Inside the Atom: the Discharge Tube and the Electron ഡിസ്ചാർജ് ട്യൂബ് പരീക്ഷണങ്ങളും ഇലക്ട്രോണിനെ കണ്ടെത്തലും
Everything around you – sugar in your chaya, the water in the well, the air you breathe – is made of atoms. But what is inside an atom? On this page you count the atoms in three familiar molecules, then switch on a glowing glass tube, just as William Crookes did in 1875, and follow the clues (a shadow, a turning wheel, a bending beam) that led to the first particle smaller than an atom: the electron.
- An atom (ആറ്റം) is the smallest particle of an element; atoms join to make molecules (തന്മാത്ര). A formula like H₂O tells you which atoms and how many (page 1).
- Like charges repel, unlike charges attract. The cathode (കാഥോഡ്) is the negative electrode (−), the anode (ആനോഡ്) the positive one (+). Air at normal pressure is an insulator (വൈദ്യുതരോധി) (page 2).
- Powers of ten: 10¹¹ = 1 followed by 11 zeros; 10⁻³¹ = 1 divided by 10³¹ (page 3).
1. Which atoms are in familiar substances? (Table 1.1)
The chapter opens with four students around a laptop that shows a picture of an atom.
പരിചയമുള്ള പദാർഥങ്ങളിൽ അടങ്ങിയിരിക്കുന്ന ആറ്റങ്ങൾ ഏതൊക്കെയെന്ന് തിരിച്ചറിയാമോ? – Can you identify the atoms present in familiar substances? Analyse Table 1.1.
| Substance പദാർഥം | Constituent elements ഘടക മൂലകങ്ങൾ | Chemical formula of the molecule തന്മാത്രയുടെ രാസസൂത്രം | Ratio of the numbers of atoms ആറ്റങ്ങളുടെ എണ്ണത്തിന്റെ അനുപാതം |
|---|---|---|---|
| Sugar പഞ്ചസാര | carbon, hydrogen, oxygen | C₁₂H₂₂O₁₁ | 12 : 22 : 11 |
| Glucose ഗ്ലൂക്കോസ് | carbon, hydrogen, oxygen | C₆H₁₂O₆ | 1 : 2 : 1 (6 : 12 : 6 simplified) |
| Water ജലം | hydrogen, oxygen | H₂O | 2 : 1 |
Type symbols with a capital first letter and the number after the symbol, e.g. CH4, NH3, H2SO4, NaCl (no brackets). The balls only count atoms – they are not drawn to size or joined as in the real molecule.
Q (page 7). പരിചയമുള്ള പദാർഥങ്ങളിൽ അടങ്ങിയിരിക്കുന്ന ആറ്റങ്ങൾ ഏതൊക്കെയെന്ന് തിരിച്ചറിയാമോ? – Can you identify the atoms in these familiar substances?
2. Molecules, atoms and still smaller particles
ഒരു പദാർഥത്തിന്റെ എല്ലാ ഗുണങ്ങളുമുള്ളതും സ്വതന്ത്രാവസ്ഥയിൽ നിലനിൽക്കാൻ കഴിയുന്നതുമായ ഏറ്റവും ചെറിയ കണികയാണ് തന്മാത്ര.
A molecule (തന്മാത്ര) is the smallest particle of a substance that has all the properties of that substance and can exist independently.
വിവിധ പദാർഥങ്ങളിലെ തന്മാത്രകൾ എങ്ങനെയെല്ലാം വ്യത്യാസപ്പെട്ടിരിക്കുന്നു? – How do the molecules of different substances differ?
- in the constituent elements they contain (തന്മാത്രയിലടങ്ങിയിരിക്കുന്ന ഘടക മൂലകങ്ങൾ) – water has H and O, sugar has C, H and O;
- in the ratio of the numbers of atoms of those elements (ഘടക മൂലക ആറ്റങ്ങളുടെ എണ്ണത്തിന്റെ അനുപാതം) – sugar 12 : 22 : 11, glucose 1 : 2 : 1.
