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ISOTOPES

The word “isotope” is derived from the Greek “isos”, “topos” and means “the same place”.

Hence, isotopes are atoms having the same atomic number, but differing in atomic weight (mass number), e.g. ¹²9C and ¹³6 C are isotopes of carbon, or one out of every 5,000 atoms of hydrogen has an atomic weight of 2.016 instead of 1.008.

This odd atom has a neutron in its nucleus as well as a proton, it being known as heavy hydrogen. The water containing it is known as heavy water.

Isotopes occur with considerably greater frequency in other elements than in hydrogen, an extreme case being chlorine, its atomic weight being 35.5. It is made up of two groups of atoms in a ratio of 3:1, the weight of one group being 35, that of the other 37.

In the case of uranium, for example, one isotope of atomic weight 235 is found in every 140 atoms of the standard weight, with the weight being 238.

The chemical properties of isotopes being identical with those of regular atoms, their discovery was of little interest to chemists. Physicists, however, got interested in them, a new way of approaching to the structure of matter being opened.

 

Words to be remembered:

 

nucleus chlorine

as well as ratio

contain identical

considerably approach

extreme case

 

Notes on the text:

odd atom -

in the case of uranium - regular atoms -

e.g. = for example

 

 

¹10

LIQUIDS (I)

The liquid state occupies an intermediate position between the gaseous and solid states, liquid having a definite volume but no definite shape.

Like a gas, a liquid can take the shape of any vessel in which it is put, but in contrast to a gas, a definite quantity of liquid is required for filling the vessel. A liquid cannot be compressed so much as a gas because its molecules are alrea­dy close together, large pressure producing small changes in volume.

Increasing the temperature increases the kinetic energy of all molecules.

The change of a liquid into a gaseous or liquid states being dependent upon the kinetic energy of the molecules, which in turn is dependent upon the temperature, there are definite temperature characteristics for most liquids at which these changes occur. They are known as transition temperatures.

If we place one liquid layer carefully on top of a layer of a more dense liquid in which it is soluble, and set the vessel where it won’t be disturbed, we shall see that two liquids be­gin gradually mixing. It is also to be taken into consideration that all liquids do not flow with the same ease, water, alcohol, gasoline flowing easily, while heavy oil glycerin flowing very slowly.

When a liquid flows, layers of molecules begin rubbing over each other, friction being generated by this rubbing of layers of particles. The greater the friction, the slower is the flow. A liquid which resists flowing, or resists the reaction of any other deforming force upon it results in a homogeneous solution. We give this example for illustration that the molecules of a liquid diffuse, though much slowly than do those of a gas.



The molecules of a liquid are much closer together than they are in a gas, because of the greater relative strength of attrac­tion, the density of liquids being much greater. Naturally as the volume of a liquid begins varying with temperature its den­sity will also start varying with temperature.

It should be noted that the closeness of the molecules also is known as viscous, the opposite of viscosity being fluidity. Viscosity diminishes and fluidity increases with temperature.

 

Words to be remembered:

intermediate soluble

shape gradually

vessel to flow

to compress gasoline

to increase rub

dependent friction

transition viscosity

layer fluidity

dense in turn

 

 

¹11

LIQUIDS (II)

The molecules within the interior of a liquid have a definite average energy of motion, and thus a definite average velocity at each temperature. Some of them, however, at any given in­stant, have a velocity sufficiently greater than the average velocity to enable them to break through the surface layer of molecules and escape. Thereafter they are free to wander about in the space above and constitute a vapour - namely a gas that can be condensed to a liquid merely by increasing the pressure upon it. (Air is not a vapour, for to condense it to a liquid it must be both compressed and cooled).

The escape of the molecules from a liquid into its vapour is called evaporation. After a sufficient number of molecules have collected in the space above the liquid, their haphazard wanderings bring them back to the surface as fast as other mo­lecules escape. Thereafter, there is a balance between evaporation and recondensation and thus a constant number of molecules within the closed space at any given moment; and these, by bombardment of the walls of the vessel set up a constant pressure, called the vapour pressure.

 

Words to be remembered:

interior volatile

average to escape

motion constant

sufficient within

surface velocity

vapour thereafter

evaporation wander

 

 

¹12

SOLID STATE (I)

If you take a paper clip and bend it would stay bent, it wouldn't, spring back and it wouldn’t break. The metal of which the clip is made is ductile. Some other materials are not duc­tile at all. If you tried to bend a glass rod (unless you are holding it in a flame), it would simply break. It is brittle. In this respect as in many others, glass behaves quite differently from a metal. The difference must lie either in the parti­cular atoms of which metals and glass are made up or in the way the are put together, probably both. There are of course many other differences between metals and glass.

Metals, for example, conduct electricity and therefore are used for electrical transmission lines, glass hardly conducts electricity at all and can serve as an insulator. Glass being transparent, it can be used in windows whereas a sheet of metal even more than a millionth of an inch thick is quite opaque. It is of course interesting to understand the reasons of these differences in behaviour.

During the past 20 years studies of this kind have been call­ed solid-state physics, or sometimes since the subject includes a great deal of chemistry, just “solid state”. It is a major branch of science that has revealed new and previously unsus­pected properties in metals. Solid-state physics has become one of the most important branches of technology. It has given rise to technological progress. Having studied this branch of technology, engineers could understand much better the phenomenon of quantum mechanics as it is applied to solids. Though solids, of course, were the subject of experimental investigation long be­fore quantum mechanics was invented.

 

Words to be remembered:

interior therefore

opaque transparent

ductile branch

break conduct

insulator previously

 

 

¹13

SOLID STATE (II)

If we consider the fact known since the earliest studies of electric currents, we should remember that metals conduct electricity well and most materials do not.

It is only the discovery of electron that could help the scientists to understand some of these facts well. With the dis­covery of electron it was assumed that in metals some or all of the atoms had lost an electron and that in insulators such as glass they had not. The electrons in a metal proved thus to move freely, whereas the electrons in insulators do not. Why did this happen in metals? This very question had to await the discovery of quantum mechanics. The next question was: “How are the electrons arranged?”

As far as this question is concerned we can say that solids can be divided into two classes: crystalline and amorphous. In the crystalline group, which is the largest and includes the metals and most minerals, the atoms are arranged in a regular way. In some metals (for instance copper and nickel) they are backed together. In other metals (such as iron, for example), they are arranged in the form of a cube. The commonest of the amorphous group of solids appear in glass, its atoms are put together in a more disordered way than those of a metal.

The structure of an amorphous material is much more difficult to discover than that of a crystalline solid and considerable effort is being made to learn more about the arrangement of atoms in such materials.

Much has been learned about solids but much is still to be learned. There is a number of problems which are to be solved. No wonder that many scientists have been working at this inte­resting, so-called “solid state” science.

 

Words to be remembered:

to consider copper

electric current considerable

discovery effort

to arrange to appear

for instance to solve

 

¹14


Date: 2015-01-12; view: 954


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