Ionization Energy

• The energy required to remove (to infinity) the outermost electron in the atom or molecule when the gas atom or molecule is isolated in free space and is in its ground electronic state.
  

• Along a group, the IE decreases & along a period, the IE increases
  
  
• Factors affecting IE are Nuclear charge, Shielding effect, Completely filled and half- filled orbitals & Principal quantum number

Trends of Ionization potential as a function of atomic number