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Published in: Geography
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ATOMIC STRUCTURE, CLASS IX, CHEMISTRY

Arun M / Faridabad

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Teaches: Mathematics, IIT JEE Mains

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  1. CHEMISTRY KEY POINTS Sub •ect : Chemistr Class : IX Cathode Rays : Board : CBSE TOPIC : STRUCTURE OF ATOM Cathode Rays consist of negatively charged particles known as ELECTRONS. J.J Thomson discovered them. Charge on the Electron is 1.6 x 10-19 coulomb ( Regarded as UNIT NEGATIVE Charge ). Mass of the electron is 9.1 x 10-31 kg ( Negligible ). Anode Rays ( or Positive Rays ) : Anode Rays consist of positively charged particles known as PROTONS. E. Goldstein discovered them. Charge on the Electron is 1.6 x 10-19 coulomb ( Regarded as UNIT POSITIVE Charge ). Mass of the proton is 1.67 x 10-27 kg. Thomson's Model of an Atom • An atom consists of a positively charged sphere and the electrons are embedded in it. • The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral Rutherford's Alpha particle scattering experiment : Observations from Alpha particle scattering experiment : Most of the fast moving Alpha — particles passed straight through the gold foil. • Some of the Alpha — particles were deflected by the foil by small angles. Surprisingly one out of every 12000 particles appeared to rebound. Conclusion from Alpha particle scattering experiment : 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 were deflected by 1800 , indicating that all the positive charge and mass of the atom were concentrated in a very small volume within the atom.
  2. Rutherford's Model of an Atom • There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus. • The electrons revolve around the nucleus in circular paths. The size of the nucleus is very small as compared to the size of the atom. Drawbacks of Rutherford's Model of an Atom • Rutherford model of could not explain the stability of the atom. Rutherford model of atom could not explain as to how the electrons are distributed in the extra nuclear portion in an atom. Bhor's Model of an 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 level represented by the letters K, L, M, N . or the numbers, n = l, 2, 3, 4 Neutrons : Neutrons have no charge. J. Chandwick discovered them. Mass of the electron is nearly equal to that of proton ( 1.675 x 10-27 kg ). They are present in the nucleus of all atoms. Distribution of Electrons in different Orbits ( Shells ): Following was suggested by Bohr and Bury The maximum number of electrons present in a shell is given by the formula 2n2, where n is the shell number or energy level index l, 2, 3 Accordingly maximum number of electrons in different shells are as follows K shell 1 ) = 2, shell (n -2) = 8, M shell (n 18, N shell (n 4 ) = 32, and so on The maximum number of electrons that can be accommodated in the outermost shell is 8. Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step - wise manner. Atomic Number . Atomic number is defined as the total number of protons present in the nucleus of an atom.. It is represented by Z . Atomic Number ( Z ) = No. of protons = No. of electrons
  3. Mass Number . Mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.. It is represented by A . Mass Number ( A ) = No. of protons + No. of neutrons Protons and neutrons are present in the nucleus of an atom. Also protons and neutrons have mass equal to 1 u ( atomic mass unit or unified mass ). Protons and neutrons present in the nucleus of an atom are also called nucleons. Representation of an Atom • The atom of an element is generally represented by the symbol of the element. The atomic no. is indicated on its lower left hand side of its symbol ( say S ) and the mass no. on its upper left hand side as shown below Mass No. ( A) Atomic No. ( Z) Valency of Elements : The combining capacity of any atom with atoms of other elements in order to acquire 8 electrons ( 2 in some exceptional cases ) is called its valency. Elements having 1, 2 and 3 valence electrons ( Electrons in outer most shell ) tend to loose these electrons. These are generally METALS. Exception H ( Hydrogen ) Elements having 5,6 and 7 valence electrons tend to gain 3, 2 and 1 electrons respectively. These are known as NON METALS. Elements which have 4 valence electrons have the properties of both metals and non - metals. These are called semi — metals or metalloids. ELECTROVALENCY : If an element combines by the loss or gain of electrons to form ELECTROVALENT or IONIC compounds, its valency is known as electrovalency. Some examples are NCI, KCI, NH4C1, MgC12, CaC12, Mgo, cao, A1203, NaOH, cuS04, etc.. COVALENCY : If an element combines by the sharing of electrons to form COVALENT or MOLECULAR compounds, its valency is known as covalency. Some examples are HCI, HO, C02, SO, CH4, NH3, C2H50H, CC14, H2S, CS2 etc. ISOTOPES : Isotopes are defined as the atoms of the same element having the same atomic number but different mass numbers. The chemical properties of isotopes are similar as because they have same number of valence electrons. But their physical properties are somewhat different because of difference in mass numbers of atoms. Some examples of Isotopes are -
  4. Hydrogen exists in the form of three Isotopes ( Atomic No. 1 ). Protium ( Mass No. 1 ), Deuterium ( Mass No. 2 ) and Tritium ( Mass No. 3 ). Carbon exists in the form of three Isotopes ( Atomic No. 6 ). Mass Nos. 12, 13 and 14. Chlorine exists in the form of two Isotopes ( Atomic No. 17 ). Mass Nos. 35 and 37. Average Atomic Masses of ISOTOPES . Let us take the case of Chlorine which occurs as two isotopes with atomic masses 3511 and 3711 in the ratio of 3 : 1 (75% : 25 % ) So average atomic mass of chlorine will be as follows --- = 35 x (75 / 100) + 37 x (25 / 100) = 35.5 u This indicates that in a sample of chlorine, both Cl - 35 and Cl — 37 isotopes are present and their average mass is 35.5u All the elements with fractional atomic masses exist as two or more isotopes. Applications of ISOTOPES . • An isotope of Uranium ( U — 235 )is used as a fuel in nuclear reactors. • An isotope of cobalt ( Co 60 ) is used in the treatment of cancer. • An isotope of iodine ( I 131 ) is used in the treatment of goitre. ISOBARS : Isobars are defined as the atoms of the different element having different atomic number but same mass numbers. They have different physical and chemical properties. For example Calcium and Argon have atomic nos. 20 and 18 respectively but their mass no. is same i.e. 40.

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