Chapter - 4
Structure of the Atom
️ Atomic :-
Atoms are the building blocks of matter. It is the smallest unit of matter that is made up of three subatomic particles: protons, neutrons and electrons .
Earlier Dalton had said that the atom is indivisible i.e. cannot be further divided which has been proved wrong by the discovery of sub-atomic particles.
️ Discovery of electron :-
Electron was discovered by JJ Thomson with the help of cathode rays. Thomson explained the presence of electrons in an atom with the help of cathode rays.
Some important facts about electron :-
- Charge on electron = -1.6×10⁻¹⁹C
- Mass on electron = 9.1 × 10⁻³¹ Kg
️ Discovery of Proton :-
E. Goldstein discovered the positively charged particle i.e. proton in the atom by using his famous anode rays or Canal rays.
Some facts about proton :-
- Charge on proton = +1.6 × 10⁻¹⁹C
- Mass of proton = 1.673 × 10⁻²⁷Kg
- Mass of proton = 1840 x mass of electron
️ Discovery of Neutron :-
James Chadwick made light elements (eg- lithium, boron, etc.) collide with particles, due to which a new particle whose mass was equal to that of a proton, and they were free of charge, proved the origin. These particles were named neutrons.
- Neutrons are not present in the protium isotope of hydrogen.
- The mass of the electron is much less than the mass of the proton and neutron, so the mass of the atom will be the sum of the masses of the proton and neutron.
️ Atomic Model :-
After the discovery of subatomic particles such as electrons, protons and neutrons, various models of the atom were given. Some of the atomic models are as follows:-
- (a) Thomson's atomic model
- (b) Rutherford's atomic model
- (c) Bohr's atomic model
These days the 'Quantum Mechanical Atomic Model', has been found to be scientifically correct and this model has been approved. This model will be taught in higher classes.
️ Thomson's Atomic Model :-
This atomic model of Thomson is called ' Sliced Watermelon Model '. In Thomson's model, the positive charge in the atom is scattered like the red part of a watermelon, while the electrons are trapped in the positively charged sphere like a watermelon seed.
Although this model discussed the chargeless characteristic of the atom, some scientists did not understand this model, so it was rejected.
️ Rutherford's Atomic Model :-
Rutherford, in his experiment, collided fast moving alpha (helium nucleus He₄) particles with gold foil.
Result of Rutherford's experiment :-
- Most of the alpha particles passed straight through the gold foil without being bent.
- Some alpha particles are bent from the following angles.
- One particle out of every 12000 particles came back.
Conclusion on the basis of Rutherford's experiment :-
- On the basis of the results of his experiment, Rutherford made the following conclusions:-
Most of the inner part of the atom is empty because most of the alpha particles leave the gold foil without folding.
A positively charged sphere in the middle of the atom called the nucleus, because one out of 12000 a-particles returned.
Because most of the particles ejected directly from the gold foil and only a few particles showed deflection, on this basis it was concluded that most of the inner part of the atom is empty and the nucleus is present in a very small part of this empty part.
- The volume of the nucleus is 10⁻5 times the volume of the atom.
The entire mass of an atom is in its nucleus.
Features of using Rutherford :-
On the basis of his experiment, Rutherford presented the model of the atom which had the following characteristics
- The center of the atom is positively charged which is called the nucleus.
- The entire mass of an atom is contained in the nucleus.
- The electrons revolve around the nucleus in a circular path.
- The size of the nucleus is much smaller than the size of the atom.
️ Drawbacks of Rutherford's Atomic Model :-
According to Rutherford, electrons revolve around the nucleus in a circular path, but being charged, these particles constantly lose their energy, due to which they eventually enter the nucleus and make the atom unstable.
This was the biggest shortcoming of the Rutherford atomic model, which Rutherford could not explain.
️ Bohr's Atomic Model :-
The shortcoming of the Rutherford model was overcome by Bohr's atomic model. Niels Bohr presented his model about the atom in 1912, in which the following facts were present: -
- Electrons can revolve only in certain orbits, which are called fixed orbitals of the electron.
- While revolving in these designated orbits, these electrons do not radiate their energy.
- The change in the energy of the electrons of any atom is due to the transfer of these electron's orbitals.
️ Atomic Number :-
The total number of protons in an atom is called its atomic number.
- Atomic number is an indication of any atom, change in it changes the nature of any atom.
- The atomic number is represented by 'z'.
- In any uncharged atom, the number of protons and electrons is equal.
️ Mass Number :-
The mass number is the sum of the number of protons and neutrons present in the nucleus of an atom.
- The mass number is represented by 'A'.
️ Connectivity :-
According to Borbury's law, we know that '8' electrons can be filled in the last shell of any atom.
Every element to fill 8 electrons in its outer shell, free electron from itself or accept electron from other elements.
8 The number of electrons that an element takes or gives to keep the electron in its last shell is used to let it take. It is called connectivity.
️ Isotopes :-
Atoms of the same element having the same atomic number but different mass number. Such atoms are called isotopes.
️ Use of isotopes :-
- Uranium isotopes are used as fuel in nuclear plants.
- Isotopes of cobalt are used in the treatment of cancer.
- Iodine isotopes are used in the treatment of goiter.
- C-14 (Carbon-14) is used in carbon dating.
️ Coordinator :-
Atoms of different elements which have same mass number but different atomic number are called isobodies.
