Exploring the Subatomic World : Electrons and Nuclei

Exploring the Subatomic World : Electrons and Nuclei

Reading Time: 6 minutes

Atoms were once thought to be indivisible and fundamental. Nineteenth century chemistry was based around atoms, culminating in Dmitry Mendeleev’s extraordinary Periodic Table. The discovery of the electron in 1897 led to a subatomic revolution, and the twentieth century has revealed the rich internal structure of the atom, culminating in the Standard Model, the subatomic counterpart of Mendeleev’s Table. In this series of brief articles, The Nerd Druid traces the wonderful history of the subatomic.

Subatomic particles

Protons, neutrons and electrons are the three most well-known subatomic particles. Earth has an abundance of free protons and electrons. All you need for the former is to somehow make electricity happen. An easy way to do that is to rub amber with fur 1. For the latter, water works pretty well, since it has plenty of free protons roaming about. Of course, it helps to keep in mind that protons are simply ionised hydrogen atoms.

The Electron

The ancient Greeks noticed that if you rub amber with fur, the amber tended to attract small light objects. This, along with lightning strikes, was the only connection humans had with electricity until modern times.

Image of Garfield (the cartoon cat character) rubbing his fur against Jon's pants. This creates static electricity. The charged fur repels each other.
Garfield rubbing his fur against Jon’s pants creates static electricity. The charged fur repels each other.

Cathode Rays

In the latter half of the nineteenth century, German and English physicists found that if you pull air out of sealed container and insert a cathode 2 into it, you see a glowing discharge. Pump out more air, lower the air pressure further, and this cathode discharge glows brighter. The pioneers of cathode ray physics were Johann Hittorf (Germany) and Eugen Goldstein (Germany); they did their work in 1869 and 1876 respectively.

William Crookes (England) created the first cathode ray tube in the 1870s by creating a high vacuum. He observed that the glow had now become a sort of a ray, moving from the cathode to the anode 3. Reckon we’d have to thank Crookes for all the televisions and older computer monitors.

Crookes also applied a magnetic field to the cathode rays and made them deflect. This showed that the rays were charged. Arthur Schuster (Germany-England) modified Crookes’ setup. He sandwiched the cathode ray between two parallel plates, one positive and one negative. The electric potential between the plates made the rays bend and strike the positive plate. This proved that the rays were negatively charged.

Discovery of the electron

Schuster didn’t stop there. By varying the current fed into his parallel capacitor, Schuster was able to vary the degree by which the cathode rays were deflected. He measured and tabulated these results, and was thus able to calculate the charge-to-mass (Q/m) ratio of the cathode rays. His results were astonishing–the Q/m ratio of the cathode rays seemed to be more than a thousand times what was expected!

Unfortunately, Schuster’s results were ignored, probably because they didn’t quite conform to what people knew about atoms. The prevalent idea at that time was that cathode rays were some sort of atoms or molecules, and thus they were expected to have a mass at least equal to the hydrogen ion.

Image of J.J. Thomson, English physicist. Experimentally obtained evidence for the existence of electrons. Thus showed that cathode rays are independent particles. Showed that these negatively charged particles are the same as the ones produced by radioactivity, by heated materials, and by illuminated materials. Suggested the plum pudding model for the distribution of positive charges and electrons within the atom. Discovery of the electron threw open the subatomic realm.
J.J. Thomson, English physicist

In 1897, J.J. Thomson (England), working with colleagues John Townsend (Ireland) and H.A. Wilson (England), showed that cathode rays were, indeed, individual charged particles. A few years later, Millikan and Fletcher performed the famous oil-drop experiment and accurately measured the charge of this new particle, the electron 4. Since the charge-to-mass ratio of the electron had already been measured, it was now simple to calculate the mass of the electron. Simply put, the electron has the same magnitude but opposite sign of the charge of a hydrogen ion. However, it had almost two thousandth its mass.

The Atomic Nucleus

Thanks to Thomson and his fellow cathode ray physicists, the atom was no longer a black box. Negatively charged cathode rays were actually particles called electrons, and they lived inside an atom. Sometimes some of these electrons would get knocked off the atom. The charged atom would then be a positive ion, a cation. At all other times, the atom would be strictly neutral. Clearly, the atom within contained an amount of positive charge equal to the negative charge it contained due to the electrons. The question was, where did the positive charge reside, and where were the electrons in relation to this?

Thomson had an answer to this. He envisaged electrons embedded within a uniform diffuse distribution of positive charge within the atom, much like plums in a pudding. The Geiger-Marsden experiment (1909) put paid to this plum pudding model soon enough.

Image of Thomson's plum pudding model (above) and Rutherford's Gold Foil experiment (below, aka the Geiger-Marsden expt.).
Thomson’s plum pudding model (above) and Rutherford’s Gold Foil experiment (below, aka the Geiger-Marsden experiment)

Hans Geiger 5 (England) and Ernest Marsden (England-New Zealand), working under the direction of Ernest Rutherford (New Zealand-England), fired positively charged alpha particles 6 at a metal foil. While most of the alpha particles whizzed through without any change in momentum, a tiny fraction (1-in-20000) deflected by almost 90 degrees. Rutherford concluded (in 1911) that this must be because all the positive charge in the atom is tightly packed inside a tiny volume at its centre. This, of course, is the atomic nucleus.

