Class 9 science Solutions English Medium

Chapter 4. Structure of The Atom

Energy Levels : There are various and definite orbits around the nucleus of an atom to revolve the electrons. These orbits or shells are called energy levels. 

      Energy Levels in An Atom 

Electronic Configuration: The distribution of electrons in various and discrete shells of an atom is known as electronic configuration.  The maximum numbers of electrons are difinite in each shell. 

The Electronic Structure of Hydrogen:

Hydrogen has only one electron, so that it's outer shell is K and has only one electron. 

The Electronic Structure of Carbon Atom: 

Carbon atom has total 6 electrons. In which it has 2 electrons in its inner most shell K and 4 electrons in outer most shell L.  

These are shells: 

K, L, M, N etc.

Number of maximum electron in various shell: 

Shell K can have maximum 2 electrons. 

Shell L can have maximum 8 electrons. 

Shell M can have maximum 18 electrons and 

Shell N can have maximum 32 electrons. 

There is used a formula to keep maximum electron in each shell : 2n²

On the basis of this formula, the maximum electrons in a shell is found.

Example, For the first shell K : 2(1)² = 2 x 1 = 2 

For the second shell L : 2(2)² = 2 x 4 = 8 

For the third shell M : 2(3)² = 2 x 9 = 18 

For the fourth shell N : 2(4)² = 2 x 16 = 32 

Valency : The number of valence electrons present in outer most shell of an atom is known as valency.

Example: 

Electronic configuration of some elements and its valency: 

(1) Sodium has 11 electrons,

Electronic configuration of sodium:  2, 8, 1 

(2) Oxygen has 8 electrons,

Electronic configuration of Oxygen: 2, 6 

Example: If there is 1 electron in outer most shell of atom of an element, then valency will be 1. If outer most shell has 2, 3 or 4 electron then valency will be 2, 3 or 4 respectively. But if outer most shell has 5, 6 or 7 electrons then valency will be find in such manner as follows. 

When outer most shell has 5 electrons then valency will be 8 - 5 = 3 

When outer most shell has 6 electrons then valency will be 8 - 6 = 2 

When outer most shell has 7 electrons then valency will be 8 - 7 = 1 

When outer most shell has 8 electrons then valency will be 8 - 8 = 0 

Now, See this table: 

Atomic  Number: The total Number of protons present in the nucleus of an atom is known as atomic number.

Example - Let be the total number of protons in the nucleus of an atom is 8 then atomic number will be 8. Similarily if the total number of protons is 11 then atomic number that atom will be 11.  

Atomic number = Number of protons

Atomic Mass: The sum of total number of protons and nutrons present in the nucleus of an atom is known as atomic Mass.   

Example - If an atom has 4 protons and 5 nutrons in its nucleus then atomic mass will be 4 + 5 = 9.  

Similarily if an atom has 7 protons and 7 nutrons the atomic mass will be 7 + 7 = 14. 

Atomic Mass = No. of protons + No. of Electrons 

Note: The number of protons in an atom is equal to the number of electron of an atom. The same number is its atomic number. The number of nutrons may be equal to the number of protons or may not be equal.