Thomson atomic model: characteristics, postulates, subatomic particles

The atomic model of Thomson was created by the famous English physicist JJ Thomson, who discovered the electron. For this discovery and his work on electrical conduction in gases, he was awarded the 1906 Nobel Prize in Physics.

From his work with cathode rays, it was clear that the atom was not an indivisible entity, as Dalton had postulated in the previous model, but contained a well-defined internal structure. 

Thomson made a model of the atom based on the results of his experiments with cathode rays. In it, he claimed that the electrically neutral atom was made up of positive and negative charges of equal magnitude. 

What was the name of the Thomson atomic model and why?

According to Thomson, the positive charge was distributed throughout the atom and the negative charges were incorporated into it like raisins in a pudding. From this comparison, the term “raisin pudding” emerged, as the model was informally known.

Although Thomson’s idea today seems quite primitive, at the time it represented a new contribution. During the model’s brief validity (from 1904 to 1910), it had the support of many scientists, although many others considered it heresy. 

Finally, in 1910, new evidence for atomic structure emerged and the Thomson model quickly fell behind. This happened as soon as Rutherford published the results of his scattering experiments, which revealed the existence of the atomic nucleus.

However, Thomson’s model was the first to postulate the existence of subatomic particles and its results were the result of fine and rigorous experimentation. In this way he set the precedent for all the discoveries that followed.

Characteristics and Postulates of the Thomson Model

Thomson arrived at his atomic model from several observations. The first was that Roentgen’s newly discovered X-rays were capable of ionizing air molecules. Until then, the only way to ionize was to chemically separate the ions in a solution.

But the English physicist was able to successfully ionize even monatomic gases like helium using X-rays. This led him to believe that the charge within the atom could be separated and therefore was not indivisible. deflected by electric and magnetic fields.

Thomson then created a model that correctly explained the fact that the atom is electrically neutral and that cathode rays are made up of negatively charged particles. 

Using experimental evidence, Thomson characterized the atom as follows:

-The atom is an electrically neutral solid sphere, with an approximate radius of 10 to 10 m.

-The positive charge is distributed more or less evenly across the entire sphere.

-The atom contains “corpuscles” with a negative charge, which guarantee its neutrality.

These corpuscles are the same for all matter.

-When the atom is in equilibrium, there are n corpuscles arranged regularly in rings within the positively charged sphere.

-The mass of the atom is evenly distributed.

cathode rays

Thomson performed his experiments using cathode rays, discovered in 1859. Cathode rays are beams of negatively charged particles. To produce them, glass tubes are used under vacuum, in which two electrodes, called cathode and anode , are placed . 

Next, an electric current is passed that heats the cathode, which emits invisible radiation that is directed directly to the opposite electrode. 

To detect radiation, which is nothing more than cathode rays, the tube wall behind the anode is covered with a fluorescent material. When the radiation gets there, the tube wall emits intense light.

If a solid object intersects the cathode rays, it casts a shadow on the tube wall. This indicates that the rays travel in a straight line and can also be easily blocked.

The nature of cathode rays was widely discussed, as their nature was unknown. Some thought they were waves of the electromagnetic type, while others held that they were particles. 

Subatomic  particles of the Thomson atomic model

Thomson’s atomic model is, as we said, the first to postulate the existence of subatomic particles. Thomson corpuscles are nothing more than electrons, the fundamental particles of the atom with a negative charge.

We now know that the other two fundamental particles are the positively charged proton and the uncharged neutron. 

But they weren’t discovered at the time Thomson produced his model. The positive charge on the atom was distributed, it did not consider any particle to carry that charge, and at the moment there was no evidence of its existence.

For this reason, his model had a fleeting existence, as, over a few years, Rutherford’s scattering experiments paved the way for the discovery of the proton. And as for the neutron, Rutherford himself proposed its existence a few years before it was finally discovered.

Crookes tube

Sir William Crookes (1832-1919) designed the tube that bears his name around 1870, with the intention of carefully studying the nature of cathode rays. He added electric fields and magnetic fields and observed that the rays were deflected by them.

In this way, Crookes and other researchers, including Thomson, found that:

  1. An electrical current was generated inside the cathode ray tube.
  2. The rays were deflected by the presence of magnetic fields, as well as negatively charged particles.
  3. Any metal used to make the cathode served equally well to produce cathode rays, and its behavior was independent of the material.

These observations fueled the discussion about the origin of cathode rays. Those who argued that they were waves relied on the fact that cathode rays could travel in a straight line. Furthermore, this hypothesis explained very well the shadow that an intermediate solid object was cast on the tube wall and, in certain circumstances, the waves caused fluorescence. 

But instead, it was not known how magnetic fields could deflect cathode rays. This was explained only if these rays were considered particles, a hypothesis that Thomson shared.

