What are Valence Electrons?

  • The electrons present in the outermost shell of an atom are called valence electrons.
  • And the shell that contains the valence electrons is called the valence shell .

18. and 21. and 24. valence electrons sodium-Teachoo-01.png

 

Example - 1

  • Helium has 2 Electrons.
  • All these are in K Shell (as K Shell can hold max 2 electrons).
  • Hence Valence Electrons in Helium are 2 (as K shell is the outermost shell).
  • Therefore, K Shell is the valence shell.

19. valence electron helium-Teachoo-01.png

Example - 2

  • Lithium has 3 Electrons.
  • 2 of them are held in K Shell (as K Shell can hold max 2 electrons) .
  • Remaining 1 is held in L Shell .
  • Hence Valence Electrons in Helium is 1 (as L shell is outermost shell).
  • Therefore, L Shell is the valence shell.

20. valence electron lithiun-Teachoo-01.png

Example - 3

  • Sodium has 11 Electrons.
  • 2 of them are held in K Shell (as K Shell can hold max 2 electrons).
  • 8 of them are held in L Shell (as L Shell can hold max 8 electrons).
  • Remaining 1 is held in M Shell .
  • Hence Valence Electrons in Sodium are 1 (as M shell is Outermost shell).
  • And M Shell is the valence shell .

18.sodium-Teachoo-01.png

Why are Valence Electrons important?

  • We know that valence electrons are the electrons present in the outermost shell of an atom.
  • Only these electrons take part in chemical reactions .
  • Those having their outermost shell full do not react with other elements



Example : -

 

  • Helium has 2 Elements in Outer shell .

 

  • All these are in K Shell (as K Shell can hold max 2 electrons).

 

  • Hence, there are no free electrons which take part in chemical reactions .

 

  • The outermost/valence shell of an atom can contain at-most 8 electrons only . If there are more than 8 electrons, then the remaining electrons will go to the next shell.
  • This is known as the octet rule .

 

For example - Calcium has 20 electrons

 

Note that only the K shell has a maximum of 2 electrons , the remaining shells can accomodate 8 electrons each . After accommodating 8 electrons, the remaining electrons move to the next shell and so on.

 

Its electrons are divided in shells as -

K shell   2 electrons

L shell   8 electrons

M shell 8 electrons

N shell 2 electrons (the valence electrons)

Even though the M shell (n = 3) can contain 2n 2 = 2 x 3 2 = 2 x 9 = 18 electrons

 

  • But since it will become the valence shell, it cannot accommodate more than 8 electrons.
  • The remaining 2 e- will go to the N shell, making the N - shell to be the valence shell.

Note:- The atoms are most stable when they've 8 electrons in their valence shell.

Hence, elements generally combine in order to exchange electrons and achieve octet.

22. calcium-Teachoo-01.png

Examples -

In text Question -

NCERT Back Exercise - Q17, Q18

 

What is Valency ?

  • It is the combining capacity of an element.

Example:

  • Valency of Hydrogen is 1.
  • Valency of Oxygen is 2.
  • Hence 2 atoms of Hydrogen are needed to combine with 1 atom of Oxygen to form 1 Atom of Water (H 2 O).

How is Valency of An Element Determined?

  • Valency depends upon Valence Electrons (Number of Elements in Outermost shells).
  • We know that electrons are negatively charged particles.
  • They revolve around the nucleus, which is a very small positive body.
  • An atom is neutral because the positive charge of the nucleus is equal to the negative charge of electrons.

23. oxygen atom-Teachoo-01.png

Now, how do we determine the valency of an atom?

  • The maximum number of electrons in a valence shell can be 8.
  • Hence, the number of electrons in a valence shell can vary from 1 to 8 .

 

  1. Now, if an atom has 1, 2 or 3 number of valence electrons

It is easier for them to lose 1, 2 or 3 e- respectively from their valence to achieve an octet.

    (Since now the previous shell would be its valence shell, containing 8 electrons)

  

      Valency of atoms losing 1 electron = 1

    Valency of atoms losing 2 electrons = 2

    Valency of atoms losing 3 electrons = 3

 

Example - 1

  • Sodium has a total 11 Electrons.
  • 2 of these are held in K Shell (as K Shell can hold max 2 electrons).
  • 8 of these are held in L Shell (as L Shell can hold max 8 electrons).
  • Remaining 1 is held in M Shell .
  • Thus Sodium needs to lose 1 electron to achieve octet (8 Electrons in the outermost shell).
  • Hence, Valency of Sodium is 1.

21. and 24. valence electrons sodium-Teachoo-01.png

Example - 2

  • Magnesium has a total 12 Electrons.
  • 2 of these are held in K Shell (as K Shell can hold max 2 electrons).
  • 8 of these are held in L Shell (as L Shell can hold max 8 electrons).
  • Remaining 2 are held in M Shell .
  • Thus Magnesium needs to lose 2 electrons to achieve an octet (8 Electrons in the outermost shell).
  • Hence Valency of Magnesium is 2.

