Structure Of The Atom PDF

Structure of the Atom  Key concepts 1. Fundamental Constituents of an Atom Electron Protons Neutrons 2. Thomson’s Model of an Atom 3. Rutherford’s Model of an Atom 4. Bohr’s Model of an Atom 5. Distribution of Electrons in Distinct Sh...

Structure of the Atom  Key concepts 1. Fundamental Constituents of an Atom Electron Protons Neutrons 2. Thomson’s Model of an Atom 3. Rutherford’s Model of an Atom 4. Bohr’s Model of an Atom 5. Distribution of Electrons in Distinct Shells (Electronic configuration) 6. Valency 7. Atomic Number (Z) 8. Mass Number (A) 9. Isotopes and Isobars Cathode rays - Discovery of electrons Discharge tube experiment - Discovery of Electron Thomson’s Model of Atom An atom consists of positively charged sphere and the electrons are embedded in it. The negative and positive charges are equal in magnitude. So atom as a whole is electrically neutral. Thomson’s atomic model Limitations of Thomson’s Atomic Model Thomson’s atomic model failed to explain how the positive charge holds on the electrons inside the atom. It also failed to explain an atom’s stability. The theory did not mention anything about the nucleus of an atom. It was unable to explain the scattering experiment of Rutherford. Rutherford’s Model of Atom Rutherford made fast moving alpha particles to fall on a thin gold foil. Observation Most of the fast moving alpha particles passed through the gold foil. Some of the alpha particles were deflected by the foil by small angles one out of every 12000 particles appeared to rebound.  Conclusion Most of the space inside the atom is empty because most of the alpha particles passed through the gold foil without getting deflected. Very few alpha particles were deflected from their path, indicating that the positive charge of the atom occupies very little space. A very small fraction of alpha particles deflected by 180o , indicating that all the positive charge and the mass of the gold atom were concentrated in a very small volume within the atom. Features of the Nuclear model of atom There is a positively charged centre in an atom called the nucleus. Nearly all the mass of the atom resides in the nucleus. The electrons revolve around the nucleus in well defined orbits. The size of the nucleus is very small as compared to the size of the atom. Rutherford’s Atom Model  Drawbacks of Rutherford’s Atomic Model Rutherford’s Atomic Model had the Following Limitations: This atomic model failed to explain the stability of atoms. According to the model, electrons revolve around the positively charged nucleus. It's not possible for the long run as we know atoms are stable while any particle in a circular orbit would undergo acceleration. During acceleration charged particles would radiate energy. Revolving electrons will lose energy and finally fall into the nucleus. Postulates of Bohr’s Model of atom Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom. While revolving in discrete orbits the electrons do not radiate energy. These orbits or shells are called energy levels. They are numbered 1,2,3,4…. Or named K, L, M, N …. When an electron moves from a lower level to a higher level it absorbs energy. When an electron moves from a higher level to a lower level, it emits energy. Electron distribution in various shells. Atomic Number (Z) and Mass Number (A) Atomic number is the number of protons present in the nucleus of an atom. It is denoted by the symbol Z Protons and neutrons are called nucleons. Mass number is the sum of the total number of protons and neutrons present in the nucleus of an atom. Isotopes Isotopes are atoms with same atomic number but different mass number For example, hydrogen atom, it has three isotopes, namely Protium (11H), Deuterium ( 21H or D) and Tritium ( 31H or T). The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively Isotopes have different number of neutrons but same number of protons and electrons. Isotopes of Hydrogen Application of Isotopes 1) An isotope of Uranium (i.e. Uranium-235) is used as a fuel in a nuclear reactor. 2) An isotope of cobalt (i.e. cobalt-60) is used in the treatment of cancer. 3) An isotope of iodine (i.e. iodine-131) is used in the treatment of goiter. 4) An isotope of carbon (i.e.carbon-14) is used in the carbon dating technique (through which the age of fossils can be determined) Isobars Isobars are atoms having same mass number but different atomic number. ex: Two elements — calcium, atomic number 20, and argon, atomic number 18. The number of electrons in these atoms is different, but the mass number of both these elements is 40. Carbon-14 (146C), Nitrogen-14 (147N), and Oxygen-14 (148O) are also isobar Uses of Isobars Nuclear reactors can use uranium isobars. Iodine’s isobars are used in goiter treatment. For cancer treatment, isobars of cobalt can be used.