Atoms and Molecules


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Matter is made up of tiny indivisible particles.
Laws of Chemical Combination:
The Law of conservation of mass:

This law states that, the total mass of reactants is equal to the total mass of products. In any chemical reaction, the total mass of the substance before and after the reaction is the same although its matter undergoes a physical change.
law of definite proportions:

According to law of definite proportions, in a chemical substance, the elements are always present in definite proportions by mass.

Law of Multiple Proportions:
When two elements combine to form different compounds, then the weight of one is constant and the other has a simple ratio.


The smallest particle of an element is called atom.
Dalton’s Postulates:

  • Matter is made of atoms
  • Atoms are indivisible and indestructible
  • Atoms of a given element are identical in mass and in properties
  • Compounds are formed by a combination of two or more different kinds of atoms
  • A chemical reaction is a rearrangement of atoms

Elements And Symbols

Dalton's  Symbols of Elements:

Dalton proposed a scheme of notation to represent elements.


The symbols used today were suggested by Berzelius.

The first letter of the symbol is always in upper case. Some elements were represented by the starting alphabet of its name. Example: Carbon is represented as C, Boron as B, Oxygen as O.

Symbols of some elements were the first two alphabets, where the second alphabet is in the lower case. Example: Aluminium is represented as Al, Chlorine as Cl
Some elements have Latin names:
Element Symbol Latin Name
Antimony Sb Stibium
Copper Cu Cuprum
Gold Au Aurum
Iron Fe Ferrum
Lead Pb Plumbum
Mercury Hg Hydragyrum
Potassium K Kalium
Silver Ag Argentum
Sodium Na Natrium
Tin Sn Stannum
Tungsten W Wolfram
Atomic mass:

Atomic mass is the mass of an atom.
The relative atomic mass  of an atom of an element is the number of times an atom of that element is heavier than  1/12th of the mass of a carbon-12 atom.

a molecule is the smallest particle of an element or compound that can exist independently.

The number of atoms constituting a molecule is referred to as atomicity.
  • Noble gases  like helium, neon, argon etc are monoatomic.
  • Hydrogen, chlorine, nitrogen are diatomic .
  • Elements like phosphorus and sulphur are called poly atomic.


Elements combine to form compounds.

An ion is a charged particle.
A positively charged particle in a molecule is called cation. Example: Na+ Ca+2 
An anion is a negatively charged particle in a molecule. Example: F-,Cl-

The combining capacity of an element is known as valency.
Formula of a compound:

A formula is the short hand representation of a compound.
The formula of potassium hydroxide is KOH, magnesium nitrate is Mg (NO3)2.


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