Atoms & Molecules

Yes, this is all about how atoms combine to form molecules. Combination of atoms to form molecules can be explained by various laws.  We not only cover these Laws but see how actually atoms combine to form molecule, symbols and formulae of atoms and molecules and various ways of expressing their masses.

1. Laws of Chemical Combinations

Elements combine together to form a compound according to a certain set of laws known laws of chemical combination.

Antoine L. Lavoisier gave such two laws of chemical combination called (i) Law of Conservation of Mass and (ii) Law of constant proportions (also known as the Law of definite proportions).  We need to get hold of these two Laws.

1. Law of Conservation of Mass

The Law states ‘Mass can neither be created nor be destroyed during a chemical reaction.

In any chemical reaction, the total mass of the reactants is equal to the total mass of the products and there is no change in mass during the chemical reaction.


2. Law of Constant Proportions/ Law of Definite Proportions

The Law states, “In a chemical compound, the elements are always present in definite proportions (or ratios) by mass.”

For e.g. in a   compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 1: 8, whatever the source of water.  Let’s understand it step-wise.

Step 1:   The formula for water is H2O.  There are two atoms of Hydrogen and one atom of Oxygen.

Step 2:   Mass of one Hydrogen atom is one unit.  Therefore, the mass of two hydrogen atoms is two units. Mass of one oxygen atom is 16 units.

Step 3:   If we consider mass, the ratio of hydrogen to oxygen is 2: 16 or in it is simplified to 1:8. Thus, if 9g of water is decomposed, 1 g of hydrogen and 8g of oxygen are always obtained.

Similarly, carbon dioxide (CO2) always contains carbon and oxygen in the ratio of 3: 8. If a sample of CO2 contains 36 g of carbon then it is compulsory that the sample has 96g oxygen.

It is pertinent here to mention the main postulates of Dalton’s Atomic Theory, which are as follows:

(i)        Every matter is made up of very small particles. called the atoms.

(ii)         Atoms are indivisible panicles which can neither be created nor be destroyed in a chemical reaction.

(iii)        Atoms of a given element are identical in mass as well as in chemical properties.

(iv)        Atoms of different elements have different masses and chemical properties.

(v)         Atoms combine in the ratio of small whole numbers to form compounds.

(vi)        The relative numbers and kinds of atoms are constant in a given compound.

2. Atoms

Atom is the smallest particles of an element which may or may not have an independent existence but takes part in a chemical reaction.  In other words, atoms are the building blocks of all matter. E.g. atoms of hydrogen, oxygen, nitrogen etc., are not capable of independent existence whereas atoms of helium, neon etc., are capable of existing independently.

(i)          Size of Atoms

Size of atoms are measured in nanometers(nm).

1 nanometer (nm) = 10-9 meter(m)

Or 1 m = 109 nanometer

A hydrogen atom is the smallest atom and its radius is 0.1 nm.

(ii)         Symbols of Different Elements

Symbols are the representation of an element. It is simple to use the symbol of an element rather writing the whole word of an element.  Dalton was the scientist who introduced the symbols.

The first letter of a symbol is always written in the capital letter and the second letter as a small letter. e.g. chlorine (Cl), zinc (Zn) or aluminium (Al).

Symbols of some elements. Have been taken from their names from different languages.  See these examples.

(i)          Iron                         Fe from Ferrum (Latin name)

(ii)         Gold                        Au from Aurum (Latin name)

(iii)        Potassium             K from  Kalium (Latin name)

(iv)        Chlorine                Cl from Chloros (Greek name)

(v)         Cobalt                    Co from Kobold (German  name)

(vi)        Sodium                  Na from Natrium (Latin name)

Let’s get to know the symbols of the first 20 elements:

Just for reference let’s look at symbols of some other elements:

Symbol of some Elements

(iii)       Atomic Mass

Each element has a characteristic atomic mass. For measuring atomic mass, we have chosen Carbon atom as a unit.  We take the 1/12th mass of 1 atom of carbon (Carbon – 12 isotope) as a unit.

atomic mass unit

(a)         Atomic Mass Unit

It is defined as the mass unit equal to exactly l/12th of the mass of the atom of Carbon-12 isotope, written as ‘u’ – unified mass.

