Canal Rays, the Proton and Thomson's Model പ്രോട്ടോൺ · ആറ്റത്തിന്റെ പ്ലം പുഡിങ് മാതൃക
If atoms contain negative electrons, but an atom as a whole has no charge, then something positive must be hiding inside it too. On this page you drill holes in the cathode as Eugen Goldstein did, watch positive rays stream out the "wrong" way, discover why hydrogen gives the lightest of them – the proton – and then build J. J. Thomson's first picture of the atom, the plum pudding. Finally you meet radioactivity, which gave scientists the "bullets" for the next great experiment.
- Cathode rays are streams of electrons – negative particles found in all matter (page 4).
- Unlike charges attract, like charges repel. A body with equal amounts of + and − charge is neutral (page 2).
- A perforated (സുഷിരങ്ങളുള്ള) electrode is a metal plate with holes in it.
1. Goldstein's canal rays (page 10)
In 1886 the German scientist Eugen Goldstein did discharge tube experiments with a perforated cathode (സുഷിരങ്ങളുള്ള കാഥോഡ്). Behind the cathode – on the side away from the anode – he found new rays streaming out through the holes. Because they come through the holes ("canals") he called them canal rays (കനാൽ രശ്മികൾ). They come from the region of the positive electrode (anode), so they are also called anode rays (ആനോഡ് രശ്മികൾ).
Goldstein studied their properties and found that they carry positive charge. Unlike cathode rays, the nature of canal rays changes with the gas used in the tube. When the tube was filled with hydrogen, the positive particles in the canal rays were the smallest and lightest of all. Ernest Rutherford found that this is a subatomic particle and named it the proton (പ്രോട്ടോൺ).
Not to scale and in slow motion; the speeds and bends are compressed so that every gas fits on the screen. The particle sizes stand for their masses (relative to hydrogen's). In the electric field all these particles (each with charge +1) bend by the same amount towards the − plate; the magnet tells them apart – the lighter the particle, the more it bends (going further: this is how scientists "weigh" atoms in a mass spectrometer).
| Cathode rays കാഥോഡ് രശ്മികൾ | Canal (anode) rays കനാൽ രശ്മികൾ / ആനോഡ് രശ്മികൾ | |
|---|---|---|
| Discovered by | Plücker, Hittorf, Goldstein, Crookes (1858–1876); Thomson identified electrons | Goldstein, 1886 |
| Travel | from the cathode towards the anode | from the anode side towards the cathode, and on through the holes of the perforated cathode |
| Charge | negative – bend towards the + plate | positive – bend towards the − plate |
| Depend on the gas? | No – always the same particles (electrons) | Yes – different gas, different particles |
| Lightest particle found | electron | with hydrogen: the proton |
Where does a proton come from?
2. The plum pudding model of the atom (Fig 1.5)
When it became clear that atoms contain negative particles, J. J. Thomson proposed the plum pudding model (പ്ലം പുഡിങ് മാതൃക) of the atom (Fig 1.5):
ഇതനുസരിച്ച് പോസിറ്റീവ് ചാർജ് ഉള്ള ഒരു ഗോളത്തിൽ നെഗറ്റീവ് ചാർജുള്ള ഇലക്ട്രോണുകൾ വിന്യസിച്ചിരിക്കുന്നു. ഗോളത്തിലെ ആകെ പോസിറ്റീവ് ചാർജുകളുടെയും നെഗറ്റീവ് ചാർജുകളുടെയും എണ്ണം തുല്യമായിരിക്കും. അതിനാൽ ആറ്റം വൈദ്യുതപരമായി നിർവീര്യമാണ്.
- The atom is a sphere of positive charge, with negatively charged electrons embedded in it – like plums in a pudding.
- The total positive charge and the total negative charge in the sphere are equal, so the atom is electrically neutral.
In Thomson's model the positive charge is spread evenly through the whole sphere; the "+" signs only show where it is. An atom that has lost or gained electrons is no longer neutral – it is an ion (going further: you will meet ions in higher classes).
എന്നാൽ പല പരീക്ഷണഫലങ്ങൾക്കും വിശദീകരണം നൽകാൻ തോംസൺ മാതൃകയ്ക്ക് സാധിച്ചില്ല. അതിനാൽ ഈ മാതൃക പിന്തള്ളപ്പെട്ടു. – Thomson's model could not explain the results of many experiments, so it was set aside. The most famous of these was Rutherford's gold foil experiment (next page): in a "pudding" atom the positive charge is spread thin, so nothing inside it could ever throw a fast α particle straight back – yet some came back!
3. Radioactivity – nature's own "bullets" (box, page 11)
Elements such as uranium and thorium give out certain radiations (വികിരണങ്ങൾ) all by themselves. This is called radioactivity (റേഡിയോ ആക്ടീവത). It was discovered by Henri Becquerel in 1896. Mainly three kinds of rays come out:
- alpha (α) rays (ആൽഫ (α) കിരണങ്ങൾ) – have positive charge and mass;
- beta (β) rays (ബീറ്റ (β) കിരണങ്ങൾ) – have negative charge;
- gamma (γ) rays (ഗാമ (γ) കിരണങ്ങൾ) – have no charge and no mass.
A sketch: the bends are exaggerated and the rays are slowed down enormously. The textbook only says which rays are charged; the sizes of the bends are going further – β particles are about 7000 times lighter than α particles, so they bend much more.
Gamma rays are dangerous, but in careful hands they save lives. At the Regional Cancer Centre (RCC), Thiruvananthapuram, machines with a shielded cobalt-60 source (and newer machines that make similar high-energy rays electrically) aim narrow beams at a tumour to destroy the cancer cells. More uses of radioactive substances are on pages 10 and 13.
- 🧪 Rutherford Scattering (PhET simulation) PhET, University of Colorado Boulder
Fire alpha particles at a gold atom: compare a plum-pudding atom with a tiny nucleus and watch which particles bounce back. - 🧪 Models of the Hydrogen Atom (PhET simulation) PhET, University of Colorado Boulder
Try the plum pudding, Rutherford (classical solar system) and Bohr models one after another and see what each can and cannot explain. - 📄 Rutherford scattering experiments Wikipedia
The 1909–1913 experiments of Geiger and Marsden under Rutherford, with drawings of the apparatus and how the nucleus was discovered. - 📄 Niels Bohr – Biographical The Nobel Prize
The life of the Danish scientist who gave the atom fixed energy levels (Nobel Prize in Physics 1922).
4. Check yourself
- Canal rays (anode rays) were found by Goldstein (1886) using a perforated cathode. They are positive, stream through the holes in the cathode away from the anode, and depend on the gas in the tube.
- With hydrogen the canal-ray particles are the smallest and lightest: Rutherford named this particle the proton.
- Thomson's plum pudding model: a sphere of positive charge with electrons embedded in it; equal + and − charges, so the atom is neutral. It could not explain many experiments and was set aside.
- Radioactivity (Becquerel, 1896): uranium, thorium… give out α (positive, with mass), β (negative) and γ (no charge, no mass) rays by themselves.