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Atoms and Molecules

Chapter: Atoms and Molecules


In the end of eighteenth century scientist recognized the difference between elements and compounds and naturally became interested in finding out how and why elements combine and what happens when they combine. Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination. One is law of conservation of mass in which mass can neither be created nor destroyed in a chemical reaction and other is law of constant proportion in which elements are always present in definite proportion by mass. After that scientist again facing the problem that how to give the appropriate explanation of these two laws. Then British chemist John Dalton was born in England explain the basic theory about the nature of matter in year 1808 and he use the term ‘atom’ and said that the smallest particles of matter are atoms. His theory was based on the laws of chemical combination and following are the postulates of theory about the nature of matter-

  1. All matter is made of small particles called atoms.
  2. Atoms neither created nor destroyed in a chemical reaction.
  3. Atoms of given elements are identical in mass and chemical properties.
  4. Atoms of different elements have different masses and chemical properties.
  5. Atoms combine in the ration of small whole numbers to form compounds.
  6. Atoms are constant in a given compound.


What is an atom?

Like a building is made up of small units i.e. bricks, all matter in this universe is made of atoms. Atoms are very small, they are smaller than anything we can imagine or compare with. More than millions of atoms when stacked would make a layer as thick as the sheet of paper. We may not be able to see them, but they are there, and constantly affecting whatever we do. Through modern techniques, we can now produce magnified images of surfaces of elements showing atoms.

How big are atoms?

Atomic radius is measured in nanometers

1/109m = 1nm

1m = 109nm

Now showing some of the radii in (nm) and its examples:

10-10                                     Atom of hydrogen

10-9                                       Molecule of water

10-8                                       Molecule of hemoglobin

10-4                                       Grain of sand

10-2                                       Ant

10-1                                       Watermelon

What are the modern day symbol of atoms of different elements?

Dalton was the first to use the symbols for elements and when he used a symbol for an element he also meant a definite quantity of that element, that is, one atom of that element. At the same time Berzelius was another scientist who suggested that the symbols of elements will be made from one or two letters of the name of the element.

In the beginning the names of the elements were derived from the name of the place where they were found for the first time. For example the name copper was taken from Cyprus. Some names were taken from specific colors. For example gold was taken from the English word meaning yellow. But now days, IUPAC (International Union of Pure and Applied Chemistry) approves the names of elements. The first letter of a symbol is always written as capital letter and second letter as a small letter, for example: Hydrogen is represented by symbol ‘H’, Helium by symbol ‘He’ and Lithium by ‘Li’ etc.

Atomic mass

According to Dalton’s atomic theory, each element had a characteristic atomic mass. Atomic mass unit (amu), but according to the latest IUPAC recommendations, it is now written as ‘u’- unified mass which is equal to the mass of one carbon atom. While searching for various atomic mass units, scientist initially took 1/16 of the mass of an atom of naturally occurring oxygen as the unit. One atomic mass unit is a mass unit equal to exactly one/12th the mass of one atom of carbon 12.

How do atoms exist?

Atoms form molecules and ion and when they aggregate in large numbers they form matter that we can see, feel or touch..


A molecule is defined as a group of two or more atoms which are chemically bonded together. It shows all the properties of a substance. It is the smallest particle of an element or a compound.

Molecules of elements

Molecules of many elements are made up of only one atom of that element. Argon, Helium comes under such category. But for non-metals that is not the case. The molecules of any element are constituted by the same type of atoms. 
Atomicity is defined as the number of atoms constituting a molecule. It is classified as monatomic, diatomic, tetra atomic and poly-atomic.
Argon and Helium come under monatomic, Oxygen, Hydrogen, Chlorine and Nitrogen come under Diatomic, Phosphorus comes under Tetra-atomic and Sulphur comes under Poly-atomic.

Molecules of compounds

Atoms of different elements join together in definite proportions and form molecules of compounds.

What is an ion?

All the compounds contain charged species for both metals and non-metals. These charged species are called as ions. An ion is a charged particle and it can be negatively or positively charged. The positively charged ion is called cation and a negatively charged ion is called anion. For example consider sodium chloride which is formulated as NaCl. In this, Sodium ions are positively charged (Na+) and Chloride ions are negatively charged (Cl-).

Writing chemical formulae

The chemical formula of any compound is the representation of constituent particles in the compound.

The rules that you have to follow while writing a chemical formula are as follows:

  1. The valencies or charges on the ion must balance.
  2. When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and  copper are metals, and are written on the left.
  3. In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. In case the number of polyatomic ion is one, the bracket is not required. For example, NaOH.


Molecular Mass and Mole Concept

Molecular mass

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).

Formula unit mass

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular mass.

Mole Concept

A mole is defined as that quantity in number having a mass equal to its atomic or molecular mass in grams. The word “mole” was introduced around 1896 by Wilhelm Ostwald.