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Wednesday, 21 October 2015

FSc Notes Chemistry Part 1 Chapter 6 Chemical Bonding Lecture 2

FSc Notes Chemistry Part 1 Chapter 6 Chemical Bonding Lecture 2




Electron Affinity:

The minimum amount of energy evolved by an atom when it gains an electron in its valence shell, in gaseous state, is known as electron affinity of that atom, Due to loss of energy by atom during gaining an electron, the value of electron affinity is taken as negative ie negative sign shows loss of energy.

Types:
Based on the no. of electrons gained by an atom in its valence shell, the Electron Affinity may be 1st Electron Affinity 2nd Electron Affinity and so on.

1st Electron Affinity:
The minimum amount of energy lost by an atom, when it gains an electron in its valence shell, is known as 1st E.A of that atom. It has always a negative value.
For example oxygen atom has 6 electrons in its valence shell and needs two electrons to complete its valence shell. When it gains a single electron in its valence shall to form a uni-negative oxide ion ( 0(-1) ), it loses 141 KJ / mole of energy. So 1st E.A of "O" is -141 KJ / mole.
O + IE(-1)---------------------------O(-1)       ( E.A )1st = -141 KJ / mole
2nd Electron Affinity:
The amount of energy involved when a uni-negative ion gains a 2nd E.A of that atom.
It is important to note that the value of 2nd E.A is always positive. It is because of the fact that when a uni-negative ion gains a 2nd electron in its valence shell, then the incoming electron is repelled by the already gained electron. Thus to overcome the electron – electron repulsion, the atom absorbs energy and that is why 2nd E.A has always a +ve value. For example when O-1 gains another electron in its valence shell to obey octet rule, it absorbs 884 KJ / mole of energy to overcome the electron – electron repulsion. Thus the 2nd E.A of oxygen is 844 KJ / mole i.e
O(-1) + IE(-1)----------------- O(-2) ( E.A )2nd = + 884 KJ / mole
K=2 L=7                                   K=2 L=8

Factors Affecting Electron Affinity:
Following are some important factors which affect the E.A of elements.

  1. Nuclear Charge: Electron Affinity directly proportional to nuclear charge.
  2. Atomic Radius: Electron Affinity inversely proportional to Atomic Radius.
  3. Shielding Effect: Electron Affinity inversely proportional to shielding effect.


Trends of Electron Affinity in the Periodic Table:

In Group:

From top to bottom in any group of the periodic table, the Electron Affinity of elements decreases.
Reason
It is because of the fact that down the group, the atomic size increases due to increase in the no. of shells due to greater valence shell has less contact with nucleus and that is why ability of gaining electron or electrons decreases. Hence we say, the Electron Affinity decreases down the group.

In Period:

From left to right in any period. Of the periodic table, the Electron Affinity of elements increases.
Reason
It is because of the fact that from left to right in any pd. Of the periodic table, the atomic size of elements decreases due to increase of nuclear charge an thus the valence shell comes closer to nucleus and thus the electron gaining power of element increases.

Electronegativity:

The ability of an atom to attract the shared pair of electrons toward it, is known as Electronegativity of that atom.
Fluorine has given the electron –negativity values to all the elements. As fluorine ( F ) is the most E.N element of the periodic table so it has been taken as standard by Pauling and has been given an electro negativity value of all other elements are compared with F.

Trends in Periodic Table:

In Group:

From top to bottom, in any group of the periodic table, the Electronegativity of elements decreases.
Reason:
It is because of the fact that as we go down the group, the atomic size of elements increases and thus the shared pair of electrons gets away from the nucleus and thus nucleus cannot attract this shared pair of electrons with great strength. Thus we say that the Electronegativity of elements decreases down the group.

In Period

From left to right in any period. Of the periodic table, the Electronegativity of elements increases.
Reason:
It is because of the fact that as we go from left to right in any pd. Of the periodic table, the atomic radius decreases due to increase in nuclear charge and thus the valence shell comes close to the nucleus, thus the ability of nucleus to attract the shared pair of electrons and hence we say that the Electronegativity of elements increases from left to right in any pd. Of the periodic table

Electropositivity:

The ability of an element to give electron or electrons is known as electro positivity, of that element.

Trends in the Periodic Table:

In Group:

From top to bottom in any group of the periodic table, the Electropositivity of elements increases.
Reason:
It is because of the fact that from to bottom, in any group of the periodic table, the atomic radius increases, and hence valence electrons get away from the nucleus and thus the electrons can get out of the atom easily. Thus from top to bottom, the Electropositivity of elements increases.

In Period

From left to right in any period of the periodic table, the Electropositivity of elements decreases.
Reason
It is because of the fact that as we go from left to right in any period of the periodic table, the atomic radius of elements decreases thus the valence electrons come close to nucleus and thus their removal from the atom becomes difficult. So from left to right in any pd. Of the periodic table, the Electropositivity of elements decreases.

Written by: Asad Hussain

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