ATOM

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 ATOM:- An atom is defined as the smallest particles of an element which may or may not have independent existence. 

Ex:-(i) Atoms of Cu, Fe etc.have independent existence. 

(ii) Atoms of hydrogen, Oxygen etc.don't have independent existence. 

MOLECULE :- A molecule is defined as the smallest particle of an element or a compound which has independent existence. 

Ex :- CO2 , O2 , N2 , etc. 

Molecules can be classified into two types

(I) Homoatomic molecules

(II) Heteroatomic molecules

Homoatomic molecules :-In this type, two or more atoms of the same element combine to form molecules. 

Ex :- i) Mono atomic molecules :- They contain single atoms of the same element. He, Ne etc.are mono atomic molecules. 

ii) Di atomic molecules  :- They contain two atoms of the same element. Oxygen (O2) , Nitrogen (N2)  etc.are diatomic molecules. 

iii) Tri atomic molecules :- They contain three atoms of the same element. Ozone (O3) is a Tri atomic molecule. 

iv) Tetra atomic molecules :- They contain four atoms of the same element. Phosphorus (P4)  is tetra atomic molecule. 

Heteroatomic molecules :- In this type , two or more atoms of different elements combine to form molecules

Ex:- i) NaCl is a di atomic molecule. 

ii) H2 O is a tri atomic molecule. 

iii) Sulphuric acid (H2 SO4 ) , Nitric acid (HNO3)  etc. are Polyatomic molecules. 

COMPOUND 

A compound is a substance which is obtained by the Union of two or more elements in the definite ratio by mass. 

Ex :- Water-H2 O (1:2), Glucose - C6 H12 O6  (1:2:1) etc. 

It is of two types. 

I) In -organic compounds:- Such compounds are obtained from non-living sources such as minerals, limestone etc. 

II) Organic compounds :- Such compounds are obtained from living sources such as plants and animals.(Ex:- Fats, Carbohydrates etc.) 

MIXTURE 

A substance obtained by mixing of two or more elements or compounds in any ratio is called a mixture. 

Ex :- Bronze (Cu+ Sn), Brass (Cu + Zn) , Air etc. 

Mixture can be classified as

(I) Homogeneous mixture:- 

Such mixture consists of a single phase  and have the same composition throughout.  

Ex :- Air,  water + sugar + salt, etc. 

(II) Heterogeneous mixture :- 

Such mixture consists of two or more phases having different composition. 

Ex :- mixture of sand and sulphur, mixture of Oli and water. 

SIZES OF ATOMS AND MOLECULES :-

The size of atoms and molecules are expressed in Angstrom units. But in modern age, the size of atoms and molecules are expressed in X- ray crystallography. 

The size of an atom can be measured as its Covalent and Vanderwaal's radius. 



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