Notes-Class 8-Science-Chapter-5-Inside the Atom-Maharashtra Board

Rutherford’s scattering experiment : Rutherford studied the inside of atom by his celebrated scattering experiment and put forth the nuclear model of atom in the year 1911.

Alpha particles emitted by radioactive element bear a positive charge. Rutherford bombarded alpha particles through a very thin gold foil. He observed the path of a  - particles by means of a fluorescent screen around the gold foil. It was expected that

• Most of the alpha particles passed through the gold foil without any deviation.
• Some alpha particles were deflected from their path through small angles.
• A few alpha particles were scattered at large angles.
• A still smaller number of same sign particles get deflected through a larger angle and one a- particle out of 20000 bounced back in the direction opposite to the original path.

• Nucleus: The atomic nucleus is positively charged. Almost entire mass of the atom is concentrated in the nucleus. The nucleus contains two types of subatomic particles together called nucleons. Protons and neutrons are the two types of nucleons.
• Proton (p) : Proton is a positively charged subatomic particle in the atomic nucleus. The positive charge on the nucleus is due to the proton in it. All the elements contain protons. A proton is represented by the symbol ‘p’. Each proton carries a positive charge of +1e. (1e = 1.6 x 10^-9 coulomb). When total positive charge on the nucleus is expressed in the unit ‘e’, its magnitude is equal to the number of proton in the nucleus. The mass of one proton is approximately 1u (1 Dalton). (1u = 1.66 x 10^-27g) (The mass of one hydrogen atom is also approximately 1u.)
• Neutron (n): Neutron is an electrically neutral subatomic particle and is denoted by the symbol ‘n’. The number of neutron in the nucleus is denoted by the symbol ‘N’. Atomic nuclei of all the elements except hydrogen with atomic mass 1u, contain neutrons. The mass of a neutron is approximately 1u, which is almost equal to that of a proton.
• Electron (e-) : Electron is a negatively charged subatomic particle and is denoted by the symbol ‘e-’. Each electron carries one unit of negative charge (—1e). Mass of an electron is 1800 times less than that of a hydrogen atom. Therefore the mass of an electron can be treated as negligible. Electron in the extra nuclear part revolve in the discrete orbits around the nucleus. The energy of an electron is determined by the shell in which it is present.

 Distribution of electron : Electrons revolve in stable shells. These shells have a definite energy. The shell nearest to the nucleus is called the first shell. The next shell is called the second shell.

A symbol ‘n’ is used for the ordinal number of a shell. The shells are referred to by the symbols K, L, M, N, corresponding to the ordinal numbers n = 1, 2, 3, 4,  The maximum number of electron a shell can contain is obtained by the formula ‘2n²’. As the magnitude of ‘n" increases, the energy of an electron in that shell increases.

Electronic configuration of elements : The electrons in an atom are distributed in the shells according to their maximum capacity. The shell wise distribution of the electron in an atom of an element is called the electronic configuration of that element.

• Each electron has a definite energy as per the shell in which it is present.
• Energy of an electron in the first shell (K shell) is the lowest.
• Energy of electron in the subsequent shells goes on increasing with ordinal number of the shell.
• The electronic configuration of an element is such that the energy of all the electrons together is the maximum possible.
• Electrons get a place in the shells in accordance with the maximum capacity of the electron shell in an atom and the increasing order of energy.

Fig-Electronic configuration of atoms

Uses of Isotopes :

• Uranium - 235 is used for nuclear fission and production of electricity
• Radioactive isotopes like Cobalt – 50 are used in the treatment of cancer.
• Iodine - 131 is used in the treatment of goitre, disease of thyroid gland.
• The radioactive isotopes are used for detection of cracks (leakage) in the underground pipes, E.g. Sodium -24,
• Radioactive isotopes are used for food preservation from microbes.
• The radioactive isotope C-14 is used for determining the age of archaeological objects.

When uranium—235 is bombarded with a slow speed neutron, it undergoes nuclear fission. Various elements are produced. For example : Krypton — 92 and

Barium - 141 along with 2 to 3 neutrons are emitted on fission, these neutron have high speed. Their speed is reduced and they are used for bombarding more Uranium—235 nuclei. The process is repeated many times.

In this way a chain reaction of nuclear fission takes place (See the figure). A large amount of nuclear energy is released during a chain reaction of fission. The chain reaction is controlled to prevent the probable explosion.

Neutrons are slowed down using graphite or heavy water as moderator. The chain reaction is controlled by absorbing neutron with the help of rods of boron, cadmium and beryllium. The heat produced in the fission is taken out by water as coolent. Water is converted into steam. The available heat is used to drive turbines to produce electricity.