CLASS :- 11
Electron gain enthalpy 👇
the energy released when extra electrons inter in in outermost cell of an
isolated gaseous atom the energy is called electron gain enthalpy and atom
change in Anion
As x + e- → x- △egH = …… KJ/mol
The energy released when electron is added to an isolated gaseous atom
Electron gain enthalpy process may be exothermic or endothermic
Example
(I) Electron gain enthalpy is exothermic process
F + e-→ F- △egH = -327.9 KJ/mol
O + e-→ O- △egH = --140.9 KJ/mol
(II) Electron gain enthalpy sometime Endothermic process
As
O- + e-→ O- - △egH = +780 KJ/mol
Successive electron gain enthalpy 👇
like ionization enthalpy the second and third electron gain enthalpy
are also possible
when extra electron inter in outer Shell or valence shell,
the electrons which already exist in outer shell always repel
incoming electrons then due to electron - electron repulsion
some energy always release It is called first electron get enthalpy
and this value
△egH = - KJ/mol ( or exothermic process )
Example
(1) oxygen first electron gain enthalpy 👇
O + e-→ O- △egH = --140.9 KJ/mol
(2) oxygen Second electron gain enthalpy :- 👇
The second electron gain enthalpy of oxygen positive or endothermic process
Because after addition of one electron oxygen change in
oxygen Monovalent anion, after that when one another electrons
enter in oxygen monovalent anions, The magnitude of electron - electron
Repulsion very high then extra force are extra energy required for
second electron which enter in monovalent anion
O- + e-→ O- - △egH = +780 KJ/mol
Therefore second electron gain enthalpy endothermic process and
first electron gain enthalpy exothermic process
Factor affecting of electron gain enthalpy👇
(i) Nuclear charge
Negative value of electron gain enthalpy depend on attractions between
incoming electrons and nuclear charge if nuclear charge more
than electron gain enthalpy have large value
(ii)Size of the atom
Electron gain enthalpy depend on attractions between incoming
electrons and nucleus, if size of the atom small then attractions between
electrons and nucleus is higher means electron gain enthalpy have
large value
(iii) Electronic configuration
17 group ( halogen family ) member have higher electron gain enthalpy
in their respect period because they have ns2 and P5 electronic configuration
only one electrons required for complete of their octet
1 group (alkali metal) have smaller electron gain enthalpy because the have
general electronic configuration ns1 they have tendency to release or
lose of one electrons instead of gain of electron
Periodic trends of electron gain enthalpy 👇
variation in A period 👇
On moving in period left to right generally electrons gain enthalpy increase
because Atomic size decreases and attraction between electrons and nucleus
increases Then incoming electrons felt more attraction towards the side of the
nucleus.
variation down a group 👇
On moving down a group the size and nuclear charge increase but the effect of
increase in atomic size it's much more compared to nuclear charge.
Then incoming electrons feel less attractions by the nucleus in large size atoms
then electron gain enthalpy decreases .
Some important point (variation) 👇Halogen have the highest negative electron gain enthalpy :- 17 group
( halogen family ) member have higher electron gain enthalpy in their
respect period because they have ns2 and P5 electronic configuration. only one electrons required for complete of their octet.
Electron gain enthalpy of noble gas are positive :- 18 group (noble gas or real gas or inert gas ) have completed octet of their valency or outer shell.
No need of of extra electrons because octet complete. if any additional
electrons inter in any noble gas definitely it occupy place in new outer valency cell and it feel less attraction by the nucleus this is the cause why electron gain enthalpy of noble gas have positive value.
Beryllium magnesium of alkaline earth metal and nitrogen Phosphorus of of 15 group have zero electron gain enthalpy :-
Electron gain enthalpy of beryllium (2s2) and magnesium (3s2) almost is
zero because they have stable ns2 electronic configuration.
electron gain enthalpy of of Nitrogen (2s 2 2p3) and phosphorus
( 3s 2 3p3 )almost is zero because they have stable ns 2 np3 electron
configuration
Electron gain enthalpy of fluorine (−333kJ/mol)is unexpectedly less negative than that of chlorine (−349kJ/mol)
it is observed that fluorine atom has unexpected less negative electron
gain enthalpy than chlorine atom because fluorine atom very small in size
therefore incoming electrons feel much more inter electronic repulsion
in the relative compact 2P subshell of fluorine atom but in the case
of chlorine outer shell third Then incoming electron not fell
more inter electronic repulsion in the relative large in 3P subshell
And also the attraction between incoming electrons and the
nucleus is much more in Cl-atom
F + e-→ F- △egH = -333. KJ/mol
Cl + e-→ Cl- △egH = -349 KJ/mol