Charged particles in uniform electric and magnetic fields

A charged particle q experiences a force Fe in the middle of a uniform electric field E of magnitude:

F e = qE

When a charged particle crosses a uniform electric field perpendicularly, such as that which occurs between two plates with opposite charges, it experiences a deviation and therefore an acceleration:

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qE = ma

a = qE / m

On the other hand, if the charged particle moves with a velocity of magnitude v, in the middle of a uniform magnetic field of magnitude B, the magnetic force F m it experiences has the following intensity:

F m = qvB

As long as the velocity and magnetic field vectors are perpendicular. When a charged particle collides perpendicularly to a homogeneous magnetic field, it also deflects and its motion is uniformly circular.

The centripetal acceleration ac in this case is:

qvB = ma c

Centripetal acceleration, in turn, is related to the velocity of particle v and to the radius R of the circular path:

c = v 2 / R

Therefore:

qvB = mv 2 / R

The radius of the circular path can be calculated as follows:

R = mv / qB

These equations will be used later to recreate how Thomson deduced the charge-to-mass ratio of the electron.

Thomson Experience

Thomson passed a beam of cathode rays, a beam of electrons, although he didn’t know it yet, through uniform electric fields. These fields are created between two charged conductive plates a short distance apart. 

He also passed cathode rays through a uniform magnetic field, noting the effect this had on the beam. In both fields, there was a deflection of the rays, which led Thomson to think, correctly, that the beam was composed of charged particles.

To verify this, Thomson performed several strategies with cathode rays:

  1. He varied the electric and magnetic fields until the forces were cancelled. In this way, the cathode rays passed without deflection. By matching the electrical and magnetic forces, Thomson was able to determine the velocity of the particles in the beam.
  2. It canceled out the strength of the electric field, so the particles followed a circular path in the middle of the magnetic field.
  3. He combined the results of steps 1 and 2 to determine the charge-to-mass ratio of the “corpuscles”.

The electron’s mass-to-charge ratio

Thomson determined that the charge-to-mass ratio of the particles that made up the cathode ray beam had the following value:

q / m = 1.758820 x 10 11 C.kg -1 .

Where q represents the charge of the “corpuscle”, which is actually the electron and m is its mass. Thomson followed the procedure described in the previous section, which we recreate step-by-step here, with the equations he used.

Step 1

Equalize the electric force and the magnetic force by passing the beam through perpendicular electric and magnetic fields:

qvB = qE

Step 2

Determine the velocity acquired by the beam particles as they pass through without deflection:

v = E / B

step 3

Cancel the electric field, leaving only the magnetic field (now there is deflection):

R = mv / qB

With v = E / B, the result is:

R = mE / qB 2

The radius of the orbit can be measured, therefore:

q / m = v / RB

The good:

q / m = E / RB 2

next steps

The next thing Thomson did was measure the q/m ratio using cathodes made of different materials. As mentioned earlier, all metals emitted cathode rays with identical characteristics.

Thomson then compared their values ​​with those of the q / m ratio of the hydrogen ion, obtained by electrolysis and whose value is approximately 1 x 10 8 C / kg. The charge-to-mass ratio of the electron is approximately 1750 times that of the hydrogen ion.

Therefore, cathode rays had a much larger charge, or perhaps a much smaller mass, than the hydrogen ion. The hydrogen ion is simply a proton, the existence of which was known long after Rutherford’s scattering experiments.

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It is now known that the proton is almost 1800 times more massive than the electron and with a charge of equal magnitude and opposite sign to the electron.

Another important detail is that Thomson’s experiments did not directly determine the electric charge of the electron, nor the value of its mass separately. These values ​​were determined by Millikan’s experiments, begun in 1906.

Thomson and Dalton model differences

The fundamental difference between these two models is that Dalton thought the atom is a sphere. Unlike Thomson, he did not propose the existence of positive or negative charges. For Dalton, an atom looked like this:

As we saw earlier, Thomson thought that the atom itself was divisible and whose structure is formed by a positive sphere and electrons around it.

Model failures and limitations

At the time, Thomson’s atomic model was able to explain the chemical behavior of substances very well. He also correctly explained the phenomena that occurred in the cathode ray tube. 

But in fact, Thomson didn’t even call his particles “electrons,” although the term was already coined by George Johnstone Stoney. Thomson simply called them “corpuscles”.

Although Thomson used all the knowledge he had at the time, there are several important limitations in his model, which became evident very soon:

– The positive charge is not distributed over the entire atom . Rutherford’s scattering experiments showed that the atom’s positive charge is necessarily confined to a small region of the atom, which later became known as the atomic nucleus. 

– Electrons have a specific distribution within each atom . The electrons are not evenly distributed, like the raisins in the famous pudding, but have an arrangement in orbitals that later models have revealed.

It is precisely the arrangement of electrons in the atom that allows the elements to be organized by their characteristics and properties on the periodic table. This was an important limitation of the Thomson model, which did not explain how it was possible to order the elements in this way.

– The atomic nucleus is what contains most of the mass. Thomson’s model postulated that the atom’s mass was evenly distributed within it. But today we know that the mass of the atom is practically concentrated in the nucleus’ protons and neutrons.

It is also important to note that this model of the atom did not allow us to infer what kind of motion the electrons had inside the atom.

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