Atoms of chemical elements having same atomic mass but a different atomic number are called Isobars. The sum of the number of protons and neutrons together form the atomic mass. Therefore, we can also say the number of nucleons present in the nucleus is equal to the atomic mass of an atom. It will have the same number of nucleons. The number of protons and neutrons alone will vary but the number of nucleons or the sum of protons and neutrons in isobars will always be same. Isobars always have different atomic structure because of the difference in atomic numbers. The number of neutrons makes up the difference in the number of nucleons. Therefore, they are always different chemical elements having same atomic masses. Thus, isobar has different chemical properties. For example : Iron and nickel have atomic number 26 and 28 respectively. However, the mass number is 58. Argon and calcium with atomic number 18 and 20, respectively. However, they have the same atomic mass 40.
Law of Chemical Combination Given by Lavoisier and Joseph L. Proust as follows: 1. Law of conservation of mass Mass can neither be created nor destroyed in a chemical reaction. e.g. A + B → C + D Reactants → Products, Mass of reactants = Mass of products 2. Law of constant proportion In a chemical substance the elements are always present in definite proportions by mass. e.g., in water, the ratio of the mass of hydrogen to the mass of oxygen is always 1 : 8 respectively. These laws lacked explanation. Hence, John Dalton gave his theory about the matter. He said that the smallest particle of matter is called ‘atom’. Dalton’s Atomic Theory · Every matter is made up of very small or tiny particles called atoms. · Atoms are not divisible and cannot be created or destroyed in a chemical reaction. · All atoms of a given element are same in size, ma...