Atomic Theory by JJ Thomson – Structure – Model – Experiment

Atomic theory is one of the most basic study in chemistry. This theory opened the possibility to discover other new theories which then will be very useful in the chemistry study in general. One of the early scientist who discovered chemistry model of atoms was J.J. Thomson. He conducted the experiment to find out the new part in atom after the Dalton’s discovery.

He was the one who successfully discovered electrons in atom. JJ Thomson redefined the structure of atom. The atomic theory of JJ Thomson is not only beneficial for atomic study but also other fields including the invention of mass spectograph. He is one of the most influential people in the atomic study. This article will cover atomic theory by JJ Thomson, his early life and other atomic theories after his discovery.

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Prior Study by Dalton that Triggered JJ Thomson 

In 1803, John Dalton released his atomic theory which will then become one of the most useful theory in chemistry. John Dalton conducted the atomic theory called the five postulates of John Dalton.  His theory has five basic principle as following points:

  1. All mater contains very small particles which are called atoms. He belived that atoms has the small shape and solid spheres form. He also said that atoms have various motions
  2. Atom can not be destructed and changed. The atoms in element can not be created, destroyed, divided or transformed. He used the Antoine Lavoisier theory to support this point
  3. The weight of atom determines the characteristic of atom. Dalton believed that all atoms in the same element must have same weights. Every single atoms in oxygen is same to another. While atoms in the different element will have the different characteristic from one to another.
  4. Atoms combine in the small and whole-all rations in chemical reactions. Based on Dalton’s experiment, he concluded that the chemical reactions will occur based on atom to atom ratios
  5. Atoms may combine in more than one-all ratios in element reactions. There were multiples number ratios in various compounds like oxygen compound.

Prior to Dalton’s atomic theory, the famous Greek philosopher, Democritus, discovered the very basic theory about atom. He created the theory from the concept of there should be the part of matter which is extremely small that we as a human can not see. Democritus called this part as the atomos. It was indivisible in Greek language. He believed that atomos have different shape and dimension. This basic atomic theory triggered other scientists including JJ Thomson to perform experiment to explain the atomic structure model.

Also read: Proton Uses

Atom Model by JJ Thompson

After the discovery of Dalton, the scientist started to develop other great theories and experiments to help us understand the chemical models of atom. With the help of better laboratory tools, scientists were able to conduct more detailed experiment in defining atom model. In 18977, J.J Thomson performed several experiments to examine the structure of atom using the cathode ray application.

He did the study about electric discharge in high vacuum cathode ray tube. Actually, there were many other scientists who conducted the experiment on the similar field. After these experiment, JJ Thomson successfully discovered the electron part in the atom. He did the experiment using the cathode ray. He represented the cathode rays as the negative charge.

Based on this experiment, he released the concept of atom model as the plum pudding. The raisins represent the negative charge electrons while the dough represents the positive charges or protons of atom. He proved that atom consists of electrons and protons. He stated that the atom has the neutral characteristic which means that the amount of electrons is same with the amount of protons in one atom. Atom has positive charged protons which are surrounded by the negative charged electrons. Thus, that’s the atomic theory by JJ Thomson.

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JJ Thomson Experiment

As we mentioned previously, JJ Thomson performed some experiment in his laboratory to find out the true model of atom. In that period of time, some other scientists also conducted similar experiment using the cathode ray tube to understand the atom model. They did pass the electric current to the vacuum tube. In this experiment, scientists could see the glowing light inside that vacuum tube.

The light moved in the straight line. However, they could not successfully find the reason to explain this condition. At that time JJ Thomson conducted more experiments to find out this case using cathode tube method. He used the two electric plates on its path. One plate is the positive charge electric plate while another plate is the negative charge plate. His experiment showed that the glowing light bent to the positive charge plate. It also resulted that the glowing materials in the light bent to the positive charge plate.

JJ Thomson identified that the glowing light were the small particles. That glowing light particles were smaller than the atom. Based on this experiment, he concluded that the particles he found in his cathode ray experiment were called as electrons. This experiment was the first step of the JJ Thomson’s atomic theory.