Molecules are made of atoms. You learnt in earlier classes that atoms in turn contain still smaller particles. ആറ്റങ്ങളിൽ അടങ്ങിയിരിക്കുന്ന പ്രധാന കണങ്ങൾ ഏതൊക്കെയാണെന്ന് അറിയാമോ? – Do you know the main particles in an atom? The book gives the first: electron (ഇലക്ട്രോൺ), ……, ……
These are called subatomic particles (സബ്അറ്റോമിക കണങ്ങൾ) – particles smaller than an atom. This page is the story of the first one to be found, the electron; the proton and neutron follow on pages 5 and 6.
3. Crookes' discharge tube (Fig 1.1)
In 1875 the scientist William Crookes took a glass tube with a metal plate – an electrode (ഇലക്ട്രോഡുകൾ) – sealed in at each end and passed electricity through it at a very high voltage (ഉയർന്ന വോൾട്ടത). Such a tube is a discharge tube (ഡിസ്ചാർജ് ട്യൂബ്). A side tube leads to a vacuum (suction) pump (നിർവാത പമ്പ്) so that the air inside can be pumped out bit by bit.
Air is an insulator (വൈദ്യുതരോധി), so at normal pressure no electricity flows through the air in the tube. But when the pressure is reduced step by step to a very low value, a current flows through the tube – an electric discharge (വൈദ്യുത ഡിസ്ചാർജ്). With a perforated anode (സുഷിരങ്ങളുള്ള – with holes), a pale-green glow (ഇളംപച്ച നിറത്തിലുള്ള തിളക്കം) appears on the glass wall behind it, which is coated with zinc sulphide (സിങ്ക് സൾഫൈഡ് (ZnS)). The glow is caused by rays coming from the cathode, so they were named cathode rays (കാഥോഡ് രശ്മികൾ).
Pressures are rough values – the exact pressure at which each change happens depends on the tube and the voltage. Cathode rays themselves are invisible; they are drawn as pink dashes so you can follow them (as in the textbook figure). Notice that the colour of the gas glow at medium pressure depends on the gas – that is why neon signs glow orange-red.
The story behind the discovery (box, page 9)
The textbook box കാഥോഡ് രശ്മികളുടെ കണ്ടെത്തൽ (Discovery of cathode rays) tells how many scientists, over about 50 years, slowly built up the picture. Step through the timeline. (Years not given in the textbook are marked "go further".)
Scientists knew in the early 1800s that electricity can pass through gases at low pressure, and Michael Faraday studied the changes it produced. But there were no good suction pumps, and it was hard to make glass tubes and pump the air out of them. Only after Heinrich Geissler built better tubes and pumps in 1854 could the experiments really begin. Science often waits for better tools!
4. Main properties of cathode rays (Fig 1.2 – 1.4)
കാഥോഡ് രശ്മികളുടെ പ്രധാന സവിശേഷതകൾ – Scientists then put different objects in the path of the rays. Try each one in the lab below: the pressure is already very low and the high voltage is on.
In the textbook's Fig 1.2 the cross itself is the anode; Fig 1.3 and 1.4 have the anode at the far end – the lab follows the book. Bending is exaggerated so you can see it. Change the gas or the cathode metal: watch whether anything about the rays changes.
| Observation | What does it tell us? |
|---|---|
| An opaque object (അതാര്യ വസ്തു) in the path casts a sharp shadow (നിഴൽ) (Fig 1.2) | |
| A light paddle wheel (നേർത്ത ഇതളുകളുള്ള ചക്രം) in the path turns (Fig 1.3) | |
| In an electric field (വൈദ്യുതമണ്ഡലം) the rays bend towards the positive plate (Fig 1.4) | |
| A magnetic field (കാന്തിക മണ്ഡലം) also changes their path | |
| Changing the gas or the electrode metal does not change the rays |
5. The electron and J. J. Thomson
Changing the gas in the tube or the metal of the electrodes made no difference to the nature of the rays. So the particles in cathode rays are present in all substances. These particles are the electrons (ഇലക്ട്രോൺ).
