Thomson’s Atomic Model: Characteristics, Postulates, Subatomic Particles

The

Thomson’s atomic model

It was created by the celebrated 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 became clear that the atom was not an indivisible entity, as Dalton had postulated in the preceding model, but that it 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 stated that the electrically neutral atom was made up of positive and negative charges of equal magnitude. 

Article index

  • one

    What was Thomson’s atomic model called and why?

  • two

    Characteristics and postulates of the Thomson model

    • 2.1

      Cathode rays

  • 3

    Subatomic particles from Thomson’s atomic model

    • 3.1

      Crookes tube

    • 3.2

      Charged particles in uniform electric and magnetic fields

  • 4

    Thomson’s experiment

    • 4.1

      The charge-mass ratio of the electron

  • 5

    Thomson and Dalton model differences

  • 6

    Model flaws and limitations

  • 7

    Articles of interest

  • 8

    References

What was Thomson’s atomic model called and why?

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

Thomson's atomic model

Joseph John Thomson

Although Thomson’s idea looks quite primitive today, for its time it represented a novel contribution. During the brief life of the model (from 1904 to 1910), it had the support of many scientists, although many others considered it heresy. 

Finally in 1910 new evidence emerged about atomic structure, and Thomson’s model quickly fell to the side. 
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 fruit 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 based on several observations. The first was that the X-rays recently discovered by Roentgen were capable of ionizing air molecules. Until then, the only way to ionize was by chemically separating ions in solution.

But the English physicist managed to successfully ionize even monatomic gases such as helium, using X-rays. This led him to believe that the charge inside the atom could be separated, and that therefore it was not indivisible. He also observed that cathode rays they could be deflected by electric and magnetic fields.

JJ Thomson, discoverer of the electron. Source: Lifeder.

So Thomson devised a model that correctly explained the fact that the atom is electrically neutral and that cathode rays are composed 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
-10
m.

-The positive charge is distributed more or less uniformly throughout the sphere.

-The atom contains negatively charged “corpuscles”, which ensure its neutrality.

-These corpuscles are the same for all matter.

-When the atom is in equilibrium, there is

n

corpuscles regularly arranged in rings within the sphere of positive charge.

-The mass of the atom is evenly distributed.

Cathode rays

Beam of electrons are directed from the cathode to the anode.

Thomson carried out his experiments using cathode rays, discovered in 1859. Cathode rays are bundles of negatively charged particles. To produce them, vacuum glass tubes are used, in which two electrodes are placed, called

cathode

Y

anode

An electric current is then passed that heats up the cathode, which in this way emits invisible radiation that is directed directly to the opposite electrode. 

To detect radiation, which is nothing other than cathode rays, the wall of the tube behind the anode is covered with a fluorescent material. When the radiation reaches there, the wall of the tube gives off an intense luminosity.

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

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

Subatomic particles of the 
Thomson’s atomic model

Thomson’s atomic model is, as we said, the first to postulate the existence of subatomic particles. Thomson’s corpuscles are nothing other than electrons, the negatively charged fundamental particles of the atom.

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

But these were not discovered at the time Thomson developed his model. The positive charge in the atom was distributed in it, it did not consider any particle to carry this charge and at the moment there was no evidence of its existence.

For this reason his model had a fleeting existence, since in the course of 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 some 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.

Cathode ray tube scheme. Source: Knight, R.

Thus, 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, in the same way that negatively charged particles were.
  3. Any metal used to make the cathode was equally good at producing cathode rays, and their behavior was independent of material.

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

But instead it was not understood how it was possible for magnetic fields to deflect cathode rays. This could only be explained if said rays were considered as particles, a hypothesis that Thomson shared.

Charged particles in uniform electric and magnetic fields

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

F
and
= qE

When a charged particle perpendicularly passes through a uniform electric field, such as that produced between two plates with opposite charges, it experiences a deflection, and consequently an acceleration:

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
that you experience has the following intensity:

F
m
= qvB

As long as the velocity and magnetic field vectors are perpendicular. When a charged particle strikes a homogeneous magnetic field perpendicularly, it also undergoes a deflection and its motion is uniform circular.