Properties of Electrons

Based on J.J Thomson’s experiment, he could compose the properties of electrons. He also included these properties to the atomic theory by JJ Thomson. Here’re some electrons properties from the JJ Thomson atomic experiments:

  1. Electrons travel in the straight path. JJ Thomson conducted this experiment using the maltese cross. When he put the maltese cross on the electrons’ path, the cross’s shadow can be seen on the wall of the tube which was on the opposite side of the cathode. Then, the maltase cross was placed in the lower position. It showed that the shadow was disappeared as the cross was placed lower. So, it showed that electrons moved in the straight direction.
  2. Electrons have kinetic energy and momentum. In the cathode rays path, you can place the light wheel of mica. Then, you will see that the wheel rotates to the anode. This showed that the cathode rays or electrons have momentum and kinetic energy.
  3. Electrons make heat when striking. In the center of cathode with concave shape, you can place the platinum strip. This platinum strip has the black / dark color. Then, when the cathode rays struck this platinum, the platinum strip will change its color to red hot. It shows that electrons can produce heat when it strikes another object.
  4. Electrons emit x-rays
  5. Electrons can deflect if the electric filed and magnetic field are present
  6. Electrons move in the high level of velocity
  7. Electrons are able to make fluorescence when they strike objects like minerals, crystals and salts.

These properties of electrons are really useful to perform further studies about electrons and atomic theory in general. JJ Thomson put the great work to other scientists so that they can work on further researches using this concept.

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Limitations of Atomic Theory by JJ Thomson

Since the atomic model constructed by JJ Thomson was one of the earliest atomic theory, it’s understandable that his atomic theory still contains some flaws. There were some inconsistency in his atomic theory and his atomic model. JJ Thomson proposed the neutrality in the atom. He believed that the total mass in atom is uniform in all atom’s particles.

However, this point was not entirely true as it showed the the different results from the experiments. After JJ Thomson’s experiment, Rutherford could prove that the most of space in the atom is empty. So that the mass of atom is not uniformly distributed. We can also conclude that the atomic model of plum pudding is no longer applicable to explain the structure model of atom.

The model of plum pudding was lack of experiment proves, so that other scientist tried to conduct more experiment to test his atomic model. Many scientists at that time were eager to perform further studies and experiments to explain the atomic model in more detail. One of the successor of atomic theory of JJ Thomson was Rutherford. He then did the great experiment which explained the flaws in the JJ Thomson’s atomic model.

After Thompson’s atomic theory, Ernest Rutherford, Thompson’s student, conducted the experiment which countered  his teacher’s atomic theory. In 1911, he performed the experiment using the alpha particles. He shoot the alpha particle through the gold foil. This experiment resulted that most of the alpha particles passed through the gold foil.

However, there were few alpha particles which are deflected back. Rutherford believed that there were the positive charge nucleus. in the center of atom and negative charge electron around the nucleus. The nucleus finding by Rutherford is really helpful to understand the chemistry study, especially the chemical models of atom. To sum up, that is the limitation of atomic theory by JJ Thomson.

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J.J. Thomson Brief History

We know Thomson as one of the greatest physics and scientist in the world history. At the early age, J.J. Thomson’s father forced him to be the an engineer. Unfortunately, Thomson’s family could not afford the education fee for becoming an engineer as the engineer position required some apprenticeships at that time. So, J.J. Thomson joined the Owens College in Manchester.

They had the great faculty of science. After studying in Manchester, he was recommended to continue his study to Trinity College in Cambridge. In that university, he successfully became the mathematical physicist. After performed some experiments in Trinity College, in 1884 he got the title of Cavendish Professorship of Experimental Physics at Cambridge.

He was known for his brilliant mind in conducting apparatus in experiment and his analysis of the problems. He was also popularly known as the great lecturer who often encouraged his students. After his many great experiments, J.J. Thomson gained some honorable recognition from different constitution. His highest achievement was the Nobel Prize in Physics in 1906 and a Knighthood in 1908.

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