Atomic Theory

Atomic Theory - Dalton's Theory and Thomson's Theory

The word Atom comes from the pristine Greek word “invisible”. In the 19th-century, chemists began using the term in connection with irreducible chemical elements. 

According to the current theoretical model of the atom, it is a dense nucleus covered by a probabilistic "cloud" of electrons. The smallest piece of an element that maintains the identity of that element is known as an atom.

John Dalton (1766-1844) was an English chemist, physicist, and meteorologist. He is known for introducing the atomic theory into physics. He is also called the creator of proposing the atomic theory. This theory explains some concept that is relevant in the observable world. 

John Dalton used a basic concept for his theory: the law of conservation of mass and the law of constant composition. 


According to the law of conversion of mass, the total mass available before chemical reaction is the same as the total mass available after the chemical reaction. In other words, we can explain this theory as“mass is conserved”. The law of conservation of mass was formulated by Antoine Lavoisier in 1743- 1794. 

Atomic theory is a theory that classifies many elementary states, facts, and properties including both postulate and axiom about atoms. According to Dalton, the atomic model consists of an atom in hard spheres. According to Rutherford, atomic model includes atoms to view in the shape of solar systems. 

The concept that atoms play a major role to get formalized by the modern atomic theory consists of the following three parts:-

• All matter is made from the atoms.
• Atoms of the different elements are different and atoms of the same elements are the same.
• Atoms are the combination of whole numbers and ratio that are combined to form the compound.

These concepts comprise of the basic building blocks of chemistry. We all know that Atom comes from the Greek word “invisible” and that the atoms are composed of smaller parts known as subatomic particles. The first part to be explored was the electron. It is also known as a small subatomic particle with a negative charge. The proton is still a tiny subatomic particle with a positive charge. It is represented as P+. The Neutron is a subatomic particle with about the same mass as that of the proton but has no charge. It is represented as n or n0. Atoms have proton and neutrons in the center and electron at the orbit in the nucleus. 

All hydrogen atoms have only one proton in their nucleus; all iron atoms have 26 protons in their nucleus. With the advancement of quantum mechanics and Schrodinger equation, the atomic theory has become an accurate mathematical science. Atomic radii are 10-8 cm. It contains three subatomic particles named electron, proton, and neutrons. The electron was discovered in the theory of cathode rays by JJ Thomson. It carried negative charges (-1.6*10-19c). Rutherford discovered proton on the basis of anode rays experiments. It carried positive charges (+1.6*10-19c). Neutrons are neutral particles and it was discovered by Chadwick. The total mass of the neutron is 1.675*10-24g or 1.008665 amu. 

94 Be+ 42He+ → 12 6C+ 1 0n

An atomic number of the elements correspond to the total number of protons available in the nucleus and the total number of electrons available in neutron atom. 

Mass number of elements= Number of protons+ Number of neutrons


The story of Atomic Theory:

Here we state the whole specification about the atomic theory. The Atomic theory is the scientific theory which defines the nature and behaviour of the Atom. The matter is composed of separate units known as Atoms. There are multiple theories given by the scientists named below:-

• Dalton's Theory
• J.J. Thomson's Theory
• Rutherford's Hypothesis
• Bohr's Theory
• Einstein, Heisenberg and Quantum Mechanics
• Quark Theory

According to this theory, everything in the world is made up of tiny particle and it is surrounded by empty space. It varies in size and shape. These particles are known as the atoms. The first theory about the atoms was invented by Dalton and his theory is named as Dalton’s Theory. He was an English physicist, born in 1766 at Eaglesfield, Cumbria, United Kingdom.


According to Dalton’s atomic Theory, every element in the space is composed of small particles that are indivisible and cannot be seen by the naked eye. These particles are known as atoms. Atoms are imperishable as “Atoms cannot be made nor be destroyed by the human being”. All the elements of the atoms are identical in mass and the behaviour of the atom of one element is different from the behaviour of the atoms of all the other elements.

There were some issues in Dalton’s theory so his idea was opposed by the prominent philosophers of that time. Aristotle at that instance disagreed completely stating instead that matter involved four elements which are earth, wind, water and fire, and after that scientists followed this suit. Atomic theory was now accepted by the scientific community, but the next advancement was through the discovery by J. J. Thomson’s in the year 1897. He defined in his theory that the atoms as uniformly packed spheres of positive charges matter filled with the negatively charged electron.
 
Thomson won Nobel Prize for this research. But his model of the atom did not go around for long. Ernest Rutherford invented a new theory. It is known as the Rutherford model and is generally compared to the Thomson’s model. According to Thomson’s model, the atom thinly disperses positive charges and there would not be enough space to recurve the particle in any one place. The effect would have been like a wad of tennis balls punching via a thin paper screen. But while most of the particle did cross some bounced right back, suggesting that the foil was more like a thick net with a very volumetric mesh. According to Rutherford, the atoms dwell largely in empty space with just a few electrons, while mass was concentrated in the center, which he termed as the nucleus. The Alpha particle crossed through the gaps but bounced back from the intensive, positively charged nucleus. 

But the atomic theory was not complete just yet. After some time another scientist named as Niels Bohr expended on Rutherford’s nuclear model. He confirmed that electrons orbit the nucleus at fixed energies and distance and had the ability to jump from one level to other levels in their orbits. However, they cannot exist in the space. Bohr’s planetary model took centre stage, but soon, it too gave rise to some difficulties. His experiment has shown that rather than simply being separate particles, electrons cumulatively behave like a wave. They are not confined to a particular point in space. 

After some time, again new research came in by Werner Heisenberg. He formulated his famous uncertainty principle. According to this, it was impossible to specify both the exact position and speed of electrons as they moved around an atom. The idea was that the electrons cannot be pinpointed but they exist within a range of eventual locations giving rise to the same quantum model of the atom, a fascinating theory whose absolute implications have yet to be fully grasped.