J. J. Thomson measured the ratio of the charge (ചാർജ്) to the mass (മാസ്) of the electron – the e/m ratio (e/m അനുപാതം). When the scientific world accepted his work on cathode rays, it became clear that particles smaller than atoms exist. For the discoveries that followed from the discharge tube experiments, Thomson received the Nobel Prize in Physics in 1906.
The e/m ratio of the electron is 1.76 × 10¹¹ C/kg. But J. J. Thomson could not find the charge and the mass separately. Later, Robert Millikan, through his famous oil-drop experiment (ഓയിൽ ഡ്രോപ്പ് പരീക്ഷണം), found that the electron has a negative charge of 1.6 × 10⁻¹⁹ C, and from this calculated the mass of the electron as 9.1 × 10⁻³¹ kg. (C = coulomb കൂളോം, the unit of charge.)
Millikan sprayed tiny oil drops between two charged plates and watched one drop through a microscope. By adjusting the voltage he could make a charged drop hang still: the electric pull up balanced its weight down. The charges on the drops always came out as whole-number multiples of one smallest charge, 1.6 × 10⁻¹⁹ C – the charge of one electron.
6. Questions from the textbook (page 10)
Q1. ഇലക്ട്രോണിന് മാസുണ്ടെന്ന് തെളിയിച്ചതെങ്ങനെ? – How was it proved that the electron has mass?
Q2. കാഥോഡ് രശ്മികളുടെ പാതയിൽ ഒരു അതാര്യ വസ്തു വച്ചാൽ നിഴൽ ഉണ്ടാകുന്നു. ഇതിൽ നിന്ന് എന്ത് മനസ്സിലാക്കാം? – An opaque object placed in the path of cathode rays casts a shadow. What does this show?
The big, deep television sets of the 1990s were cathode ray tubes (CRTs). At the narrow back end, a hot cathode shot out a beam of electrons; magnets (coils) bent the beam left-right and up-down hundreds of times a second, and wherever it struck the coated screen it glowed – painting the picture line by line. The colourful signboards of a Kerala town at night are discharge tubes too: neon glows orange-red, other gases and coatings give other colours (page 13).
- 📄 Crookes tube Wikipedia
Photos and diagrams of the discharge tube in your textbook, including the Maltese-cross shadow and the paddle-wheel tube. - 📄 Cathode ray Wikipedia
How cathode rays were studied by Plücker, Hittorf, Goldstein, Crookes and Thomson, and why they turned out to be electrons. - 📄 Anode ray (canal rays) Wikipedia
Goldstein's canal rays: positive particles that pass through holes in a perforated cathode. - 📄 Joseph John “J. J.” Thomson Science History Institute
A short, readable biography of the scientist who identified the electron and proposed the plum pudding model. - 📄 Oil drop experiment Wikipedia
How Robert Millikan measured the charge of the electron (about 1.6 × 10⁻¹⁹ C) using tiny charged oil drops. - 📄 James Chadwick – Biographical The Nobel Prize
The discoverer of the neutron (1932) and his Nobel Prize in Physics 1935.
7. Check yourself
- A molecule is the smallest particle of a substance that has all its properties and can exist independently. Molecules differ in their elements and in the ratio of their atoms.
- Atoms contain smaller subatomic particles: electrons, protons and neutrons.
- Discharge tube (Crookes, 1875): no current at normal pressure (air is an insulator); at very low pressure and high voltage, cathode rays from the cathode make the ZnS-coated glass glow pale green.
- Cathode rays travel in straight lines (shadow), have mass (paddle wheel), are negative (bend towards +) and are bent by a magnet; they are the same for every gas and metal → electrons are in all matter.
- Thomson measured e/m = 1.76 × 10¹¹ C/kg (Nobel Prize 1906); Millikan found e = 1.6 × 10⁻¹⁹ C; so the mass of an electron = 9.1 × 10⁻³¹ kg.
Next: Canal rays, the proton and Thomson's plum pudding model ▶