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Home › Grade9 › Grade 9 CBSE › Chemistry › Structure of The Atom › Structure of The Atom Notes › Electronic configuration of elements

Electronic configuration of elements

ELECTRONIC CONFIGURATION OF ELEMENTS

  • The arrangement of electrons in the various shells (or energy levels) of an atom of the element is known as its electronic configuration.
  • The maximum number of electrons which can be put in a particular energy level or shell was given by Bohr and Bury.

 

According to Bohr-Bury scheme:

  1. The maximum number of electrons which can be accommodated in any energy level of the atom is given by 2n2 (where n is the number of that energy level)
  1. For 1st energy level, n = 1

So, the maximum number of electrons in 1st energy level = 2n2

= 2 × (1)2

= 2 × 1

= 2

  1. For 2nd energy level, n = 2

So, the maximum number of electrons in 2nd energy level = 2n2

= 2 × (2)2

= 2 × 4

= 8

  1. For 3rd energy level, n = 3

So, the maximum number of electrons in 3rd energy level = 2n2

= 2 × (3)2

= 2 × 9

= 18

  1. The maximum number of electrons that can be accommodated in the outermost shell of an atom is 8.
  2. Electrons in an atom do not occupy a new shell unless all the inner shells are completely filled electrons.

 

Valence Electrons (Valency Electrons):

  • The electrons present in the outermost shell of an atom are known as valence electrons (or valency electrons).
  • The combining capacity of an element is called its valency.

 

For example:

Sodium

  • The atomic number of sodium is 11, which means that one atom of sodium has 11 electrons in it.
  • So, the electronic configuration of sodium atom will be 2, 8, 1.

  • There is 1 electron in the outermost shell/valence shell of sodium atom. So the valency of sodium is 1.


Magnesium

Atomic number of Magnesium is 12

Number of protons = 12

Number of electrons = 12

Electronic configuration-2, 8, 2

Valency of Magnesium is 2

 

Relation between Valency and Valence Electrons:

  • The valency of an element is either equal to the number of valence electrons in its atom or equal to the number of electrons required to complete its octet or duet in case of hydrogen.
  • The valency of a metal element is equal to the number of valence electrons in its atom.
  • Valency of a metal = No. of valence electrons in its atom
  • The valency of a non-metal element is usually equal to eight minus the number of valence electrons in its atom. That is,

Valency of a non-metal = 8 – No. of valence electrons in its atom

Atomic number of Aluminium is 13

Electronic configuration – 2,8,3

Valency = 3

Atomic number of Silicon is 14

Electronic configuration – 2,8,4

Valency = 4

Atomic number of Phosphorous is 15

Electronic configuration –2, 8, 5

Valency – 3(8 – 5 = 3 )

Atomic number of Sulphur is 16

Electronic configuration –2, 8, 6

Valency = 2(8 – 6 = 2)

Atomic number of Chlorine is 17

Electronic configuration –2, 8, 7

Valency –1(8 – 7 = 1)

Atomic number of Argon is 18

Electronic configuration –2, 8, 8

 

Types of Valency:

There are two types of valency:

  1. Electrovalency
  2. Covalency
  1. Electrovalency:
  • The number of electrons lost or gained by one atom of an element to attain the nearest noble gas electronic configuration is known as its electrovalency.

1.  Valency of Sodium:

  • The atomic number of sodium is 11, so its electronic configuration is 2, 8, 1.
  • It has 1 electron in its outermost shell. Sodium atom can lose this electron to form a sodium ion, Na+ having an inert gas electron arrangement of 2, 8.
  • Since a sodium atom loses 1 electron to attain the inert gas electronic configuration, the valency of sodium is 1.
  • The atomic number of potassium is 19 and its valency is also 1.
  • Similarly, the atomic number of lithium is 3 and its valency is also 1.

2.  Valency of Magnesium:

  • The atomic number of magnesium is 12, and its electronic configuration is 2, 8, 2. It has 2 electrons in its valence shell.
  • The magnesium atom can lose these two outermost electrons to form a magnesium ion, Mg2+, having an inert gas electron configuration of 2, 8.
  • Since one magnesium atom loses 2 electrons to achieve the inert gas electron configuration, therefore, the valency of magnesium is 2.
  • The atomic number of calcium is 20 and its valency is also 2.
  • Similarly, the atomic number of beryllium (Be) is 4 and its valency is also 2.

 

  1. Covalency:
  • The number of electrons shared by one atom of an element to attain the nearest inert gas electron configuration is known as its covalency.
  • If an atom shares 1 electron with the other atom, its covalency will be 1; if it shares 2 electrons with the other atom, then its covalency will be 2, and so on.
 
  1. Covalency of Chlorine:
  • A chlorine atom has 7 electrons in its outermost shell, so it can share its 1 electron with one electron of another atom to complete its octet.
  • Since one chlorine atom shares 1 electron to achieve the nearest inert gas electron arrangement, therefore, the covalency (or just valency) of chlorine is 1.

For example:

  • In the formation of a chlorine molecule, Cl2, each chlorine atom shares its 1 electron with the other atom, so the valency of chlorine in Cl2 molecule is 1.
 
  1. Covalency of Oxygen:
  • An oxygen atom has 6 valence electrons, so it can share its 2 electrons with two electrons of another atom to complete its octet.
  • Since once oxygen atom shares 2 electrons to achieve the nearest inert gas electron arrangement the covalency (or just valency) of oxygen is 2.

For example:

  • In the formation of an oxygen molecule, O2 each oxygen atom shares its 2 electrons with the other atom, so the valency of oxygen in the O2 molecule is 2.