Introduction
Ionisation Energy Ionisation energy is one of the important properties of elements . If energy is supplied to an atom , electrons are promoted to higher energy levels . If sufficient energy is supplied , an element in the outer most shell can be completely removed from the atom , resulting in the formation of a positive ion .
The minimum energy required to remove the most loosely bound electron from an isolated gaseous atom is called ionisation energy . It is also called the first ionisation energy . ( I1 )
M (g) + I1 `|->` M+ (g) + e-
The minimum energy required to remove another electron affinity from the uni positive ion is called second ionisation energy ( I2 ) .
M (g) + I2 `|->` M2+ (g) + e-
The second ionisation energy ( I2 ) is greater than the first ionisation energy . On removing an electron from an atom , the uni positive ion formed will have more effective nuclear charge than the number of electrons .
This decreases the repulsions between the electrons and increases the nuclear attraction on the electrons . As a result , more energy is required to remove an electron from the uni positive ion . Hence the second ionisation energy ( I2 ) is greater than the ionisation energy ( I1 ) .
Similarly the third ionisation energy ( I3 ) is greater than the second ionisation energy . An atom has as many ionisation energies as the number of electrons present in it . The order of ionization energies
I1 < I2 < I3 < ........In
where n is the number of electrons in the atoms . Ionisation energies are determined from spectral studies as well as from discharge tube experiments . They are measured in electron volts (eV) atom-1 .
1 eV = 1.602 * 10-19 J , hence 1 mol of eV has energy 1.602 *10-19 * 6.023*1023
= 96.45 k j mol-1 .
The discharge tube is filled with gas whose ionisation energy is required . At low voltages , there is no flow of electricity . But , on increasing the voltage between anode and cathode , the gas ionizes at a particular voltage , which is indicated by sudden increase in the flow of electricity . That particular voltage is called ionization energy .
The magnitude of ionisation of an atom depends on the following factors
Electron Affinity
Electron affinity is another important property of elements like ionisation energy is required to remove an electron from an atom . Conversely , when electron is added to an atom , energy is released .
Electron affinity of an element is the energy released when an electron is added to a neutral gaseous atom of that element .
Energy is released when only one electron is added to an atom forming a uni negative ion . The negative ion prevents entry of further electrons due to repulsive forces between negative charges . Thus energy is needed to overcome these repulsive forces between uni-negative ion ( X- ) and electron ( e- ) and to add one more electron energy is required . Hence , second electron affinity value usually has a positive value .
Electron affinity depend on the size and the effective nuclear charge of atom . They cannot be determined directly , but are obtained indirectly using the Born-Haber cycle . Electron affinity are measured in kj mol-1 .
Halogens have high values for electron affinity due to their small atomic sizes and requirement of only one electron to get the nearest inert gas configuration . Depending on thermodynamic notation that energy liberated is shown by negative sign , the electron affinity values are mentioned with numerals carrying negative sign before them . If energy is absored during the addition of an electron to an atom , then it is shown by positive sign .
Ionisation Energy Ionisation energy is one of the important properties of elements . If energy is supplied to an atom , electrons are promoted to higher energy levels . If sufficient energy is supplied , an element in the outer most shell can be completely removed from the atom , resulting in the formation of a positive ion .
The minimum energy required to remove the most loosely bound electron from an isolated gaseous atom is called ionisation energy . It is also called the first ionisation energy . ( I1 )
M (g) + I1 `|->` M+ (g) + e-
The minimum energy required to remove another electron affinity from the uni positive ion is called second ionisation energy ( I2 ) .
M (g) + I2 `|->` M2+ (g) + e-
The second ionisation energy ( I2 ) is greater than the first ionisation energy . On removing an electron from an atom , the uni positive ion formed will have more effective nuclear charge than the number of electrons .
This decreases the repulsions between the electrons and increases the nuclear attraction on the electrons . As a result , more energy is required to remove an electron from the uni positive ion . Hence the second ionisation energy ( I2 ) is greater than the ionisation energy ( I1 ) .
Similarly the third ionisation energy ( I3 ) is greater than the second ionisation energy . An atom has as many ionisation energies as the number of electrons present in it . The order of ionization energies
I1 < I2 < I3 < ........In
where n is the number of electrons in the atoms . Ionisation energies are determined from spectral studies as well as from discharge tube experiments . They are measured in electron volts (eV) atom-1 .
1 eV = 1.602 * 10-19 J , hence 1 mol of eV has energy 1.602 *10-19 * 6.023*1023
= 96.45 k j mol-1 .
The discharge tube is filled with gas whose ionisation energy is required . At low voltages , there is no flow of electricity . But , on increasing the voltage between anode and cathode , the gas ionizes at a particular voltage , which is indicated by sudden increase in the flow of electricity . That particular voltage is called ionization energy .
The magnitude of ionisation of an atom depends on the following factors
- Atomic radius
- Nuclear charge
- Screening or shielding effect on the outer most electrons from the attraction of the nucleus .
- Completely filled or half filled nature of sub shells .
Ionisation potentials of some elements
Electron Affinity
Electron Affinity
Electron affinity is another important property of elements like ionisation energy is required to remove an electron from an atom . Conversely , when electron is added to an atom , energy is released .
Electron affinity of an element is the energy released when an electron is added to a neutral gaseous atom of that element .
Energy is released when only one electron is added to an atom forming a uni negative ion . The negative ion prevents entry of further electrons due to repulsive forces between negative charges . Thus energy is needed to overcome these repulsive forces between uni-negative ion ( X- ) and electron ( e- ) and to add one more electron energy is required . Hence , second electron affinity value usually has a positive value .
Electron affinity depend on the size and the effective nuclear charge of atom . They cannot be determined directly , but are obtained indirectly using the Born-Haber cycle . Electron affinity are measured in kj mol-1 .
Halogens have high values for electron affinity due to their small atomic sizes and requirement of only one electron to get the nearest inert gas configuration . Depending on thermodynamic notation that energy liberated is shown by negative sign , the electron affinity values are mentioned with numerals carrying negative sign before them . If energy is absored during the addition of an electron to an atom , then it is shown by positive sign .
Variation of electron affinity in a group and in a period .
Variation of Electron affinity in a group
From
top to bottom as the atomic size increases , the electron affinity
decreases , in a period . But the electron affinity of the second
element in the group is greater than the first one . For example , in
halogens , the electron affinity of fluorine is -333 kj mol-1 while the electron affinity of chlorine is -348 kj mol-1
. It is because fluorine atom is smaller in size than chlorine atom
and has strong inter repulsions . During the addition of electrons of
fluorine atom the electronic repulsions are overcome at the expense of
some liberated energy and hence , the overall energy liberated is less
than that of chlorine atom .
Similarly phosphorus has bigger electron affinity value than nitrogen and sulphur has greater electron affinity than oxygen in V and VI groups .
Similarly phosphorus has bigger electron affinity value than nitrogen and sulphur has greater electron affinity than oxygen in V and VI groups .
Variation of electron affinity in a period
In
a period , as we move from left to right , the atomic size decreases
and the nature of the element changes from metallic to non metallic and
this results in an increase in the electron affinity values . In a
given period halogen has the highest electron affinity . Since the
outer shells of zero group elements are filled with electrons to attain
octet structure sp2sp6 , they do not accept electron and their electron affinities are treated as zero .