STRUCTURE OF ATOM XI CHEMISTRY NOTES

STRUCTURE OF ATOM

Sub-atomic particles

Discovery of Electron

Electron was discovered by J J Thomson by Cathode ray discharge tube experiment. A cathode ray tube is made of glass containing two thin pieces of metal (electrodes) sealed in it. The electrical discharge through the gases could be observed only at very low pressures and at very high voltages.When a very high voltage (about 10,000 volts) is applied between the two electrodes, no electric discharge occurs at normal pressure. When the pressure of the gas inside the tube is less than 1 mm of mercury, a dark space appears near the cathode. When the pressure is reduced to 0.01 mm Hg, it fills the whole tube. When the pressure is further reduced, the electric discharge passes between the electrodes and the tube begins to glow. This is due to the striking of some invisible rays from the cathode. These rays which start from the cathode and move away from it, in straight lines are called cathode Rays or cathode ray particles.

These rays can be further checked by making a hole in the anode and coating the tube behind anode with phosphorescent material like zinc sulphide. When these rays strike the zinc sulphide coating, a bright spot on the coating is developed.

Properties of Cathode Rays

(i) The cathode rays start from cathode and move towards the anode.

(ii) They are invisible, but their behaviour can be observed with the help of fluorescent or phosphorescent materials.

(iii) In the absence of electrical or magnetic field, these rays travel in straight lines.

(iv) In the presence of electrical or magnetic field, the cathode rays behave similar to that of negatively charged particles.From this it is clear that the cathode rays consist of negatively charged particles called electrons.

(v) The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube.

Charge to Mass Ratio of Electron

J.J. Thomson measured the ratio of electrical charge (e) to the mass of electron (me ) by using cathode ray tube and applying electrical and magnetic field perpendicular to each other as well as to the path of electrons.The amount of deviation of the particles from their path in the presence of electrical or magnetic field depends upon:

(i) the magnitude of the negative charge on the particle (greater the magnitude of the charge on the particle, greater is the deflection).

(ii) the mass of the particle (lighter the particle, greater the deflection).If an element X has the atomic number Z and the mass number A, it is denoted as: or _ZX^A

Isotopes, Isobars and Isotones

Isotopes are atoms with same atomic number but different mass number. That is they contain same number of protons but different number of neutrons. Hydrogen has three isotopes Protium (₁H¹), Deuterium (₁H² or ₁D²) and Tritium (₁H³ or ₁T³). Among these Protium is the ordinary hydrogen and Tritium is the radioactive isotope of Hydrogen.The number of protons, neutrons and electrons present in the 3 types of hydrogen are:Isotope Number of protons Number of electrons Number of Neutrons respectively

                                    P                            e                             n

Protium                      1                      1                     0                    

Deuterium                  1                     1                      1         

Tritium                        1                     1                     2

Almost all the elements have isotopes. All the isotopes of a given element have same chemical properties, but they differ in their physical properties.

Isobars are atoms of different elements having same mass number but different atomic number. i.e. they have different number of protons but have equal sum of the protons and neutrons. e.g. ₆C¹⁴ and ₇N¹⁴ ₁₈Ar⁴⁰ and ₂₀Ca⁴⁰

Isotones are atoms having same number of neutrons but have different atomic numbers. Some examples are:

Isotones                     p                     e                      n

₆C¹⁴                            6                     6                     8        

₇N¹⁵                            7                      7                      8         

 ₈O¹⁶                            8                      8                     8

ATOM  MODELS

1.Thomson’s Model of Atom

J. J. Thomson proposed the first atom model, which is known as the plum pudding or raisin pudding or watermelon model. According to this model, an atom has a spherical shape in which the positive charge is uniformly distributed. The electrons are distributed in it, just like the seeds are distributed in a water melon or plums are distributed in a pudding. An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom. Also the total positive charge in an atom is equal to the total negative charge and hence the atom is electrically neutral.

Atom is a sphere of positive charge in which number of electrons are embedded.

Limitations: - Could not satisfactorily explain the results of scattering experiment carried out by Rutherford.

2.Rutherford’s Nuclear Model of Atom

Rutherford proposed an atom model based on his α–particle scattering experiment. He bombarded a very thin gold foil with α–particles. Almost all the positive charge and mass of atom is present in its nucleus.Electrons revolve around the nucleus in circular orbits.There is strong electrostatic attraction between nucleus and electrons. 

Observations: The important observations made by Rutherford are:

1. Most of the α– particles passed through the gold foil without any deviation.

2. A small fraction of the α–particles was deflected by small angles.

3. A very few α– particles (∼1 in 20,000) bounced back, that is, were deflected by nearly 180°.

Conclusions: From the above observations, Rutherford made the following conclusions:

1. Since most of the α–particles passed through the foil without any deviation, most space in the atom is empty.

2. A few positively charged α– particles were deflected. This is because the positive charge of the atom is concentrated in a very small volume at the centre called nucleus.

3. The volume occupied by the nucleus is negligibly small as compared to the total volume of the atom. The radius of the atom is about 10–10 m, while that of nucleus is 10–15 m.On the basis of above observations and conclusions, Rutherford proposed the Nuclear model (Planetary model) of atom. According to this model:

1. All the positive charge and most of the mass of the atom were concentrated in an extremely small region called nucleus.

2. Electrons are revolving round the nucleus with a very high speed in circular paths called orbits.

3. Electrons and the nucleus are held together by electrostatic forces of attraction.

Drawbacks or Limitations of Rutherford’s atom model

1. According to the electromagnetic theory of Max Planck, charged particles in accelerated motion should emit energy in the form of radiation. So an electron in orbit will emit radiation. So the energy of the electron decreases and finally it falls to the nucleus through a spiral path. But this does not happen. So Rutherford’s model cannot explain the stability of the atom.

2. He cannot explain the electronic structure of atom. That is about the distribution of electrons around the nucleus and the energies of the electrons.

Limitations: - Could not explain stability and electronic structure of atom.

 

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Subatomic Particles:

Name	Electron	Proton	Neutron
Symbol	e-	p	n
Approximate relative mass	1/1836	1	1
Mass in kg	9.109×10–31	1.673×10–27	1.675×10–27
Mass in amu	5.485×10–4	1.007	1.008
Charge (coulomb)	1.602×10–19	1.602×10–19	0
Actual Charge (e.s.u)	4.8 × 10–10	4.8 × 10–10	0