Additional reading the world of the atom

More than 2,000 years ago Greek philosophers discussed the structure of matter and some argued that if a substance were cut repeatedly into smaller and smaller pieces, after many cuts only solid particles; which were exactly alike and could not be cut again, would remain.

They called these particles atoms (Greek for “uncut”) and believed them to be invisible, indivisible and inde­structible – the smallest possible bodies. Each substance was be­lieved to have a characteristic type of atom which was shaped differently from the atoms of any other substance.

In 1804 John Dalton, an English scientist, believed the atom to be indivisible – the smallest particles of matter in existence.

Then, after many years of careful research the discovery of the electron, a part of an atom, was announced.

The discovery of one kind of charged atomic particle suggested the presence of a second particle with an equal, but opposite electric charge. The existence of such a posi­tively charged particle, named the proton, was proved in 1911. The third basic atomic particle, the electrically charged neutron, was not discovered until 1932.

After the parts of an atom were known, scientists learned how they fit together. Protons and neutrons are grouped together in the centre or core of the atom and form its nucleus and are called “nuclear particles” or “nucleons”. Electrons orbit around the nucleus, although they are so widely sepa­rated from it that the atom consists largely of empty space.

Notes:

solid particles …… indestructible …… charge …………... core ……………...

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Natural radioactivity

By the beginning of the 20^th century man had begun to learn something of the structure of the atom. In 1896, a French scientist Becquerel made an observation that a piece of ura­nium ore gave out a mysterious radiation. This radiation was even more powerful than the X-rays which Roentgen had recently discovered. Year in and year out, uranium and all of its compounds send out their highly penetrating radiation. Nothing you can do in any way prevents it. Undisturbed the uranium just keeps on giving off its radiation. We call this action radioactivity.

The discovery that uranium and other elements possessed radioactive properties led to a systematic search by the Curies for this state in numerous natural ores and chemical compounds. They soon found that thorium and its compounds acted like uranium, too. They also noticed that pitchblende, an ore of uranium, was more radioactive than pure uranium itself. So they decided that pitchblende must contain an un­known substance much more radioactive than uranium.

Many years of untiring work passed before Irene Curie, after the death of her husband, succeeded in separating about 1/10 of gram of radium from many tons of pitchblende. She found that this element, radium, was more than 100,000,000 times as radioactive as uranium. The era of ra­dioactive chemistry had arrived.

What is radioactivity? A British scientist, Ernest Ruther­ford, made a thorough study of radioactivity. He showed that three kinds of rays come from radioactive elements such as uranium and radium. One kind called alpha particles, consists of positively charged nuclei of helium atoms. A second kind, beta particles, is a beam of electrons, travelling nearly as fast as light. The rest of radiation consists of gamma rays. Gamma rays are similar to powerful X-rays.

It was soon realized that the atom must contain a tremen­dous amount of energy. For example scientists calculated that the heat given off every hour by a pound of radium would be more than enough to heat a pound of water from freezing to boiling.

Notes:

to give out …………….. penetrating radiation…... to possess ……………... thorium ……………….. thorough ……….……...

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