Structure of Atoms Specially design for IIT-JEE and Other engineering entrance examination aspirants
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Dalton's Atomic Theory John Dalton, in 1803, put forward his atomic theory. important points of this theory are (i) Matter is composed of particles called atoms. ii) All the atoms of an element are similar in mass and properties. (ii Atom is indivisible. (iv) Atoms of different elements combine in a simple ratio to give "compound atom" The discovery of phenomena of Radioactivity by Prof. Henri Becquerel showed that atom is divisible The discovery of isotopes of an element by Soddy proved that the atoms of an element are different in mass and properties. The modern researches, such as discharge tube experiments have conclusively proved that atom is no longer an indivisible particle. It has definitely a complicated structure. Recent researches have shown that atom is composed of three, elementary, fundamental or sub- atomic particles, namely electron, proton and neutron.
Electron Ge): It is (discovered by J.J. Thomson), negatively charged particle. It is the component particle of cathode rays. IMPORTANT POINTS ABOUT ELECTRON o Cathode rays were discovered by William Crooke's Cathode rays originate from cathode in the discharge tube. o Cathode rays are composed of stream of negatively charged particles o The magnitude of negative charge on electron was first determined by Millikan (oil drop experiment) o The specific charge (e/m) on electron was first of all determined by J.J. Thomson using mass spectrometer. a The value of of cathode ray particle does not depend upon the material of the cathode or the nature of the gas taken in the discharge tube. o The name electron was suggested by J.S. Stoney a Magnitude of charge on electron 1.6x 101 coulomb 4.8 x 1010 e.s.u o Amount of charge on one mole of electrons is one Faraday
28 Rest mass of electron 0 gram -9.1 x 103 kg -31 = 9.1 10-33 Quintal Mass ofelectron on atomic scale = 0.000549 a mu of the mass ofhydrogen atom. 1837 im rest o The mass of electron moving with a velocity v' is given as: m (in motion) Mass of a substance in motion increases with increase in velocity. o Mass of electron moving with a velocity of light will become infinite Mass of one mole of electrons is 0.55 mg. o Electron is universal component of matter o o Electron is the fundamental particle which takes part in chemical combination. o The physical and chemical properties of an element depend upon the distribution of electrons in outer shells o Flow of electrons in a conductor means flow of electricity which can be detected by means of a Galvanometer.
Proton (iH1, Ht,P) It was discovered by Rutherford. It is a positively charged particle. The magnitude of charge on proton is the same as that on an electron. Proton, like electron, is a fundamental particle and is universal component of matter. MORE ABOUT PROTON o charge on proton = 1.6 10-19 coulomb = 4.8 10-10 e.s.u. Mass of proton = mass of hydrogen atom = 1837 times the mass of electron 1.00747 a.m.u. = 1.6726 10-24 gram = 1.6726 10-27 kg.
4 Volume of a pnis roughly close to 1.5 x 1038 cm3. Mass of one mole of protons is about 1.007 gm. Proton is protium nucleus Proton is ionized hydrogen atom (H) Removal of solitary electron from hydrogen atom gives proton. Hydrogen atom minus electron is proton
Neutron (on , N) It is a chargeless particle i.e., it is neutral. It was discovered by James Chadwick. The reason of its late discovery is its charglessness When beryllium or boron is bombarded with o-particles, highly penetrating radiations are obtained These radiations are not deflected from their path by strong magnetic or electric field. These radiations are composed of chargeless particles, known as neutrons. IMPORTANT POINTS ABOUT NEUTRON Mass of neutron1.00899 a.m.u - 1.6749 x 1027 kg 3 Actual mass of neutron is slightly greater than that of proton. Density of neutron is of the order 10" kg/cc 12 3 Out of the three types of fundamental particles, neutron is the most unstable. It decays as follows: 0 Neutron Proton Electron Antineutrino
Niranjay Kumar Dwivedi
(N.K.D.Sir) is a Complete Chemistry faculty for IIT~JEE, and NEET examination. with more then 8 year of Teaching experience