25. magnesium-Teachoo-01.png

2.    If an atom has 5, 6 or 7 number of valence electrons.

It is easier for them to gain 3, 2 or 1 electron respectively in their valence to achieve an octet.

(Since after gaining 3, 2 and 1 electron respectively, their valence shell will have 8 electrons).

     Valency of atoms gaining 1 electron = 1

Valency of atoms gaining 2 electron = 2

Valency of atoms gaining 3 electron = 3

 

Example - 1

  • Oxygen has a total 8 Electrons.
  • 2 of these are held in K Shell (as K Shell can hold max 2 electrons).
  • Remaining 6 are held in L Shell.
  • Hence Valence Electrons in Oxygen are 6 (as L shell is outermost shell).
  • Thus Oxygen needs to gain 2 electrons to achieve an octet (8 Electrons in the outermost shell).

Hence Valency of Oxygen is 2.

26. oxygen-Teachoo-01.png

Example - 2

  • Nitrogen has a total 7 Electrons.
  • 2 of these are held in K Shell (as K Shell can hold max 2 electrons).
  • Remaining 5 are held in L Shell.
  • Hence Valence Electrons in Nitrogen are 5 (as L shell is outermost shell).
  • Thus Nitrogen needs to gain 3 electrons to achieve octet (8 Electrons in outermost shell).
  • Hence Valency of Nitrogen is 3.

27. nitrogen-Teachoo-01.png

3.  If an atom has 4 electrons in its valence shell , then it has an equal chance of gaining, losing or sharing it's valence electrons in order to  achieve octet.

   

           In any case, the valency of such atoms is 4

Example - 3

  • Silicon has a total 14 Electrons.
  • 2 of these are held in K Shell (as K Shell can hold max 2 electrons)
  • 8 are held in L Shell .
  • The remaining 4 are held in the M shell.
  • Hence Valence Electrons in silicon are 4 (as L shell is outermost shell).
  • Thus silicon needs to gain 4 electrons or lose 4 electrons to achieve octet (8 Electrons in the outermost shell).
  • Hence, the Valency of silicon is 4.



Note: - If an atom has 8 electrons in its valence shell

    Then it has already achieved octet and is stable

    Hence, it won't lose, gain or share electrons.

     Their valency will be 0.

28. silicon-Teachoo-01.png

Examples -

In text Question - Page 52 Q1 Left

NCERT Back Exercise - Q7

 

What are Inert Gases or Noble Gases?

  • They are those gases which do not combine with other elements to form compounds.
  • Valency of these elements is 0.

Different Noble Gases are: -

 

Element

Atomic Number

Valency

Helium

2

0

Neon

10

0

Argon

18

0

Krypton

36

0

Xenon

54

0

Radon

86

0

Why do the Noble Gases not react?

  • It is because the electron arrangement in these gases is very stable.
  • These normally have 8 electrons in their outermost shells (except helium).
  • Hence, they do not need to gain or lose electrons to achieve an octet.




Example - 1

  • Neon has total 10 Electrons
  • 2 of these are held in K Shell (as K Shell can hold max 2 electrons).
  • 8 of these are held in L Shell (as L Shell can hold max 8 electrons).
  • Hence in the outermost shell, already octet (8) achieved.
  • It also means they do not need to gain or lose electrons to achieve an octet.
  • Hence, Valency of Neon is 0 and it is called inert gas or noble gas .

29. neon-Teachoo-01.png

Example - 2

  • Argon has 18 electrons
  • 2 of these electrons are held in K Shell (as K Shell can hold max 2 electrons).
  • 8 of these electrons are held in L Shell (as L Shell can hold max 8 electrons).
  • Remaining 8 electrons are held in M Shell .
  • Hence in the outermost shell, already octet (8) achieved.
  • It means they do not need to gain or lose electrons to achieve an octet.
  • Hence, Valency of Argon is 0 and it is called inert gas or noble gas.

30. argon atom has 18 electrons-Teachoo-01.png

Note:-

All Noble Gases Achieve Octet except Helium

 

Examples -

In text Question -

NCERT Back Exercise -

 

Example 3

  • Helium has 2 Electrons.
  • All these are in K Shell (as K Shell can hold max 2 electrons).
  • Hence the outermost shell is the K shell which is already full.
  • Now, they do not need to gain or lose electrons.
  • Hence, Valency of Helium is 0 and it is called inert gas or noble gas .

 

  1. Class 9
  2. Chapter 4 Class 9 - Structure of Atom (Term 2)

About the Author

CA Maninder Singh's photo - Founder at Teachoo
CA Maninder Singh
CA Maninder Singh is a Chartered Accountant for the past 11 years and a teacher from the past 11 years. He teaches Science, Accounts and English at Teachoo