There were quite a few experiments performed by Geiger and Marsden. The most famous one is also referred to as Rutherford’s Gold foil experiment.

Nationalities

A quick aside about the nationalities of the people involved. A majority seem to be from England and Germany. The only two scientists from the US are Millikan and Fletcher, and they did their work in the 20th century. This seems to suggest that the nerve-centre of cutting edge physics in the nineteenth century was very much a few nations in Europe.

Nuclear density and pressure

The atom is mostly empty space 7 Atomic diameters are usually of ångström order (1 Å = 10-10 m). Nuclear diameters are a hundred thousand times smaller, usually of femtometer order (1 fm = 10-15 m). However, nuclei carry most of the atom’s mass. Due to their small sizes, they are incredibly dense objects.

A quick back-of-the-envelope calculation for carbon (C-12 isotope) shows that its nuclear density is approximately 1.25×1015 gm/cc. To put that into perspective, the density of ordinary water, under normal atmospheric pressure and standard room temperature, is approximately 1 gm/cc. Atomic nuclei are hundreds of trillion times denser than tap water.

Before we proceed, I’m going to presume that you are aware that atomic nuclei contain protons and neutrons. I’ll get to them in the next part of this series.

You’d think that such high densities would mean that the pressure inside the nucleus would be immense. You’d be correct, but there are places in this universe which make nuclear pressures seem like cotton candy. I’m talking about the interior of neutron stars, where pressures reach absurd values of 1034 pascal. In comparison, standard atmospheric pressure is about a hundred thousand pascal, 105. This of course makes neutron star cores very very hot. The story of how they cool down is quite interesting.

Up until a few days ago, this was the highest pressure found in the universe. Recently however, scientists at Jefferson Lab (the USA) have found that proton pressure is ten times that inside neutron stars. That is a mind-boggling million trillion trillion times that of standard Earth sea-level pressure.

Yes. A million trillion trillion times. 10 followed by 30 zeros.

In the next article, we encounter nucleons.


References

List of People : Who Did What

  1. Dmitry Mendeleev : Russian chemist
    • Designed the Periodic Table of Elements
  2. Johann Wilhelm Hittorf : German physicist
    • Discovered that a cathode within an evacuated chamber emitted a glow
    • Found that the intensity of the glow increased as pressure is lowered
  3. Eugen Goldstein : German physicist
    • Showed that the rays from Hittorf’s glow cast a shadow
    • Named the rays cathode rays
  4. Sir William Crookes : English chemist and physicist
    • Invented the extreme low pressure cathode ray tube
    • Showed that cathode rays travel as a straight beam between a cathode and anode
    • Showed that cathode rays deviate in a magnetic field, proving that they are charged
  5. Arthur Schuster : German-British physicist
    • Placed the cathode ray between the plates of a parallel plate capacitor
    • Showed that the beam deviated towards the positive plate
    • Thus showed that cathode rays are negatively charged
  6. J.J. Thomson : English physicist
    • Experimentally obtained evidence for the existence of electrons
    • Thus showed that cathode rays are independent particles
    • Showed that these negatively charged particles are the same as the ones produced by radioactivity, by heated materials, and by illuminated materials
    • Suggested the plum pudding model for the distribution of positive charges and electrons within the atom
    • Discovery of the electron enabled the subatomic world to be probed
  7. George Johnstone Stoney : Irish physicist
    • Introduced the term electron for units of electricity
  8. Robert Millikan : American physicist
    • Performed the famous oil-drop experiment and measured the charge of the electron
  9. Harvey Fletcher : American physicist
    • Performed the famous oil-drop experiment and measured the charge of the electron
  10. Ernest Rutherford : New Zealand-British physicist
    • The father of subatomic and nuclear physics
    • Called the greatest experimentalist since Faraday
    • Supervised the Geiger-Marsden experiment or the Gold foil experiment that disproved the plum-pudding model and discovered the atomic nucleus
    • Discovered the proton
    • Suggested that protons within the nucleus have a neutral partner
    • Named them neutrons
  11. Hans Geiger : German physicist
    • Performed the Geiger-Marsden experiment that disproved the plum-pudding model and discovered the atomic nucleus
    • Invented the Geiger counter, a detector of radioactivity
  12. Ernest Marsden : New Zealand-English physicist
    • Performed the Geiger-Marsden experiment that disproved the plum-pudding model and discovered the atomic nucleus

Original papers

  1. Thomson, J.J. : Cathode Rays, Philosophical Magazine (1897)
  2. Rutherford, Ernest : The scattering of α and β particles by matter and the structure of the atom, Philosophical Magazine (1911)
  3. Millikan, R.A. : On the Elementary Electrical Charge and the Avogadro Constant
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Footnotes

  1. Or at least that is how the ancient Greeks did it
  2. A negatively charged electrode
  3. A positively charged electrode
  4. Named as such by George Stoney (Ireland)
  5. Yes, he of the Geiger counter
  6. Helium nuclei, consisting of two protons and two neutrons, are called alpha particles. Thus they have a charge of +2.
  7. Classically speaking. If you invoke quantum mechanics, things tend to get spread out.

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