CHEMISTRY Class 9 Atomic structure part 1
CHEMISTRY
ATOMIC STRUCTURE
1 DALTON ATOMIC THEORY
- Matter is made up of minute particles called atom
- Atom cannot be divided during chemical reactions.Atoms can neither be created nor destroyed.
- Atoms of the same elements are identical in properties,size and mass.
- Atoms of different elements differ in their properties and mass.
- Atom is the smallest particle that can take part in chemical reactions.
- Compounds are formed when atoms of two or more elements combine in a small ratio.
2 Discovery of electron -Discharge tube experiment -Conducted by J J Thomson
The experiment conducted by using a simple apparatus known as discharge tube. It consists of a sealed glass tube of about 50 cm legth with two metal electrodes fused to the ends and a side tube connected to a vacuum pump.The discharge tube is filled with the gas under study and two electrodes are connected to a high voltage DC source.
At normal temperature and the pressure, electric current does not pass through the gases. Due to this, pressure in the discharge tube decreases . Connect a battery to the induction coil for getting DC voltage. The electrode which is connected to the positive terminal of the electrode has positive change and the electrode connected to the negative terminal has negative charge.
JJ Thomson noticed that, when electricity was passed through the discharge tube at low pressure, the gas walls of the tube at side of the positive plate (anode)were glowing.He found out that some particles formed inside the tube caused this glow.
In the discharge tube,some invisible rays flow from negative electrode to positive electrode (Cathode to Anode). These invisible rays make the fluorescence which is the reason for glowing.
If a light paddle wheel of a papers is placed in the discharge tube,the paddle wheel rotates. This indicates that cathode rays consist of material particles and produced mechanical effect.
Whenever an object is placed in the cathode rays.It causes a shadow on the walls of the opposite to the negative electrode.This experiment showed that cathode rays travels in straight.
When the beam of cathode rays are allowed to pass between electrically charged plate,they are deviated towards the positively charged plate.This shows that particle in the cathode rays are negatively charged.
3 Inference arrived by J J Thomson
- The cause of the glow is the invisible rays emitted from the negative metal plate of the discharge tube
- It is a flow of particles with negative charge
- There are particles carrying negative charge in all substances
4 ELECTRON
Electron is the first identified particles in an atom.The existence of electrons in an atom was shown by J J Thomson by passing electricity at high voltage through a gas at low pressure.This great discovery proved Dalton Atomic Theory was wrong.
Charge of an electron - Negative
charge - 1.6 x 10^-19 C
Mass - 1/1837th mass of Hydrogen atom
9.1x10^-31 Kg (9.1 x 10^-28 g )
Thomson model of atom(plum pudding model)
• The negatively charged electrons are embedded in over a sphere of positive charge.
• The total number of units of positive charge of the sphere are equal to the total number of negatively charged electrons. Hence an atom is electrically neutral.
Proton
Observation and inference of Rutherford gold foil experiment
Observation
. Most of the alpha particles passed through the gold foil without any deviation. . Some alpha particles deviated in small angles
.Only small number of alpha particles bounced back.
Inference
. Most of the space in an atom is empty
. Since positively charged alpha particle are deflected,we can infer that there are +vely charged parts in an atom.These positively charged parts are concentrated in a small
space in atoms.
. There is a small center in an atom which
consists of the entire mass and positive charge.The positive charged alpha particles moving towards this center reflect.
Proton
Charge : positive charge (1.6 x 10^-19 C)
mass : Same as Hydrogen atom (1.67 x 10^-27 kg)
Rutherford atom model
The structure of the atom presented by Rutherford is similar to that of the solar system. This model is known as the planetary model of atom.
According to electro magnetic theory put forward by James Clerk Maxwell, a charged body in circular motion continuously emits energy as radiations. Electrons which revolve around the nucleus are negatively charged. Hence, while electrons revove around the nucleus, they should emit electromagnetic radiations, lose energy and come closer tothe nucleus. Finally the negatively charged electrons should collapse in to the nucleus. But this does not happen in an atom.Rutherford model failed to explain this.
James Chadwick confirmed the presence of neutral particle within the nucleus of an atom. These particles are called neutron. The mass of neutrons determined is slightly higher than that of protons.
1 അഭിപ്രായങ്ങള്:
Effective notes
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