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.
- Nuclear Charge: Electron Affinity directly proportional to nuclear charge.
- Atomic Radius: Electron Affinity inversely proportional to Atomic Radius.
- 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.
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