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  • Chemistry – Structure Of The Atom – Notes For W.B.C.S Examination.
    Posted on December 22nd, 2018 in Chemistry
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    Chemistry – Structure of the Atom – Notes For W.B.C.S Examination.

    রসায়ন – পরমাণুর কাঠামো – WBCS পরীক্ষার জন্য নোট।

    Introduction

    • By 1900, it was discovered that the atom was not a simple, indivisible particle, but rather it contains sub-atomic particles.
    • J.J. Thomson discovered the sub-atomic particle namely ‘electron.’
    • J.J. Thomson was the first person who proposed a model for the structure of an atom.
    • In 1886, E. Goldstein discovered the presence of new radiations in a gas discharge and named them canal rays.
    • Another positively charged sub-atomic particle was discovered with experiments of canal rays and named it proton.

    Thomson’s Model of Atom

    • Thomson proposed that an atom consists of a positively charged sphere and the electrons (negative charge) are embedded in it.
    • Further, Thomson said that the negative and positive charges are equal in magnitude. Thus, the atom as a whole is electrically neutral.

    Rutherford’s Model of Atom

    • E. Rutherford is popular as the ‘Father’ of nuclear physics.
    • Rutherford is largely known for his work on radioactivity and the discovery of the nucleus of an atom with the gold foil experiment.
    • Rutherford said that in an atom, there is a positively charged center known as the nucleus.
    • Rutherford said that nearly all the mass of an atom exists in in the nucleus.
    • According to Rutherford, the electrons revolve around the nucleus in well-defined orbits.

    Bohr’s Model of Atom

    • Neils Bohr further extended Rutherford’s model and improved his drawbacks.
    • According to Bohr, only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
    • Bohr said that electrons do not radiate energy while revolving in discrete orbits.
    • Bohr named orbits or shells as energy levels.
    • Bohr represented these orbits or shells are by the letters K, L, M, N,… or the numbers, n = 1,2,3,4,….

    Neutron

    • In 1932, J. Chadwick discovered a new sub-atomic particle i.e. neutron.
    • Neutron has no charge and a mass nearly equal to that of a proton.
    • Neutrons are present in the nucleus of all atoms, except hydrogen.

    Electrons Distributed in Different Orbits (Shells)

    • The maximum number of electrons that can be present in a shell is given by the formula 2n2.
    • ‘n’ is the orbit number or energy level index, i.e. 1, 2, 3,….
    • According to the given formula −
      • First orbit i.e. K-shell will be = 2 × 12 = 2
      • Second orbit i.e. L-shell will be = 2 × 22 = 8
      • Third orbit i.e. M-shell will be = 2 × 32 = 18
      • Fourth orbit i.e. N-shell will be = 2 × 42 = 32
    • Likewise, the maximum number of electrons that can be accommodated in the outermost orbit is 8.
    • Electrons are not filled in a given shell, unless the inner shells are filled. It means, the shells are filled in a step-wise manner; starting from inner shell to outer shell.

    Valence

    • The electrons, those are present in the outermost shell of an atom, are known as the valence electrons.
    • According to Bohr-Bury model, the outermost shell of an atom can have a maximum of 8 electrons.

    Atomic Number

    • The total number of protons, present in the nucleus of an atom, is known as atomic number.
    • The number of protons of an atom determines the atomic number.
    • Atomic number is denoted by ‘Z’.
    • Protons and neutrons collectively are known as nucleons.

    Mass Number

    • The sum of the total number of protons and neutrons, present in the nucleus of an atom, is known as mass number.

    Isotopes

    • The atoms of the same element, having the same atomic number but different mass numbers, is known as isotopes. E.g. Hydrogen atom has three isotopes namely protium, deuterium, and tritium.
    • The chemical properties of isotopes of an atom are similar but their physical properties are different.

    Isobars

    • Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars. E.g. calcium’s atomic number is 20and argon’s atomic number is 18; further, the number of electrons in these atoms is different, but the mass number of both these elements is 40.

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