Ionization Energy and Periodic Trends

Definition and Fundamentals of Ionization Energy

• Definition: Ionization energy is the amount of energy required to remove the most loosely bound electron from the valence shell of an isolated gaseous atom.

• First Ionization Energy: When there is only one electron present in the valence shell, the energy required to remove it is called the first ionization energy.

• Unit of Measurement: Ionization energy is expressed in units of $kJ\,mol^{-1}$.

• Sodium Example:

  • The first ionization energy of a gaseous sodium atom ($Na$) is $+496\,kJ\,mol^{-1}$.

  • Ionization equation:     $Na_{(g)} \rightarrow Na^+_{(g)} + e^-$

  • Enthalpy change:     $\Delta H = +496\,kJ\,mol^{-1}$

Successive Ionization Energies

• Multiple Valence Electrons: When an atom has more than one electron in its valence shell, the electrons can be removed sequentially (one by one) by providing increasing amounts of energy.

• Comparison of Successive Removal: It is easier to remove the first electron from an atom than to remove the second electron.

• Magnesium Example:

  • A magnesium ($Mg$) atom has $2$ electrons in its outermost valence shell.

  • First Ionization Step:

  • Chemical equation:       $Mg_{(g)} \rightarrow Mg^+_{(g)} + e^-$

  • Enthalpy change:       $\Delta H = +737\,kJ\,mol^{-1}$

  • Second Ionization Step:

  • Chemical equation:       $Mg^+_{(g)} \rightarrow Mg^{2+}_{(g)} + e^-$

  • Enthalpy change:       $\Delta H = +1450\,kJ\,mol^{-1}$

Factors and Periodic Trends Governing Ionization Energy

• Atomic Size Relationship:

  • Ionization energy value is directly related to atomic size.

  • The smaller the radius of an atom, the stronger the electrostatic attraction between the nucleus and the outer electrons.

  • Stronger attraction results in a higher ionization energy value required to remove a valence electron.

• Trend Across a Period:

  • Ionization energy values increase from left to right across a period in the periodic table.

• Trend Down a Group:

  • Ionization energy values decrease from top to bottom down a group in the periodic table.

Ionization Energy Values of Second Period Elements

• Overview: Ionization energies of elements across the second period generally increase from left to right as shown in Table (8.6).

• Element Values (Table 8.6):

  • Lithium ($Li$): $520\,kJ\,mol^{-1}$

  • Beryllium ($Be$): $899\,kJ\,mol^{-1}$

  • Boron ($B$): $801\,kJ\,mol^{-1}$

  • Carbon ($C$): $1086\,kJ\,mol^{-1}$

  • Nitrogen ($N$): $1402\,kJ\,mol^{-1}$

  • Oxygen ($O$): $1314\,kJ\,mol^{-1}$

  • Fluorine ($F$): $1681\,kJ\,mol^{-1}$

  • Neon ($Ne$): $2081\,kJ\,mol^{-1}$

Ionization Energy Values of First Group Elements

• Overview: Ionization energies of elements in Group 1 decrease from top to bottom down the group as shown in Table (8.7).

• Element Values (Table 8.7):

  • Lithium ($Li$): $520\,kJ\,mol^{-1}$

  • Sodium ($Na$): $496\,kJ\,mol^{-1}$

  • Potassium ($K$): $419\,kJ\,mol^{-1}$

  • Rubidium ($Rb$): $403\,kJ\,mol^{-1}$

  • Cesium ($Cs$): $377\,kJ\,mol^{-1}$

Introduction to Electron Affinity

• Definition: Electron affinity represents the energy change that occurs when an electron is added to the outer shell of an isolated gaseous atom.