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Atoms and Molecules


Consider an element, say iron. If a piece of iron is broken into smaller pieces, there will come a stage when it can no longer be sub-divided. Thus, an ultimate particle of iron is obtained. It is called an atom of iron.


An atom is defined as the smallest particle of an element which may or may not have an independent existence and which displays the properties of that element.

Symbol of some elements as proposed by John Dalton




A molecule is defined as the smallest particle of an element or a compound which have an independent existence.

We know that a compound is formed by the combination of two or more elements combined in the fixed ratio by weight. The ultimate particle of a compound is called molecule.



The number of atoms present in a molecule is called atomicity.


Example : Oxygen, Hydrogen, Nitrogen are examples of diatomic molecules because their molecules contains two atoms.

If a molecule contains 3 atoms it is called as a triatomic molecule.


Example : Ozone (O3). Metals like Sodium, Iron, Silver, Gold, etc., are monoatomic in nature. 


Name of the Molecule Formula Atomicity
Sodium Na Mono atomic
Helium He Mono atomic
Hydrogen H2 Diatomic
Nitrogen N2 Diatomic
Chlorine Cl2 Diatomic
Ozone O3 Tri atomic
Phosphorus P4 Tetra atomic
Sulphur S8 Poly atomic


Ions are charged particles (positive or negative). Ions may consist of a single charged atom (mono atomic ion) or a group of atoms that have a net charge on them (poly atomic ion). Compounds composed of metals and non-metals contain cations (positively charged) and anions (negatively charged).

A few examples are tabulated below: 


Name of the compound Formula  Cation present Anion present
Sodium Chloride NaCl Na+ Cl-
Potassium Nitrate KNO3 K+ NO3-
Copper Sulphate CuSO4 Cu2+ SO42-
Calcium Carbonate CaCO3 Ca2+ CO32-
Sodium bicarbonate NaHCO3 Na+ HCO3-
Calcium Phosphate Ca3(PO4)2 Ca2+ PO43-
Potassium Sulphite K2SO3 K+ SO32-


Valency Name of the ion Symbol Non-metallic element Symbol Polyatomic ions Symbol
1. Sodium Na+ Hydrogen H+ Ammonium NH4+
  Potassium K+ Hydride H- Hydroxide OH-
  Silver Ag+ Chloride Cl- Nitrate NO3-
  Copper (I)* Cu+ Bromide Br- Hydrogen  
      Iodide I- Carbonate HCO3-
2. Magnesium Mg2+ Oxide O2- Carbonate CO32-
  Calcium Ca2+ Sulphide S2- Sulphite SO32-
  Zinc Zn2+     Sulphate SO42-
  Iron (II)* Fe2+        
  Copper (II)* Cu2+        
3. Aluminium Al3+ Nitride N3- Phospohate PO43-
  Iron (III)* Fe3+        


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