We will limit our discussions to protons, neutrons and electrons.
Niels Bohr was a Danish scientist who introduced the model of an atom in 1913. Bohr's model consists of a central nucleus surrounded by tiny particles called electrons that are orbiting the nucleus in a cloud. These electrons are spinning so fast around the nucleus of the atom that they would be just a blur if we could see particles that small. In our pictures and exercises the electron appears to orbit in the same path around the nucleus much like the planets orbit the Sun. But, please be aware that electrons do not really orbit in the same path. The electrons actually change their orbit with each revolution.
UNIT 3
Vocabulary list
penetrate 1) а) проникать внутрь, п–оходить сквозь, пронизывать The water has penetrated into the bedrooms. Вода проникла в спальни. These new ideas are penetrating into the framework of society. – Эти новые идеи проникают в общество.
penetration 1) внедрение 2) пенетрация 3) проникание 4) проникновение 5) проницательность 6) проплавление
scatter 1) затрусить 2) натрясти 3) разбрасывать 4) разброс 5) разбросать 6) развеивать 7) разволакивать 8) разворошить
scatter electrons рассеивать электроны
scatter plot график рассеяния
breakthrough 1) крупное научное / техническое достижение; прорыв (в науке) 2) внезапный рост цен
capture 1) захват 2) захватывание 3) захватывать 4) каптаж 5) каптировать 6) уловить
capture electron захватывать электрон
compelling необоримый, неодолимый, неотразимый, непреодолимый
compelling reason неопровержимые доводы
attraction 1) привлечение 2) притяжение 3) притяжимость
attractive interaction взаимное притяжение
Coulomb’s attraction кулоновское притяжение
determine 1) определять, устанавливать (с помощью расчетов, рассуждений, проведения расследования и т. п.) to determine the answer to the problem – определить ответ на поставленный вопрос
By the early 20th century, there was rather compelling evidence that matter could be described by an atomic theory. That is, matter is composed of relatively few building blocks that we refer to as atoms. This theory provided a consistent and unified picture for all known chemical processes at that time. However, some mysteries could not be explained by this atomic theory. In 1896, A.H. Becquerel discovered penetrating radiation. In 1897, J J. Thomson showed that electrons have negative electric charge and come from ordinary matter. For matter to be electrically neutral, there must also be positive charges lurking somewhere. Where are and what carries these positive charges?
A monumental breakthrough came in 1911 when Ernest Rutherford and his coworkers conducted an experiment intended to determine the angles through which a beam of alpha particles (helium nuclei) would scatter after passing through a thin foil of gold.
Models of the atom. The dot at the center of the Rutherford atom is the nucleus. The size of the dot is enlarged so that it can be seen in the figure.
What results would be expected for such an experiment? It depends on how the atom is organized. A prevailing model of the atom at the time (the Thomson, or “plum-pudding,” atom) proposed that the negatively charged electrons (the plums) were mixed with smeared-out positive charges (the pudding). This model explained the neutrality of bulk material, yet still allowed the description of the flow of electric charges. In this model, it would be very unlikely for an alpha particle to scatter through an angle greater than a small fraction of a degree, and the vast majority should undergo almost no scattering at all.
The results from Rutherford’s experiment were astounding. The vast majority of alpha particles behaved as expected, and hardly scattered at all. But there were alpha particles that scattered through angles greater than 90 degrees, incredible in light of expectations for a “plum-pudding” atom. It was largely the evidence from this type of experiment that led to the model of the atom as having a nucleus. The only model of the atom consistent with this Rutherford experiment is that a small central core (the nucleus) houses the positive charge and most of the mass of the atom, while the majority of the atom’s volume contains discrete electrons orbiting about the central nucleus.
Under classical electromagnetic theory, a charge that is moving in a circular path, loses energy. In Rutherford’s model, the electrons orbit the nucleus similar to the orbit of planets about the sun. However, under this model, there is nothing to prevent the electrons from losing energy and falling into the nucleus under the influence of its Coulomb attraction. This stability problem was solved by Niels Bohr in 1913 with a new model in which there are particular orbits in which the electrons do not lose energy and therefore do not spiral into the nucleus. This model was the beginning of quantum mechanics, which successfully explains many properties of atoms. Bohr’s model of the atom is still a convenient description of the energy levels of the hydrogen atom.
Ex. 1 Make the following sentenses negative.
1. Becquerel decided to develop his photographic plates. 2. In this model, it would be very unlikely for an alpha particle to scatter through an angle greater than a small fraction of a degree. 3. The size of the dot is enlarged so that it can be seen in the figure. 4. The results from Rutherford’s experiment were astounding. 5. By the early 20th century, there was rather compelling evidence that matter could be described by an atomic theory.
Ex. 2 Make up questions to which the following sentences are the answers.
1. At high enough energy, the addition of energy creates new particles rather than frees the quarks. 2. Bohr’s model of the atom is still a convenient description of the energy levels of the hydrogen atom. 3. Energy brought into a nucleus to try to separate quarks increases the force between them. 4. This hypothesis was disproved on the 26-27th of February, when his experiment “failed” because it was overcast in Paris. 5. Because gamma rays carry no electric charge, they can penetrate large distances through materials. 6. A sheet of aluminum one millimeter
