Science: Atoms and Molecules: Law of Chemical Combination and Atom (For CBSE, ICSE, IAS, NET, NRA 2022)

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Law of Chemical Combination

Given by Lavoisier and Joseph L. Proust as follows:

Law of Conservation of Mass

In every chemical reaction, total masses of all the reactants is equal to the masses of all the products. Mass can neither be created nor destroyed in a chemical reaction.

e. g. , A + B-C + D

Reactants R Products

Mass of reactants = Mass of products

Law of Constant Proportion

  • In a given chemical compound, the proportions by mass of the elements that compose it are fixed, independent of the origin of the compound or its mode of preparation.
  • 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 it are same in size, ″ mass and chemical properties.
  • Atoms of different elements are different in size, mass and chemical properties.
  • Atoms combine in the ratio of small whole number to form compounds.
  • The relative number and kinds of atoms are constant in a given compound.


Atom is a building block of matter. Atoms are the smallest particles of an element which can take reaction.

Size of an Atom

Atomic radius is measured in nanometers.

Atomic radii of hydrogen atom = 1 × 10 – 10 m.

You cannot see atoms with your naked eyes but by using modern techniques. The technique is known as Scanning Tunneling Microscopy (STM) .

Size of an Atom

Atomic Mass

atoms of the same element have same atomic masses, but atoms of different elements have different atomic masses. Since Dalton could not weigh individual atoms, he measured relative masses of the elements required to form a compound.


  • Atoms of an element that have the same atomic number but different mass number (A) are called isotopes.
  • For example, one type of oxygen atom contains 8 protons and 8 neutrons in one atom, second type 8 protons and 9 neutrons and the third type contains 8 protons and 10 neutrons. We represent these oxygen atoms as 168O , 178O.
Atom: Isotopes
  • An atom consists of several fundamental particles such as: electrons, protons and neutrons. An electron is negatively charged, and a proton is positively charged particle. Number of electrons and protons in an atom is equal. Since charge on an electron is equal and opposite to charge of a proton, therefore an atom is electrically neutral.
  • Total mass of the nucleus = mass of protons + mass of neutrons


  • A molecule is an aggregate of two or more than two atoms of the same or different elements in a definite arrangement.
  • e. g. Oxygen molecule, with which we are familiar, is made of two atoms of oxygen and therefore it is a diatomic molecule (represented by O2) , hydrogen, nitrogen, fluorine, chlorine, bromine and iodine are other examples of diatomic molecules and are represented as H2, N2, F2, Cl2 , Br2 and I2 respectively.

Molecular Mass

The molecular mass is the sum of atomic masses of all the atoms present in that molecule Molecular formula of a compound is normally used for determining the molecular mass of that substance. molecular mass is the sum of atomic masses of all the atoms present in that molecule. The molecular mass of CO2 is obtained as

Molecular Mass

Hence, we write molecular mass of CO2 = 44.0 u.

Mole Concept

  • Atoms and molecules are extremely small in size, but their numbers are very large even in a small amount of any substance.
  • In order to handle such large numbers, a unit of similar magnitude is required.
  • We use unit dozen to denote 12 items: score for 20 items and so one. To count the entities at microscopic level mole concept was introduced.
  • In SI system, mole was introduced as seventh base quantity for the amount of a substance.
  • One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g (or 0.012 kg) of the 12C isotope.
  • The mass of a carbon – 12 atom was determined by a mass spectrometer and found to be equal to (1.992648 × 10 – 23) g.
  • We know that one mole of carbon weighs 12g, the number of atoms in it is equal to:-

  • This number of entities in 1 mol is so important that it is given a separate name and symbol.
  • It is known as ‘Avogadro constant’ , denoted by NA in honor of Amedeo Avogadro.
  • If a number is written without using the powers of ten 602213670000000000000000, so many entities (atoms, molecules or any other particle) constitute one mole of a particular substance.

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