1st Year Chemistry Chapter 6 Notes

• Define Atomic Size: The atomic radius means the average distance between the nucleus of the atom and its outermost electronic shell. Reasons of less precision of radius determination The radius of an atom cannot be determined precisely due to the following reasons: (i) There is no sharp boundary of an atom. The probability of finding an electron never becomes exactly zero even at large distances from the nucleus. (ii) The electronic probability distribution is affected by neighbouring atoms. For this reason, the size of an atom may change from one compound to another. Techniques for radius measurement Atomic radii can be determined, by measuring the distances between the centres of adjacent atoms with the help of X-rays or by spectroscopic measurements. Variation of Atomic Radii in the Periodic Table Along periods The atomic radii decrease from left to the right in a period. The decreasing trend in a period is due to: 1. The increase in the nuclear charge. As the nuclear charge increases the pull on the electrons is increased and size of an atom decreases. 2. The shielding effect remains the same from left to right in a period. Shielding effect/screening effect The screening effect is also called shielding effect. This is responsible for the decrease in force of attraction of the nucleus for the electrons present in the valence shell. Decrease of atomic radii in higher periods The decrease of atomic radii is very prominent in second period, but less in higher periods. The decrease is small when we travel from left to right in transition elements Sc(21)-Zn(30), Y(39)-Cd(48) due to the intervening electrons.
• Reasons of less precision of radius determination
• Variation of Atomic Radii in the Periodic Table
• Decrease of atomic radii in higher periods
• Definition of  Ionic Radii and Ionic radii of cations and anions
• Decrease in radius with increasing positive charge
• Increase in radius with increasing negative charge
• Definition of Covalent Radii with example
• Determination of covalent radius
• Definition of Ionization Energy
• Factors Influencing Ionization Energies
• Definition of Electron Affinity with example
• Definition of Electronegativity: The tendency of an atom to attract a shared electron pair towards itself is called its electronegativity. Factors influencing electronegativity Following factors influence the electronegativity: 1. Atomic radius 2. Nuclear charge 3. Shielding effect It is related to the ionization energy and the electron affinity of the element. Thus, fluorine atom is more electronegative than hydrogen atom. Calculation of electronegativity values. Pauling calculated the electronegativity values of elements from the difference between the expected bond energies for their normal covalent bond and the experimentally determined values. He devised an electronegativity scale on which fluorine is given an arbitrary standard value 4.0. It is the most electronegative element. The electronegativity values of other elements are compared with fluorine. Units Electronegativity has no units.
• Factors influencing electronegativity
• Calculation of electronegativity values 
  
• Variation in the Periodic Table
• Types of Bonds Chemical bonds can be classified as: (i) Ionic bond (ii) Covalent bond (iii) Coordinate covalent bond
• Definition of Ionic Bond with example
• Definition of Covalent Bond and Types of covalent bond
• Definition of Coordinate Covalent Bond and Types of covalent bond
• Valence Shell Electron Pair Repulsion Theory (VSEPR)
• Valence Bond Theory(VBT)
• Definition of Hybridization and its types
• Explanation of Structure of Methane
• Explanation of Structure of Ammonia
• Explanation of Structure of Water
• Explanation of Structure of ethene
• Define electron affinity. Name the factors affecting it and trends in the periodic table.
• Discuss the valence bond theory. How are the sigma and pi bonds formed?
• Define dipole moment. Give its units. How is it used to determine the geometry of molecule? Give an example.