The centripetal acceleration at
c
in this case it is:

qvB = ma c

In turn, the centripetal acceleration is related to the speed of the particle v and the radius R of the circular path:

a c = v
two
/ R

Therefore:

qvB = mv 2
/ R

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

R = mv / qB

These equations will be used later to recreate the way Thomson derived the charge-mass relationship of the electron.

Thomson’s experiment

Thomson passed a beam of cathode rays, a beam of electrons, although he did not know it yet, through uniform electric fields. These fields are created between two charged conductive plates separated by a small distance. 

He also passed cathode rays through a uniform magnetic field, observing the effect this had on the beam. In one field as well as the other, there was a deflection in the rays, which led Thomson to think, correctly, that the beam was composed of charged particles.

To verify this, Thomson carried out several strategies with cathode rays:

  1. He varied the electric and magnetic fields until the forces canceled out. In this way the cathode rays passed through without experiencing deflection. By equating the electric and magnetic forces, Thomson was able to determine the velocity of the particles in the beam.
  2. It annulled the intensity of the electric field, in this way 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-mass relationship of the “corpuscles.”

The charge-mass ratio of the electron

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

q / m = 1.758820 x 10
eleven
C.kg
-one
.

Where q represents the charge of the “corpuscle”, which is actually the electron and

m

is the mass of it. Thomson followed the procedure described in the previous section, which we recreate here step by step, with the equations he used.

When cathode rays pass through crossed electric and magnetic fields, they pass without deflection. When the electric field is canceled, they hit the upper part of the tube (the magnetic field is indicated by the blue dots between the electrodes). Source: Knight, R.

Step 1

Equalizing the electric force and the magnetic force, passing the beam through the perpendicular electric and magnetic fields:

qvB = qE

Step 2

Determine the speed acquired by the beam particles when they pass directly 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 it results:

R = mE / qB 2

The radius of the orbit can be measured, therefore:

q / m = v / RB

O well:

q / m = E / RB 2

Next steps

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

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

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

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

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

Thomson and Dalton model differences

The fundamental difference of these two models is that Dalton thought that 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:

Dalton Atom
As we have seen previously, Thomson thought that the atom was divisible, and whose structure is formed by a positive sphere and electrons around it.

Model flaws and limitations

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

But in fact Thomson did not even call his particles “electrons”, although the term had already been coined previously by George Johnstone Stoney. Thomson simply called them “corpuscles.”

Although Thomson made use of all the knowledge available to him at the time, there are several important limitations in his model, which became apparent very early on:

The positive charge is not distributed throughout the atom

. The Rutherford scattering experiments showed that the positive charge of the atom 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 instead have an arrangement in orbitals that later models revealed.

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

The atomic nucleus is the one that contains most of the mass.

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

It is also important to note that this model of the atom did not allow us to infer the type of movement that the electrons had within the atom.

Articles of interest

Schrödinger’s atomic model.

Atomic de Broglie model.

Atomic model of Chadwick.

Heisenberg atomic model.

Perrin’s atomic model.

Dalton’s atomic model.

Dirac Jordan atomic model.

Atomic model of Democritus.

Atomic model of Leucippus.

Bohr’s atomic model.

Sommerfeld atomic model.

Current atomic model.

References

  1. Andriessen, M. 2001. HSC Course. Physics 2. Jacaranda HSC Science.
  2. Arfken, G. 1984. University Physics. Academic Press.
  3. Knight, R. 2017. Physics for Scientists and Engineering: a Strategy Approach. Pearson.
  4. Rex, A. 2011.  Fundamentals of Physics . Pearson.
  5. Wikipedia. Thomson’s atomic model. Recovered from: es.wikipedia.org.

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