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الثلاثاء، 26 مارس، 2013

كيمياء لغات - الباب الأول| Greek philosophers

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كيمياء لغات - الباب الأول| Greek philosophers

02 Thomsom's Model Of Atom Observations

الدرس في شكل نص مقروء

كيمياء لغات - الباب الأول| Greek philosophers
Greek philosophers
*They imagined the possibility of dividing any piece of matter to smaller parts
which is divided into smaller one until reach to A tom
[A] means No but [tom ] means divided.
* Aristotle
* He rejected the concept of [the atom] and believed that:-
*All matters consists of four components [water , air , dust and fire]
* It was belived that cheap metal as Iron or Copper can be changed into precious
one as a Gold by changing the ratio of the constituents.
2-Boyle *He refused Aristole's idea about the nature of substances.
*He put the first definition about the element.
Element: The simplest pure substance can't be divided into simple one
by a traditional chemical method . [ exam 2006]
3- Dalton's atomic theory
* Dalton carried out many researches and experiments.
* Dalton stated the first theory about the atomic structure.
*He postulated that
1-Matter is composed of very minute particles named atoms.
2- An atom is a one solid part and undividable.
3- Atoms of the different elements are different.
Discovery of cathode rays
From some exp. about electric discharge in 1897 scientists reached to :-
1-All gases under a normal conditions from pressure and temp. are insulators
So cathode ray tube must be evacuated G.R.F
2-If a glass tube is discharged from a gas and the pressure inside it ranges from
[ 0.01 - 0.001 ] mmHg then exposed to suitable potential difference
it becomes a good conductor
3-When a potential difference between the two poles increased to 10000 volt
The scientists observed that:-
*Stream of invisible rays were emitted from the cathode and the glass tube glow
*These rays are called cathode rays
Question Define :- [ Cathode rays]
properties of cathode rays
1-They consist of a very small particles.
2-They have a thermal effect.
3-They transfer in a straight lines .
4- They are effected by electric and magnetic field
5- They are negatively charged particles.
6-Their nature do not depend on
-Substance of cathode. or type of a gas inside the discharge tube
* Means they do not differ either in behavior or in nature of the material of cathode or the gas which is used
* This is strong evidence that.
So [ They are fundamental constituent of all matters.] G.R.F
4-Thomson's model of the atom [Exam 2007]
* He suggested a new atomic model of the atom that took into consideration the
existence of electron
*Atom is considered a symmetrical sphere from positive electricity contains
negative electrons inside it
So atom is electrically neutral.
Rutherford's exp.
*By Giger and Marceden
Experiment 1- A deep lead box with a small hole .
2- A piece of radioactive element which produces an alpha particle was placed inside the box.
3- A metal sheet in the form of incomplete circle was plated by
a layer of zinc sulphide then placed in the pass of alpha particles .
4- A very thin gold foil was placed between the beam of alpha
particles and the metal sheet.
Observations and conclusions
Observations Conclusions
Atom has a vast space and it is not uniformly dense. A) Most of the alpha particles appears on the same place.
The atom must contain a tiny part of a very high density [ was named the nucleus ] B) Very small % of alpha particles
was reflected back [ some flashes appeared in front of the foil ]
The dense part of the atom where most of the mass is present , appears to have a similar positive charge to that of alpha particles. C) Some of (alpha) particles penetrated the foil but deflected
* What is the role of rutherford ?
Rutherford’s model
* Postulates of the theory
The atom: - 1) It’s similar to Solar system .
2) It’s extremely small in size.
3) It has a very complicated structure.
The nucleus: - 1) At the center.
2) Very small in size compared to an atom.
(There is a vast space between it and electron’s orbits)
3) An atom is not uniformly dense .
4) The mass of an atom is concentrated in the nucleus G.R.F. Because mass of protons and neutrons inside the nucleus greater than mass of electrons which can be neglected
The electrons: - 1) Thery orbit around the nucleus in a great speeds at
different distances from it.
2) They have a negligible mass compared to the mass of
nucleus .
3) They have –Ve charges and the sum of –Ve charges equal to the sum of +Ve charges so that the atom is electrically neutral. G.R.F.
4) They keep their orbits around the nucleus due to
the centrifugal force = attraction force
1- Rutherford's concept was contradicated by Maxwell's theory
[An electromagnetic theory based on classic Newton’s laws for slightly big particles. This is called Maxwell’s theory. ]
Maxwell’s theory: -
“when an electrically charged particle rotates around the opposite charged particle, it will lose some of its energy gradually by emission of radiations, leading to a gradual decrease of its speed and its orbit’s radius until hit in the centre.
