III. Text 2. Discovery of electrons.

Fill the gaps using Passive Voice where it is necessary:

Atoms (to think)……………to be the smallest possible division of matter until 1897 when J.J. Thomson (to discover)……………..the electron through his work on cathode rays. A Crookes tube* is a sealed glass container in which two electrodes (to separate) ………………….by a vacuum. When a voltage (to apply) ………………….across the electrodes, cathode rays (to generate)…………………, creating a glowing patch where they (to strike)…………….. the glass at the opposite end of the tube. Through experimentation, Thomson discovered that the rays could (to deflect)……………by an electric field (in addition to magnetic fields, which …..already (to know)……………). He (to conclude) ………………that these rays, rather than being waves, (to compose) ………………….. of negatively charged particles he (to call)…………………."corpuscles" ['kɔ: pʌsl] (they would later (to rename) ..…….......electrons by other scientists).

Thomson (to believe) …………….that the corpuscles (to emerge) ………………from the very atoms of the electrode. He thus (to conclude) ……………..that atoms (to be)…………… divisible, and that the corpuscles (to be)……………. their building blocks. To explain the overall neutral charge of the atom, he (to propose) …………that the corpuscles (to distribute) ………………….in a uniform sea of positive charge; this was the plum pudding model as the electrons (to embed) ………………..in the positive charge like plums in a plum pudding (although in Thomson's model they were not stationary).

Fig 1. Thomson's Crookes tube* in which he observed the deflection of cathode rays by an electric field. The purple line represents the deflected electron stream.

*A Crookes tube is an early experimental electrical discharge tube, invented by British physicist William Crookes and others around 1869-1875, in which cathode rays, that is electrons, were discovered.

IV. Text 3. Discovery of the nucleus.

Thomson's plum pudding model was disproved in 1909 by one of his former students, Ernest Rutherford, who discovered that most of the mass and positive charge of an atom is concentrated in a very small fraction of its volume, which he assumed to be at the very centre.

In the gold foil experiment, Rutherford with his colleagues shot alpha particles at a thin sheet of gold surrounded by a fluorescent screen. Given the very small mass of the electrons, the high momentum of the alpha particles and the unconcentrated distribution of positive charge of the plum pudding model, the experimenters expected all the alpha particles to either pass through the gold sheet without significant deflection or be absorbed. To their astonishment, only a number of the alpha particles passed through the gold sheet without deflection, some of them were slightly deflected and a small fraction of the alpha particles experienced heavy deflection. These particles were thrown back as if they suddenly collided with something hard and powerful.

This led Rutherford to propose a planetary model in which a cloud of electrons surrounded a small, compact nucleus of positive charge. Only such a concentration of charge could produce the electric field strong enough to cause the heavy deflection.