Dalton’s postulate that atom is indivisible tiny, hard solid ball was no more valid with the discovery of nucleus and electrons through cathode ray tubes experiment of J. J. Thompson. Plum pudding model of Thompson was one step ahead that of Dalton’s atomic theory.
No doubt that Dalton’s atomic theory was pioneer in the understanding matter in the form of atoms and theirs properties. It laid the foundation for modern atomic theory.
Who was J. J. Thompson?
Joseph John Thomson was a British physicist. His cathode ray experiments successfully discovered that atom is divisible. Earlier in the early 19th century, it was stated that atom is indivisible and basic unit of matter.
He was born on December 18, 1856, in England. He conducted cathode ray tubes experiments and discovered the election. It was a path breaking step. Later, he came up with plum pudding model of atom.
In 1906, J J Thompson awarded Nobel prize for his investigation on the conduction of electricity by gases.
Cathode ray tubes experiments by J. J. Thompson
- Cathode tube is a glass tube from which gases are evaluated. Air is at very low pressure.
- Two electrodes are placed on the either side of the tube.
- Cathode is negatively charged, while anode is positively charged.
- Both cathode and anode are connected with battery.
5.High voltage is passed through to ionize the air and make it a conductor of electricity. - It is observed that cathode ray travels in straight line.
- If we place positive and negative pole on the either side of the tube, it is observed that cathode ray gets deflected away from negative pole.
- Cathode rays are observed with the help of fluorescent screens (ZnS)
Final conclusion of cathode ray tubes experiments
- Cathode rays are negatively charged as they repelled away from nagative pole.
- It must be part of the atom itself.
- It is later named as basic fundamental particles “Electron”.
- Thompson concluded that electrons are the fundamental particles of all the atoms by experiment through changing cathode material.
Understanding plum pudding model of Thompson
In 1904, Thompson proposed the plum pudding model of atom. It was based on the discovery of electrons by the experiment of cathode ray tubes experiments.
As per the plum pudding model, atom is a sphere of positive charge, and negatively charged electrons are embedded in it. It is due to balance the positive charge as he stated that atom is electrically neutral.
Postulates of plum pudding model
- Atom is a sphere of positive charge, and negatively charged electrons are embedded in it.
- Atom is electrically neutral as the negative and positive charges are equal.
If you cut watermelon in two halves, you will see black seeds embedded in the red flesh or mesocarp and endocarp. Plum pudding model of Thompson was compared with watermelon.
Black seed are compared with negatively charged particles, and red flesh is compared with positively charged particles.
Surely, plum pudding model of Thompson challenged the validity of postulate of Dalton that atom is indivisible tiny hard sphere.
Thompson model on the basis of cathode ray tubes experiments, proved that atom is divisible and it is made up of negatively charged and positively charged particles.
But, he failed to explain other aspects that successfully addressed by Rutherford.
Drawbacks of plum pudding model of Thompson
Though Thompson stated that atom is electrically neutral. But, he failed to explain the stability of atom. He had no answer why and how positive charge holds together the negatively charged electrons in a atom to ensure stability and neutrality.
Also, his model failed to explain the scattering of alpha particles as was done by Rutherford.
His model wasn’t certain about the position of nucleus in atom.
All these limitations are successfully addressed by the Rutherford hypothesis that I’m going to explain in the next article.
Last words on plum pudding model of Thompson
So far, I have explained about Thompson, his cathode ray tubes experiments, plum pudding model, and limitations.
British physicist who was awarded Nobel prize for his investigation of on the conduction of electricity by gases, conducted cathode ray tubes experiments.
Second, he explained that the atom is not basic unit of matter but negatively charged and positively charged particles embedded in it.
Third, he challenged the validity of postulate of Dalton that atom is indivisible tiny hard particle.
Fourth, Thompson proved that atom is electrically neutral as the negative charge is equal to positive.
But, he failed to explain the stability of atom and position of nucleus in atom. Later, it was explained by the Rutherford hypothesis.
Solved questions on plum pudding model of Thompson
For better clarity of topic, here, I’m going to provide some important solved questions.
Q. 1. Who was J. J. Thompson
Ans: He was British physicist. He discovered negatively charged particles by the means of cathode ray tubes experiments. He was awarded Nobel prize for his investigation on the conduction of electricity by gases. Later, he proposed his plum pudding model of atom.
Q. 2. What is mean by cathode ray tubes experiments?
Ans: It is a scientific experiment done by using glass tube with low air pressure. It discovered negatively charged cathode ray after applying high voltage through cathode to anode. Cathode ray repelled away from nagative pole proved that there is negatively charged particles. Later, it was named as electrons.
Q. 3. What is plum pudding model of Thompson?
Ans: Plum pudding model of Thompson was compared with watermelon. Black seed are compared with negatively charged particles, and red flesh is compared with positively charged particles. He stated that atom is electrically neutral.
4. What are the limitations of plum pudding model?
Ans: Thompson failed to explain the stability of atom and position of nucleus in atom. He had no answer why and how positive charge holds together the negatively charged electrons in a atom to ensure stability and neutrality.
At last,This is all about the cathode ray tubes experiments, plum pudding model of Thompson, short summary and solved questions. I hope that this subject matter will help students in their preparation for exams. In the next article, I’m going to provide a detailed explanation of Rutherford hypothesis.
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