III. Text 2. Discovery of electrons.

ATOMS

I. Definition from the encyclopedia:

In chemistry and physics, atomic theory is a theory of the nature of matter, which states that matter is composed of discrete units called atoms, as opposed to the obsolete notion that matter could be divided into any arbitrarily small quantity. It began as a philosophical concept in ancient Greece and India and entered the scientific mainstream in the early 19th century when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of particles.

The word "atom" (from the ancient Greek adjective atomos, 'uncuttable') was applied to the basic particle that constituted a chemical element, because the chemists of the era believed that these were the fundamental particles of matter. However, around the turn of the 20th century, through various experiments with electromagnetism and radioactivity, physicists discovered that the so-called "indivisible atom" was actually a conglomerate of various subatomic particles (chiefly, electrons, protons and neutrons) which can exist separately from each other. In fact, in certain extreme environments such as neutron stars, extreme temperature and pressure prevents atoms from existing at all. Since atoms were found to be actually divisible, physicists later invented the term "elementary particles" to describe indivisible particles. The field of science which studies subatomic particles is particle physics, and it is in this field that physicists hope to discover the true fundamental nature of matter.

II. Text 1. History.

Read correctly:

Empedocles — [em‘pedə,kli:z] Leucippus — [‘lu:səpəs] - Левкипп

Democritus — [di’mokrətəs] Heraclites — [hi ‘ræklətəs]

Parmenides — [‘pa:mənids] Lucretius — [lu:‘kri:∫jəs]

In the 6th century BCE Greek philosophers would speculate about the structure of the world and the nature of matter. There were a lot of different theories trying to explain the variety of matter on the Earth. For example, Empedocles suggested that there were four basic types of matter: earth, air, fire and water. These four elements were combined to form familiar things through the agency of two universal powers: love and strive. (5 line)

The first atomic theory was put forward by the Greek philosophers Leucippus and Democritus between 450 and 420 BCE. Leucippus and his pupil Democritus proposed that all matter was composed of small indivisible particles called atoms, in order to reconcile two conflicting schools of thought on the nature of reality. On one side was Heraclites, who believed that the nature of all existence is change. On the other side was Parmenides, who believed instead that all change is illusion. (11 line)

Our detailed knowledge of the theory is based on the later source: the long Latin poem “De rerum nature” (“On the Nature of Things) written in the first century BC by Lucretius, the great Roman poet and philosopher. He set out to abolish superstitious fears about the arbitrary intervention of the gods in the affairs of men, maintaining instead that the world is governed by the laws of nature. All things, Lucretius wrote, are made up of invisible, indivisible particles called atoms (from the Greek word for “indivisible”). Atoms exist in a ubiquitous void; a void must be inferred because one can have no direct experience of it. Atoms are small but finite in size. They are in constant motion. There are various species, or shapes of atoms. (19 line)

Although the number of species is finite, the number of atoms of each species is unlimited. Atoms are capable of combining, but the number of combination is finite. (21 line)

Atoms of various shapes moving and combining in various ways fall at last into certain arrangements of which the world of things is created. Solids exist because certain atoms unite, entangled by their own “close-locking shapes ”. Substances that are hard and compact (diamond, iron, flint, and brass, for example) must have particles that are more tightly hooked together than others. Comparing the flowing of wine and of olive oil, Lucretius concluded that the oil must be made up of particles that are larger or “more hooked and entangled one with the other” than the atoms of wine. Substances with a pleasant taste have smooth, round atoms; those that are bitter or harsh have particles that are more irregular. (29 line)

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.

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