(b)         Relative Atomic Mass

As the atomic mass of all elements is expressed as 1/12 mass of C-12 atom i.e. relative to the mass of the carbon atom, the mass of elements are referred to as relative atomic mass.  Relative atomic mass is defined as the number of times a given atom is heavier than 1/12th of the mass of 1 atom of carbon -12 atom.

3. Molecules

A molecule is a group of two or more atoms that are chemically bonded together.  Atoms of the same element or of different elements can join together to form molecules.

Molecules can be divided into two categories ie (i) Molecules of Elements; and (ii) Molecules of Compounds.  We take a look.

1.  Molecules of Elements

The molecules of an element contain the same type of atoms. E.g. a molecule of oxygen (02) consists of two atoms of oxygen and is known as a diatomic molecule.  Ozone (03) consists of three atoms of oxygen is known as triatomic molecules.

Molecules of many elements are made up of only one atom of that element. e.g. noble gases like  Argon (Ar), Helium (He) etc. The molecules of most of the non-metals are made up of more than one atom.

2.  Molecules of Compounds

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

Let’s look as some of the examples.


Atomicity: It is defined as the number of atoms present in a molecule. On the basis of atomicity, molecules can be classified as:

4. Ions

When atoms, groups of atoms or molecules lose or gain an electron(s) they become charged. These charged species are known as ions. Atoms in solution generally exist in the form of ions. These can be negatively or positively charged.


5. Valency

The combining capacity of an element is called its valency.

Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. Further, the valency of an ion is equal to the charge on the ion.

6. Writing Chemical Formula

The shortest way to represent a compound with the help of symbols and valency of elements is known as chemical formula. Chemical formula of a compound shows its constituent elements and the number of atoms of each combining element.  In ionic compounds, the charge on each ion is used to determine the chemical formula of a compound.

Basic Rules keep in mind before we start to write chemical formulae:

(i)          The valencies or charges on the ion must be balanced.

(ii)         When a compound consists of a metal and a non-metal, the symbol of the metal is written first and on the left whereas of non-metal on its right. e.g. 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 left.

(iii)       When a compound is formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio e.g. Ca (OH)2. In case if the number of polyatomic ions is one, the bracket is not required. e.g. NaOH.

Method of writing chemical formula:

(i)          Step 1 – write the symbols of constituent elements and their valencies.

(ii)         Step 2 – write the symbol of cation first followed by the symbol of an anion.

(iii)       Step 3 – then criss-cross their charges or valencies to get the formula.

(iv)        Step 4 – the positive and negative charges must balance each other and the overall structure must be neutral.

7. 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).  

E.g. the relative molecular mass of water (H2O) is 18 u, which can  be calculated as

atomic mass of hydrogen = 1 u

atomic mass of oxygen = I6 u

H2O contains two hydrogen atoms and one oxygen atom.

Therefore, molecular mass of water is= 2 ×1u+ 1×16u = 18u

Formula Unit Mass

It is the sum of the atomic masses of all atoms present in a formula unit of a compound.

Formula unit mass is calculated in the same manner as we calculate the molecular mass. e.g. formula unit mass for sodium chloride (NaCl)

= 1 × 23u + 1 × 35.5u = 58.5 u

8. Mole Concept

The mole is the amount of a substance which contains as many particles (atoms/ions/molecules/formula units etc.) as in 12 g of Carbon-12 atom. Thus, one mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.

     E.g. 1mole of carbon (C, atomic mass= 12) is equal to 12 g.

    1 mole of oxygen (O2 , molecular mass = 2 ×  16) is equal to 32 g.

    1 mole of water (H2O, molecular mass= 2 × 1+ 1 × 16) is equal to 18 g.

The number of particles present in 1 mole of any substance is the same and fixed, which is equal to 6.022 × 1023.   This is a  constant, known as Avogadro constant or Avogadro number in honour of the Italian scientist, Amedeo Avogadro.

Mole is also defined as the number of particles equal to the Avogadro constant, NA (6.022 × 1023 ).                          

1 mole= 6.022 × 1023 particles, in number.

Molar Mass and Moles

The mass of one mole of a substance is equal to its relative atomic or molecular mass in gram.

Molar mass of atoms is also known as gram atomic mass. To find the gram molecular mass or molar mass we keep the numerical value same but change the units from ‘u’ to ‘g’.

e.g. we know the molecular mass of water (H2O) is 18 u,

   i.e.  18 u water =  one molecule of water.

          18 g water = one mole of water

                                            = 6.022 × 1023 molecules of water

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