2.6.4 Electronic Configuration and Position in Periodic Table by Using Successive Ionization Energy Data

• Successive ionization energy values are an index of the valence of an atom.
• Big gaps show that the next electron is being removed from a lower shell.

• First ionization energy: 495 kJ/mol
• Second ionization energy: 4560 kJ/mol
• There is a big gap between the first and second ionization energies.
• This indicates that there is only one electron in the valence shell, and this element must be a part of Group 1.

• First ionization energy: 735 kJ/mol
• Second ionization energy: 1445 kJ/mol
• Third ionization energy: 7730 kJ/mol
• There is a small gap between the first and second values, but a very large gap between the second and third values.
• This indicates that the third electron is being removed from the inner shell.
• There are only two electrons in the valence shell, so this element must be a part of Group 2.

Table: Successive Ionization Energies of Period 3 Elements (kJ/mol)

Intermediate Level

Periodic Trends

Factors Influencing Ionization Energy

• Nuclear charge: More protons → higher I.E.
• Atomic radii: Smaller radius → higher I.E.
• Shielding effect: More inner shells → lower I.E.
• Spin pair repulsion: Paired electrons → lower I.E.

Using Successive Ionization Energies

• Big gap between successive I.E. values → electron removed from a lower shell.
• Example: Sodium has a big gap between I₁ and I₂ → 1 valence electron → Group 1.
• Example: Magnesium has a big gap between I₂ and I₃ → 2 valence electrons → Group 2.

Important Points

• 1. Ionization energy = energy to remove an electron from a gaseous atom.
• 2. Across a period: I.E. increases.
• 3. Down a group: I.E. decreases.
• 4. Factors: Nuclear charge, atomic radius, shielding effect, spin pair repulsion.
• 5. Big gaps in successive I.E. indicate removal from a lower shell.
• 6. Successive I.E. values can determine the group of an element.
• 7. Example: Sodium → Group 1; Magnesium → Group 2.

Remember

• - Ionization energy: Energy to remove an electron
• - Across a period: Increases
• - Down a group: Decreases
• - Factors: Nuclear charge, radius, shielding, spin pair repulsion
• - Big gap: Electron removed from lower shell
• - Group determination: Number of valence electrons

Common Mistakes

• 1. Thinking I.E. always increases across a period: There are anomalies (e.g., Be → B, N → O).
• 2. Forgetting the shielding effect: It decreases I.E. down a group.
• 3. Ignoring spin pair repulsion: It causes a slight decrease in I.E. (e.g., N → O).
• 4. Confusing I.E. with electronegativity: I.E. is about removing an electron; electronegativity is about attracting an electron.

Quick Revision

• Ionization energy = energy to remove an electron from a gaseous atom.
• Across a period: I.E. increases.
• Down a group: I.E. decreases.
• Factors: Nuclear charge, atomic radius, shielding effect, spin pair repulsion.
• Big gaps in successive I.E. indicate removal from a lower shell.
• Successive I.E. values can determine the group of an element.
• Example: Sodium → Group 1; Magnesium → Group 2.

60-Second Revision

• Ionization energy: Energy to remove an electron
• Across a period: Increases
• Down a group: Decreases
• Factors: Nuclear charge, radius, shielding, spin pair repulsion
• Big gap: Electron removed from lower shell
• Group determination: Number of valence electrons

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