* By applying this theory on the electron movement.
* We would expect that electrons are in a state of continous emission of radiation with a gradual decrease in the orbit radius, thus leading the electrons to spiral inwords until they hit in the nucleus.
* That was the obvious contradication classical mechanical laws and Rutherford's concept.
2- Failed to explain the spectral line of elements although it depends on atomic structure.
Atomic spectra and its explanation
* The study of atomic spectra is considered the key which solved the puzzle of
the atomic structure .
* On heating a mass of very closed atoms [ solid – liquid – compressed gases ]
to high temperature it radiates light .
* On examining this light by spectroscope we observe rainbow of mixed colours
without any separation in between.
* On heating gases or vapours under reduced pressure to high temp. or by electric
spark , they radiate another type of radiations .
* By examining this light by spectroscope it was found to be composed of a limited
numbers of coloured lines named line spectra.
* Spectral lines are essential characteristic for each element as finger print
* Study of line spectra of the sunlight indicated Hydrogen and Helium, are the main
components of the sun .
Spectroscope :- It is an optical instrument for light analysis.
Secondly: Bohr’s atomic model
Postulates from Rutherford Added other postulates Advantages of Bohr’s model Defects of Bohr’s model Notes
* He adapted some of Rutherford’s postulates: -
1) The nucleus is small in size , heavy and positively charged.
2) No. of +Ve charges = No. of –Ve charges so atom is (electrically neutral).
3) The centrifugal force = attraction force.
* And he added other postulates: -
4) Electrons orbit around nucleus in a definite allowed energy levels (they can’t be found in between).
5) Each electron has a definite energy, depending on the distance from the
nucleus. * This energy increases and gaps decrease further from nucleus.
6) It’s possible to determine both speed and location of electrons practically at the same time.
7) Electrons orbit around nucleus without emission or absorption of energy under normal conditions (ground state).
8) Electron remains in the lowest allowed energy level till it absorbs an amount of energy (quantum), by heating or by electric discharge, it becomes excited and jumps to higher enrgy level, depending on the amount of energy which gained. And return to its original one by losing the same amount of energy
which appears as spectral line.
* Excited atom :- An atom which gained amount of energy and electron jumps
to higher enegy level .
Quantum: - It’s the amount of energy absorbed or emitted when an electron jumps from a level to another.
* The quantum required to transfer an electron from one level to another is not equal means the gaps between the levels are not equal.
It decreases further from the nucleus.
* The electron doesn’t transfer to higher energy level unless the absorbed or emitted energy is equal to the difference between the two levels.
Question Compare between quantum and exitation energy
*Energy increases
*Gaps decrease
* Distance from nucleus increases
Adequacy [Advantages ]of Bohr’s model [ Result]: -
1) It explained the Hydrogen atom spectrum .
2) It introduced a new quantaized energy called (quantum) in an atom.
3) It emphasized that [electrons when moving around the nucleus in the ground state doesn’t radiate energy].
[ Bohr succeded in a reconciliation between Rutherford and Maxwell ]
* Inadequacy [disadvantages ]of Bohr’s atomic model: -
*Despite the great effort of Bohr to construct his atomic model the quantitative calculations of this theory didn't agree with all exp. Data.
1) It failed to explain the spectral line of any atom except Hydrogen.
[ till Helium atom which contains only two electrons.]
2) It considered the electron is a particle only, and did not considered that it also has wave properties.
3) It postulated that it’s possible to determine the location and speed of electron at the same time but this pracically impossible. G.R.F
4) It described that Hydrogen atom is a planar, but it has three dimentional coordinates ( x , y , z ).
1) Electrons never fall inside the nucleus
* That’s due to the presence of two forces equal in magnitude but opposite in direction which are: -
a) Centripetal force .[The attraction force between the nucleus and the electrons.]
b) Centrifugal force [ arising while the electron orbit the nucleus]
2) The quantum of energy required to transfer an electron from one
energy level to another is not equal.
* Because the difference in energy between them is not equal since the
energy gaps decrease further from the nucleus.
3) The electron does not move from energy level to another if the energy absorbed or emitted is less than quantum.
* Because quantum is a limited amount of energy can’t be divided or multiplied.

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