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Structure of Atom

Thomson's Atomic Model

The Atomic Model 


Atom Is Divisible

Do you recall Dalton’s atomic theory? Dalton postulated in his theory that an atom is indivisible. However, the later discoveries of [[mn:glossary]]protons[[/mn:glossary]] and [[mn:glossary]]electrons[[/mn:glossary]] proved this to be erroneous. 

In 1886, while carrying out an experiment in a gas discharge tube, E. Goldstein discovered positively charged radiations which led to the discovery of the subatomic particles called protons. Later, in 1897, J. J. Thomson discovered another type of subatomic particle—the negatively charged electron. Consequent to these discoveries, an atom was no longer indivisible; rather, it became a sum total of differently charged subatomic particles.

We know that an atom is neutral. It is made up of an equal number of oppositely charged particles—protons and electrons. Now, the question that arises is this:

How are the subatomic particles arranged inside an atom?

Many scientists performed varied experiments to develop different models for the structure of an atom. The first such model was proposed by J. J. Thomson. His atomic model is compared to a plum pudding and a watermelon; hence, it is known by the names ‘the plum-pudding model’.


The Plum-Pudding Model of an Atom 

Let us understand Thomson’s atomic model with the help of a slice of a watermelon. The slice consists of a red edible portion with embedded black seeds. Now, if we liken this watermelon to an atom, then (as per Thomson’s model) the positive charge in the atom is spread all over the red edible part; and the negatively charged particles, like the seeds, are embedded in this positively charged space.

In the same way, we can liken an atom to a plum pudding. In this case, the positive charge is spread all over the pudding, while the negatively charged particles are embedded like plums in this positively charged space. 

According to Thomson’s atomic model:

1. An atom consists of a positively charged sphere with electrons embedded in it.

2. The negative and positive charges present inside an atom are equal in magnitude. Therefore, an atom as a whole is electrically neutral.



Cathode Rays

J.J Thomson discovered that there are small particle present in the atom and that atom is divisible. J.J Thomson and his colleagues conducted experiments using discharge tube apparatus.
A discharge tube apparatus consists of a glass tube of about 15 cm length and 3 cm in diameter, filled with gas at low pressure. The tube is connected with the vacuum pump and two metal electrodes are fitted to the ends of the tube.

Low pressure was created inside the tube and high voltage was applied to the electrodes of the tube. This produced greenish glow at the anode end of the tube. The greenish glow at anode was produced due to the emission of the streams of rays from the cathode. These rays are known as cathode rays. Cathode rays will emit with blue glow.
When J.J Thomson placed a light paddle wheel inside the tube in the path of the cathode rays, the wheel started rotating. This led him to conclude that cathode rays are particulate in nature.


Properties of Cathode Rays

When J.J Thomson applied an electric field in the direction parallel to the path of cathode rays , he observed that the rays were deflected towards the anode.

This observation led to the conclusion that cathode rays are negatively charged.

When the above experiment was conducted with different gases, same observation were made a...

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