Atomic terms a) Atomic Number (Z): The atomic number of an element is the number of protons contained in the nucleus of the atom of that element. b) Nucleons: Protons and neutrons are present in a nucleus. So, these fundamental particles are collectively known as nucleons. c) Mass Number (A): The total number of protons and neutrons i.e,  the number of nucleons present in the nucleus is called the mass number of the element. d) Nuclide: Various species of atoms in general. A nuclide has specific values of atomic number and mass number. IUPAC notation of an atom (nuclide) Let X be the symbol of the element. Its atomic number be Z and mass number be A. Then  the element can be represented as   e) Isotopes:   Atoms of the element with same atomic number but different mass number e.g. 1 H 1 , 1 H 2 , 1 H 3 . There are three isotopes of hydrogen. f) Isobars: Atoms having the same mass number but different atomic numbers, e.g. 15 P 32  and 16 S ...

  Principle of Atom Conservation (POAC): This principle states that the moles of atoms of an element are conserved throughout the reaction. This means, moles of an element in reactants = moles of the element in products. eg. CO + O 2  -----> CO 2 In this moles of C in CO = moles of C in CO 2 This means, moles of CO = moles of CO 2 . Similarly, for O, moles of O in CO + moles of O in O 2  = moles of CO 2 Thus, moles of CO + 2(moles of O 2 ) = 2(moles of CO 2 )   Advantages of Mole Method over other Methods: (1) Balancing of chemical equations is not required in the majority of problems as the method of balancing the chemical equation is based on the principle of conservation of atom conservation.   (2) Number of reactions and their sequences, leading from reactants to products, need not be given.   Note: Whenever balanced chemical equation is given, mole method is very useful. eg. 2KClO 3  -----> 2KCl + 3O 2 Thus, 2(moles of KClO 3 ) = 2(moles of ...