ii ) The ionisation enthalpy decreases on moving top to bottom in a group .
iii ) The ionisation enthalpy increases on moving from left to right in a period .
ELECTRON GAIN ENTHALPY : The electron gain enthalpy (( ∆egH ) is the molar enthalpy change when an isolated gaseous atom or ion in its ground state adds an electron to form the corresponding anion thus the enthalpy change for the reaction ; X ( g ) + e - → X - ( g )
Is called the electron gain enthalpy ( ∆eg H ) of the element X . The∆eg H may be positive or negative .
The successive values for the addition of second , third etc . Electron , these are called second , third etc . electron gain enthalpies . For example ,
X ( g ) + |
e - |
→ |
X - ( g ) |
∆H = ∆eg H1 |
is called first electron gain |
enthalpy X - ( g ) + |
e - |
→ |
X 2- ( g ) |
∆H = ∆eg H2 |
is called second electron gain |
enthalpy |
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|
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X 2- ( g ) + e - → X 3- ( g ) ∆H = ∆eg H3 is called third electron gain enthalpy Usually the term electron gain enthalpy ( ∆egH ) means the first electron gain enthalpy .
Periodicity :
( i ) In period- The electron gain enthalpy increases from left to right in a period .
( ii ) In group- The electron gain enthalpy decreases from top to bottom in a group .
ELECTRONEGATIVITY : “ The relative tendency of an atom in a molecule to attract the shared pair of electrons towards itself is termed as its electronegativity .”