About Atoms and Molecules

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    The building blocks of matter:

    Many of theancient Greekphilosophersbelieved thatallsubstanceswere composof tiny particor ; butatomswas not untilthe nineteentcentury thatthis idea

    developed ina useful theofor explaininsome of thechemical and

    physical properties of matter.

    In 1808, the English chemist, John Dalton (6 September 1766 27 July 1844), produced experimental evidence show that chemical compounds consist of molecules which are groups of atoms of various elements united in the

    same simple numerical proportion.

    is a substance which cannot be split into simpler substances, while is the smallest portion of An element an atomelement which can take part in a chemical change.

    Some scientists thought of atoms as being like tiny billiard-balls, but since then we have learned a great deal abouthe nature of the atoms themselves.

    The structure of atoms will be discussed later. In this post we shall show how the is used inmolecular theoryphysics to explain some of the elementary properties of gases, liquids and solids.

    So let's begin...

    Brownian movement

    I do not want to repeat myself. So please read :Einsteins theory for Brownian motion . It is discussed in detaiwith gif picture:) .

    Nature of the force between atoms and molecules

    Newton's law of universal gravitation which works so well in calculations of the force between two pieces ofmatter whose distance apart is large compared with their sizes, to give the right answer when applied to twofailsmolecules which are very close together. This does not mean that the gravitational attraction no longer acts but that

    incomparably greater forces of a different kind come into action.

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    Now we shall deal with this subject very briefly. As most students know ( ) that electricity can produceI hopeelectric and magnetic forces of attraction and repulsion.

    Atoms themselves contain particles of electricity in motion and so we get electric and magnetic forces betweenthem.

    We shall, therefore, the situation by saying that, when atoms are very close together, the forces between thsum upare in nature.electromagnetic The net result is that, when their centers are a certain distance apart, the resultantforce between two atoms of molecules is . When closer than this they one another and when further aparzero repelthey one another.attract

    Furthermore, these differ from one kind of atom to another and even between atoms of theelectromagnetic forcessame kind depending on whether a substance is in the liquid state or some particular kind of solid state. A goodexample of a substance which can exist in more than one kind of solid state is carbon ( graphite or, of course,diamond in thepictureabove).

    Three states of matter, or four?

    Matter commonly exists in either the solid, liquid or gas state.of course there is a plasma state of matterwhich could not be

    discussed now ). In a solid substance the molecules about their zerovibrateresultant force position, alternately attraction and repelling oneanother.

    crystalline structureAll true solids have a in which the atoms are arranged in a regular pattern called the lattice.There is, however, a borderline class of materials which appear to be solids but actually are very viscous liquids.Pitch is a good example. When struck with a hammer, it readily splinters, but if placed in a funnel and left for severyears it slowly flows out.

    In a liquid , the molecules are also vibrating to and fro alternately attracting and repelling one another with forcewhich can be just as strong as those in a solid.

    At the same time, however, the liquid molecules can move freely among one another, exchanging partners as theygo. It is this which enables a liquid to take up the shape of any vessel in which it is placed.freedom of movement

    It is worth mentioning that experimental evidence indicates that small groups of liquid molecules can arrangethemselves for very short period of time into the same kind of regular pattern found in solids. In a gas high velocitiesthe molecules are much further apart than those in solids and liquids. They move atcolliding with one another and with the walls of their containing vessel.

    Except at the moment of collision, the short-range we have been describing do not come ininter-molecularforcesaction.

    Unless the gas is highly compressed, the molecules are, for the greater part of the time, so far apart that theattractive force is effectively negligible.

    Consequently, a gas is perfectly free to expand and completely fill the vessel containing it.

    average distanceThe moved by a molecule between collisions is called its .mean free pathRudolf Clausius (2 January 1822 24 August 1888)applied the laws of mechanics to these collisions and showed

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    how they explained the relation between the pressure and volume of a gas.

    Coming up: Surface tension, diffusion, and more. :)

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