Учебное пособие Казань-2016 Сайфуллина М. Н., Хабирова Н. М. English for Physicists



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Posobie dlya fizikov Habirova Sajfullina ispravv

 
 
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The search for the atom began as a philosophical question. It was the 
natural philosophers of ancient Greece that began the search for the atom by 
asking such questions as: What is stuff composed of? What is the structure 
of material objects? Is there a basic unit from which all objects are made?
As early as 400 B.C., some Greek philosophers proposed that matter is 
made of indivisible building blocks known as atoms (the word «atom» in 
Greek means indivisible). To these early Greeks, matter could not be 
continuously broken down and divided indefinitely. Rather, there was a 
basic unit or building block that was indivisible and foundational to its 
structure. This indivisible building block of which all matter was composed 
became known as the atom. 
The early Greeks were simply philosophers. They did not perform 
experiments to test their theories. In fact, science as an experimental 
discipline did not emerge as a credible and popular practice until sometime 
during the 1600s. So, the search for the atom remained a philosophical 
inquiry for a couple of millennia. From the 1600s to the present century, the 


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search for the atom became an experimental pursuit. Several scientists are 
notable, among them are Robert Boyle, John Dalton, J.J. Thomson, Ernest 
Rutherford, and Neils Bohr. 
Boyle's studies (middle to late 1600s) of gaseous substances promoted 
the idea that there were different types of atoms known as elements. Dalton 
(early 1800s) conducted a variety of experiments to show that different 
elements can combine in fixed ratios of masses to form compounds. Dalton 
subsequently proposed one of the first theories of atomic behavior that was 
supported by actual experimental evidence. 
English scientist J.J. Thomson's cathode ray experiments (end of the 
19th century) led to the discovery of the negatively charged electron and the 
first ideas of the structure of these indivisible atoms. Thomson proposed the 
Plum Pudding Model, suggesting that an atom's structure resembles the 
favorite English dessert - plum pudding. The raisins dispersed amidst the 
plum pudding are analogous to negatively charged electrons immersed in a 
sea of positive charge. 
Nearly a decade after Thomson, Ernest Rutherford's famous gold foil 
experiments led to the nuclear model of atomic structure. Rutherford's 
model suggested that the atom consisted of a densely packed core of 
positive charge known as the nucleus surrounded by negatively charged 
electrons. While the nucleus was unique to the Rutherford atom, even more 
surprising was the proposal that an atom consisted mostly of empty space. 
Most the mass was packed into the nucleus that was abnormally small 
compared to the actual size of the atom. 
Neils Bohr improved upon Rutherford's nuclear model (1913) by 
explaining that the electrons were present in orbits outside the nucleus. The 
electrons were confined to specific orbits of fixed radius, each characterized 
by their own discrete levels of energy. While electrons could be forced from 
one orbit to another orbit, it could never occupy the space between orbits. 
Bohr's view of quantized energy levels was the precursor to modern 
quantum mechanical views of the atoms. The mathematical nature of 
quantum mechanics prohibits a discussion of its details and restricts us to a 
brief conceptual description of its features. Quantum mechanics suggests 
that an atom is composed of a variety of subatomic particles. The three 
main subatomic particles are the proton, electron and neutron. The proton 
and neutron are the most massive of the three subatomic particles; they are 
located in the nucleus of the atom, forming the dense core of the atom. 


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The proton is charged positively. The neutron does not possess a
charge and is said to be neutral. The protons and neutrons are bound tightly 
together within the nucleus of the atom. Outside the nucleus are concentric 
spherical regions of space known as electron shells. The shells are the home 
of the negatively charged electrons. Each shell is characterized by a distinct 
energy level. Outer shells have higher energy levels and are characterized as 
being lower in stability. Electrons in higher energy shells can move down to 
lower energy shells; this movement is accompanied by the release of
energy. Similarly, electrons in lower energy shells can be induced to move 
to the higher energy outer shells by the addition of energy to the atom. If 
provided sufficient energy, an electron can be removed from an atom and be 
freed from its attraction to the nucleus. 
(Adopted from 
www.physicsclassroom.com 

 
 
 
 


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