Exercise 8.3
Why did Thomson conclude that electrons are present in all atoms? Testing Thomson’s model: The gold foil experiment In , Geiger and Marsden, working under Ernest Rutherford, tested Thomson’s model of the atom through what became famous as the gold foil experiment. They aimed a narrow beam of alpha particles at an extremely thin sheet of gold foil. Alpha (symbol α) particles are tiny, positively charged particles emitted from certain radioactive elements. Later in this chapter, you will learn that an alpha particle is actually a nucleus of a helium atom containing two protons and two neutrons. According to Thomson’s model, the positive charge in the atom was spread out evenly. So they expected the alpha particles to pass straight through the gold foil or be deflected only slightly. But to their surprise, while most particles passed through undeflected, some were sharply deflected (Fig. ), and a few even bounced back. This deflection from the straight path is called scattering. Hence, the gold foil experiment is also called an α-ray scattering experiment. T homson’s model failed to explain the results of the gold foil experiment, particularly the deflection of some α-particles through large angles and that most of the α-particles passed undeflected.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
Because cathode rays turned out to be exactly the same whatever atoms they came from.
Thomson changed the material of the cathode and changed the gas filled in the tube, and the nature of the cathode rays did not change.
In every case the rays were streams of negatively charged particles with a mass much smaller than that of an atom.
These particles were being emitted out of atoms, so they had to be inside them to begin with.
Since identical particles came out of every element tried, the electron must be a fundamental component of all atoms, not a feature of one particular substance.