5.2 Trends in Ionization Potential

  Periodic Table

• In a period: The value of ionization potential normally increase on going from left to right in a period, because effective nuclear charge increases and atomic size decreases.
• Exceptions
• In the second period ionization potential of Be is greater than that of B, and in the third period, ionization potential of Mg is greater than that of Al due to the high stability of fully filled orbitals.
• In the second period ionization potential of N is greater than O and in the third period ionization potential of P is greater than that of S, due to high stability of half filled orbitals.
• The increasing order of the values of ionization potential of the second period elements is,
  

• The increasing order of the values of ionization potential of the third period elements is

• In a group: The value of ionization potential normally decreases on going from top to bottom in a group because both atomic size and shielding effect increases.
• Exceptions
• The value of ionization potential remains almost constant from $Al$ to $Ga$ in the $III A$ group. $(B > Al \sim Ga > In)$
• In $IV B$ group i.e. $Ti, Zr \:and\: Hf,$ the I.P. of $Hf$ is higher than that of $Zr$ due to lanthanide contraction. Thus the I.P. of $IV B$ group varies as $Ti > Zr < Hf$
• In the periodic table, the element having highest value of I.P. is $He$
• The value of ionization potential of noble gases are extremely high, because the orbitals of outermost orbit are fully-filled $(ns^2, np^6)$ and provide great stability.
• In a period, the element having least value of I.P. is an alkali metal (group $I A$) and that having highest value is inert gas (group 0).