Evolution concept of the atomic structure , Atomic theory & Properties of cathode rays
Long time ago man was asking about the nature of matter and its structure , while the trials done by the scientists to answer this question across different eras , the concept of the atomic structure is evolved , The scientists were interested in studying the atomic structure such as Heisenberg , Pauli , Schrodinger , Bohr , De Broglie , Einstein & Planck .
Democritus’s ( Greek philosopher ) idea
He imagined the possibility of dividing any piece of matter to smaller parts , then dividing those parts into smaller particles and so on , until we reach to an undividable fragment , he named it an atom , In the Greek language : A means no and tom means divide .
Aristotle’s idea ( 4th century B.C )
He rejected the concept of the atom and believed that all matter – whatever their nature – are composed of four components , which are water , air , dust and fire .
It was believed that cheap metals as iron or copper can be changed into precious ones as gold by changing the ratios of these four constituents .
This miserable idea caused a retard of development in chemistry science for more than thousand years because the scientists were busy by changing cheap metals into precious ones .
Boyle’s idea ( 1661 )
The Irish scientist Boyle refused Aristotle’s idea about the name of substance and gave the first definition of the element , The element is a pure simple substance that can not be changed to simpler forms by the traditional chemical methods .
Dalton’s model of the atom ( 1803 )
The English scientist John Dalton stated the first theory about the atomic structure .
The main postulates of Dalton’s atomic theory :
- The element is composed of very minute particles , which are named atoms .
- The atom is a very minute undividable solid particle .
- The atoms of the same element are similar in the mass , but they are different from the atoms of any other element .
- The compounds are formed by the combination of atoms of different elements in simple numerical ratios .
Thomson’s model of the atom ( 1897 )
The scientist Thomson carried out many experiments on the electric discharge through gases from which he had discovered the cathode rays .
Discovery of cathode rays :
It was known that gases do not conduct electricity under normal conditions of pressure and temperature , However , gases conduct electricity in a discharge tube whose two poles are connected to an electric source of a suitable high potential difference and under a very low pressure .
If the potential difference between the two poles of the vacuum glass tube exceeds 10000 volts , a steam of invisible rays was emitted from the cathode ( the negative pole ) , causing a fluorescent glow on hitting the tube wall , These rays were named by cathode rays .
Cathode rays : A stream of invisible rays was emitted from cathode , causing a fluorescent glow hitting the discharge tube wall under very low pressure and potential difference of about 10000 volts , It was later known that they are composed of minute particles named electrons .
Properties of cathode rays
- They consist of very fine negatively charged particles ( electrons ) .
- They move in a straight lines .
- They have a thermal effect .
- They are affected by both electric and magnetic fields .
- They do not differ either in behavior or in nature if the material of the cathode or the used gas is changed , this is a strong evidence that it is a fundamental constituent of any matter .
In the light of the electrical discharge experiment , Thomson suggested a new atomic model for the atom .
The postulate of Thomson’s model :
Rutherford’s model of the atom ( 1911 )
Rutherford‘s students Geiger and Marsden performed his famous laboratory experiment .
The used tools : A deep lead box containing a source of alpha particles ( α ) inside it , A metal sheet covered from the inside with a layer of zinc sulphide ZnS which is used to detect the alpha particles , because it glows at the site of collision with alpha particles and a very thin gold foil .
It was allowed for alpha particles to collide a metal sheet , where it was possible to define the location and number of alpha particles by counting the glows which appeared on the metal sheet , A very thin gold foil was placed between the beam of alpha particles and the metal sheet .
The vast majority of α-particles penetrated the gold foil and hit the same places in which they appeared before placing the gold foil .
A very small percentage of α-particles did not penetrate the gold foil and reflected back , where some flashes appeared in front of the foil , Some of α-particles penetrated the foil , but were deflected .
The dense part of the atom ( where most of the mass is present ) appears to have a similar positive charge to that of α-particles , On the basis of his experiment and from the experiments of other scientists , Rutherford designed his atomic model as follows :
The postulates of Rutherford’s atomic theory :
Atom is an extremely small sized particles , It has a complicated structure resembles the solar system , since it’s composed of a central nucleus ( representing the Sun ) , where the electrons revolve around ( representing the planets ) .
Nucleus is much smaller than the atom and most of the atomic mass is concentrated in it , There is a vast space between the nucleus and the orbits of electrons ( the atom is not uniformly dense ) , It is positively charged .
Electrons have negligible mass compared to that of the nucleus , Their charge is negative and equals the nuclear positive charge ( the atom is electrically neutral ) .
Electrons travel around the nucleus at a tremendous speed in special orbits , despite of the mutual attraction between them and the nucleus , This attraction force is equal in quantity and opposite in direction to the centrifugal force resulted from the electron revolving around the nucleus .
Electron does not fall into the nucleus in spite of the attraction force between them , because the attraction force equals the centrifugal force resulted from the revolving of electron around the nucleus .
Rutherford’s theory had failed to explain the atomic structure because it did not explain the system in which electrons revolve around the nucleus .