Chemistry Class 11 3. Classification of Elements and Periodicity in P | Page 5

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
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.”