Engineeringdev

CHEMICAL BONDING  Definition : The attractive force present between the atoms that holds the atoms together as a molecule is known as “ Chemical Bond” ·       1) Why do atoms combine to form a molecule? ·       2) Why do atoms combine in a fixed ratio? ·       3) How can the properties of compounds be understood in term of attractive forces? CAUSE OF CHEMICAL COMBINATION ·       Tendency to acquire inert gas electronic configuration (ns2np6). Atoms of elements acquire eight electrons in their valence shell by involving in chemical combination is known as “ Octet rule” ·       Tendency to acquire minimum energy 1) The atoms are unstable and possess high energy 2) During bond formation energy is released. This energy is known as bond energy. 3) Bond formation between atoms leads to stability with lesser energy. OCTET RULE ·       Noble gas elements, except He(1s2), contain 8 electrons in the valence shell. The stability of these elements is due to its octet configuration. ·       Ot

Hund’s Rule ·       Hund’s rule of maximum multiplicity states that electron pairing takes place only after all the available degenerate orbitals are occupied by one electron each. Ex: Carbon atom (At.No.: 6) ·       Carbon atom has six electrons ·       The first electron goes into the ‘1s’ orbital of k-shell. ·       The second electron will be paired up with the first in the same ‘1s’ orbital. ·       Similarly the third and fourth electrons occupy the ‘2s’ orbital of the L-shell. ·       The fifth electron goes into one of the three ‘2p’ orbitals of L-shell. (2Px) ·       Since the three ‘p’ orbitals (2Px, 2Py, 2P3 are degenerate) the sixth electron goes into 2Py or 2Pz but not 2Px IONIZATION POTENTIAL or IONIZATION ENERGY ·       Definition : “Ionization energy or Ionization potential is defined as the minimum energy required to remove an electron from the outermost orbital of an atom in the gaseous state.” ·       Expressed in units of electron volt or K.J.Mole-1  ·       Ioniza

PERIODIC TABLE ·       Why do elements are to be classified? ·       It is very difficult to study the chemistry of each and every element ·       Classification of elements enables a better under standing of elements and their properties. ·       The fundamental properties which govern them. ·       Properties of elements repeat periodically like seasons in the nature ·       Atomic number or Electronic configuration is basis for the periodic arrangement of the elements   MENDELEEV’S PERIODIC TABLE ·       In 1861 Mendeleev for the first time arranged the elements in the increasing of order of their atomic weights in the form of a table ·       He divided the elements into groups (vertical columns) and periods (horizontal rows) MODERN PERIODIC LAW ·       Mosely (1911) proposed the long form of periodic table ·       The physical and chemical properties of elements are periodic functions of their Atomic Numbers or electronic configurations. MODERN PERIODIC TABLE ·       It is based on

QUANTUM NUMBERS ·       Just as a person is identified by his postal address, the electron in a given atom can be defined with the help of quantum numbers Definition   ·       Quantum numbers are small integers by means of which any electron in the atom is completely defined ·       Every Electron in an atom is characterized by a set of four quantum numbers. The four quantum numbers are ·       1. Principal quantum numbers (n) ·       2. Azimuthal quantum numbers (l) ·       3. Magnetic quantum numbers (m) ·       4. Spin quantum numbers (s)   The set of quantum numbers are unique PRINCIPAL QUANTUM NUMBERS (n) ·       Introduced by Neils Bohr ·       Denoted by the letter n ·       Indicates (a) Energy of electron (b) Size of the orbit ·       It has values 1,2,3,4 etc or also indicated by the letters K,L,M,N etc. ·       Higher the number, the larger the energy possessed by the electron. ·       Maximum number of electrons in are orbit is  2n 2 Azimuthal quantum numbers (ℓ) ·       In

Atomic structure The arrangement of fundamental particles in the atom is called atomic structure. Various atomic models were proposed by different scientists. Rutherford proposed atomic model based on the α- ray scattering experiment. QUANTUM THEORY OF RADIATION In 1901 Max Plank proposed Quantum theory of Radiation from hot (black) body studies. A hot body radiates (emits) energy not in continuous waves but in small units of waves. When atoms or molecules absorb or emit radiant energy in quanta (a small packet of energy) or photons.     RUTHERFORD’S MODEL OF ATOM Atom is spherical and mostly hollow. The positive charge of the atom is concentrated in a small region called as nucleus. Protons and neutrons are present in the nucleus. Protons and neutrons are called Nucleons The electrons revolve round the nucleus like planets do around the Sun. Nuclear model fails to explain            (a) Stability of atom, and           (b) Line spectrum of atoms     BOHR’S ATOMIC THEO