Chapter 1 · Section 3Textbook pages 11–12

Rutherford's Gold Foil Experiment and the Neutron റഥർഫോർഡിന്റെ ഗോൾഡ് ഫോയിൽ പരീക്ഷണം · ന്യൂട്രോൺ

Fire a cannon shell at a sheet of tissue paper – and it comes straight back at you! That is how Ernest Rutherford described the most surprising result of his life. On this page you fire α particles at a gold foil a few hundred atoms thick, count where they land, zoom in to see why a few bounce back, build Rutherford's "solar system" atom, watch why it should have collapsed, and finally weigh the nucleus to discover the missing particle – the neutron.

📖 What you need (pages 2, 3 and 5 of this guide)

1. The gold foil experiment (Fig 1.6)

In 1911, scientists led by Ernest Rutherford – Hans Geiger and Ernest Marsden – aimed α particles at a very thin gold foil (സ്വർണ്ണത്തകിട്). These experiments made the structure of the atom much clearer. Positive α particles coming from a radioactive substance were sent through the thin gold foil, and Rutherford tried to find the changes in their paths. The α particles coming out of the foil were recorded on a circular photographic film (ഫോട്ടോഗ്രാഫിക് ഫിലിം) placed all round it (Fig 1.6).

☢ The α source is radioactive. In the real experiment it sat in a lead block inside a sealed, air-free box and was handled only by trained scientists – this is never a school experiment.
🧪 Virtual lab: fire α particles at the gold foil (Fig 1.6, seen from above)

Scaled: to see the rare events within a minute, the deflected and bounced-back α particles are made about 100 times more common here than in the real experiment. In Rutherford's experiment only about 1 in 20 000 bounced back. (Go further: the real detector was a zinc sulphide screen that gave a tiny flash for each α particle, watched through a microscope in a dark room; the textbook calls it a photographic film.)

Rutherford made these observations (page 11):

Zoom in: what happens inside the foil?

🔍 Zoom into a few gold atoms

The paths are worked out from the push between like charges (stronger the closer they get). Not to scale: the nucleus is drawn as a dot, but even that dot is far too big – if the atom were as big as a cricket ground, its nucleus would be about the size of a small pea at the centre (approx.). The electrons hardly affect an α particle, which is about 7000 times heavier than an electron.

From observations to inferences

ഈ നിരീക്ഷണങ്ങളിൽ നിന്ന് അദ്ദേഹം ചില അനുമാനങ്ങളിൽ എത്തിച്ചേർന്നു. – From these observations he reached some inferences. Match each observation to its inference.

1. Most α particles went straight through, without any deflection.
2. Some α particles were deflected by small angles.
3. About 1 in 20 000 came back at 180°.

• ആറ്റത്തിന്റെ ഭൂരിഭാഗവും ശൂന്യമായതിനാലാണ് ഭൂരിപക്ഷം ആൽഫാകണങ്ങളും വ്യതിയാനം കൂടാതെ കടന്നുപോയത്. – Most α particles went straight through because most of the atom is empty space.

• പോസിറ്റീവ് ചാർജ് ഉള്ള ആൽഫാകണങ്ങളിൽ ചിലത് ആറ്റത്തിനുള്ളിലെ പോസിറ്റീവ് ചാർജ് ഉള്ള ഭാഗത്തിന് സമീപത്ത് കൂടി കടന്നുപോയപ്പോൾ വികർഷിക്കപ്പെട്ടതിനാലാണ് അവ ചെറിയ കോണളവിൽ വ്യതിചലിച്ചത്. – Some positive α particles passed near the positive part of the atom and were repelled, so they were deflected by small angles.

• ആറ്റത്തിലെ മുഴുവൻ പോസിറ്റീവ് ചാർജും കേന്ദ്രീകരിച്ചിരിക്കുന്നത് ആറ്റത്തിന്റെ മധ്യഭാഗത്തുള്ള ഒരു ചെറിയ വ്യാപ്തത്തിലാണ്. ... ഈ കേന്ദ്രത്തെ അദ്ദേഹം ന്യൂക്ലിയസ് എന്ന് വിളിച്ചു. – All the positive charge of the atom is concentrated in a very small region at the centre, very small compared with the size of the atom. The α particles that came straight at it bounced back at 180°. He called this centre the nucleus (ന്യൂക്ലിയസ്).

💡 Why couldn't Thomson's atom do this?

In Thomson's atom the positive charge is spread thinly through the whole atom, and the electrons sit all through it, so the charges nearly cancel everywhere. A fast, heavy α particle would feel only a gentle push and go almost straight – none could ever come back (try "Thomson's atom" in both labs). To turn an α particle round, the positive charge must be packed into a tiny, heavy centre.

2. Rutherford's atom model – the solar system model

⭐ Key idea (textbook page 12)

This model is known as the solar system model (സൗരയൂഥ മാതൃക) – the nucleus is like the Sun and the electrons like the planets.

🎬 Rutherford's solar system atom

3. Limitations of Rutherford's model (page 12)

വൈദ്യുതകാന്തിക സിദ്ധാന്തപ്രകാരം, ചലിക്കുന്ന ചാർജുള്ള കണങ്ങൾ തുടർച്ചയായി ഊർജം പുറത്തുവിടേണ്ടതാണ്. – According to electromagnetic theory (വൈദ്യുതകാന്തിക സിദ്ധാന്തം), a charged particle moving in a circle must continuously give out energy. So, in Rutherford's model, the negative electrons revolving round the nucleus should keep losing energy and finally fall into the nucleus. But this does not happen. So Rutherford's model could not explain the stability (സ്ഥിരത) of the atom – this is its main limitation (പരിമിതികൾ).

🎬 What the theory predicted vs. what really happens

Hugely slowed down. By the old theory the collapse would take far less than a millionth of a second – every atom in the universe would have collapsed long ago. Yet atoms are stable. Niels Bohr solved this puzzle in 1913 (next page).

4. The neutron (page 12)

The real mass of the nucleus was found to be much more than the mass Rutherford calculated from the number of protons in it. But he could not prove the reason by experiment. Later, in 1932, James Chadwick found that the nucleus also contains some neutral particles (നിർവീര്യ കണങ്ങൾ), each with a mass about equal to that of a hydrogen atom. Because they have no charge, they were named neutrons (ന്യൂട്രോൺ).

⚖ Step by step: the case of the missing mass (example: helium)

5. Who's who so far

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6. Check yourself

🧭 Summary

Next: Bohr's atom model and the three particles ▶