s Law

This law deals with the relation between pressure and temperature of a gas. It states that at constant volume, the pressure of an ideal gas is directly proportional to its absolute temperature. 

Imagine you have a container with gas-filled inside is covered with a lid that cannot slide inside it. If we increase the temperature of the gas inside the container, its energy will start to increase. As the energy of molecules increases, they get more excited, and their random motion will also increase, which will increase collision among them and with the wall of the container. This will increase the pressure inside the container because the lid cannot move, and hence the volume of the container will remain constant. And thus, at constant volume, pressure increases as temperature increases. Also, if the temperature starts decreasing, molecules start to cool down and lose their energy and hence pressure is reduced. 

This is the first theory that gives the relation between pressure and temperature for a fixed volume of gas.

Evolution of Current Molecular Nature of Matter

All of these theories were proved to be successful until the late 19^th century, when many other scientists found demerits in Dalton’s theory. It is yet considered to be the fundamental law for any elements, but the further discovery of subatomic particles led to its downfall. Subatomic particles like protons, neutrons and electrons existed in different proportions in atoms, and with the help of them, we were able to successfully explain the existence of isotopes, isobars, isomers, allotropes, etc. based on their existing proportions and arrangement. An electron microscope was made, and we were more clearly able to visualize these tiny particles, and new theories emerged. This discovery proved Dalton’s statement incorrect; as mentioned earlier, those atoms are the fundamental elementary particles. But later on, it was also seen that the protons, neutrons and electrons are made up of even smaller particles known as quarks. 

Further, other different particles like muons, mesons, pions, neutrino, etc., led to quantum theory which deals with physics at the subatomic level. Also, all the matter in the universe are further represented as waves which give us the dual nature of matter, and further scientists are trying to represent elementary particles as a string which is known as the string theory.

Summary

Reading this article, we came to know in detail about the molecular theory for a matter which helped us to understand how matter is studied as molecules. With the help of the arrangement of molecules, three different states of matter were discovered that are solid liquid and gas. Further, to explain atoms at the microscopic level came Dalton’s theory which explained to us about atoms, their existence, and we got to know about the law of chemical combinations, which helped us a lot in understanding many chemical reactions and the proportion by which the compounds were made. Yet, due to advancements in technology, limitations were also found in Dalton’s theory.

Then we saw Avogadro’s law which gave us a fine understanding of the number of molecules in a gas that helped to simplify things further, and Gay-Lussac’s law, which explained about the relation of pressure and temperature of a gas at constant volume. 

Finally, we had an overview of the evolution of the current theories, which is helping us understand the functions at the subatomic level. 

Frequently Asked Questions

Q.1. What are the three states of matter?
Ans: The three fundamental states of matter are solid, liquid and gas.

Q.2. What are the molecular properties of matter?
Ans: The three basic properties are (1) they are very small in size, (2) they have space between them and (3) they are in constant motion and possess kinetic energy.

Q.3. How does the behaviour of molecules affect the state of matter?
Ans: For solid, the space between molecules is very less to negligible, and hence the molecules don’t have much motion. In liquid, the space between them is a little bit more than solid, and hence there is some space for them to move, and thus a liquid can flow. Whereas in gas, there is a lot of space between the molecules, and hence, they are to move anywhere.

Q.4. What are the two natures of matter?
Ans: The nature of matter is classified into two categories viz. (1) pure substance and (2) mixture or compound.

Q.5. State Avogadro’s law.
Ans: Avogadro’s law states that at constant temperature, pressure and volume, the amount of substance of any gas remains constant.

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