3. Answer the following

vi. Explain trends in ionisation enthalpies of d block elements.


The ionization enthalpy, also known as ionization energy or ionization potential, is the energy required to remove an electron from an atom or ion in the gaseous state. When examining the trends in ionization enthalpies of d-block elements (transition metals), several important factors influence these trends:

  1. Electron Shielding (Effective Nuclear Charge):

    • As you move across a period (from left to right) in the periodic table, the number of protons in the nucleus increases, resulting in a higher positive charge in the nucleus.
    • However, the increase in the number of electrons is not uniform. Electrons are added to the same energy level (principal quantum number, n), but the shielding effect of inner-shell electrons remains relatively constant.
    • This leads to an increase in the effective nuclear charge (the positive charge experienced by the outermost electrons), which makes it more difficult to remove an electron.
    • Consequently, there is generally an increase in ionization enthalpy as you move from left to right across a period in the d-block.
  2. Atomic Radius:

    • As you move across a period, the atomic radius of the d-block elements generally decreases.
    • A smaller atomic radius means that the outermost electrons are closer to the nucleus, experiencing a stronger attractive force. As a result, it requires more energy to remove an electron, leading to higher ionization enthalpy.
  3. Electron Configuration:

    • The electron configuration of transition metals often involves the gradual filling of d orbitals. As more electrons are added to a particular orbital, it becomes more difficult to remove an electron from that orbital.
    • Half-filled and fully-filled d orbitals are more stable due to electron-electron repulsion and exchange energy considerations. Consequently, it is more challenging to remove electrons from these orbitals, resulting in higher ionization enthalpies for elements with half-filled or fully-filled d orbitals.
  4. Group Trend (Down a Group):

    • As you move down a group in the periodic table (from top to bottom), the ionization enthalpy generally decreases.
    • This is primarily because additional energy levels (principal quantum levels, n) are being added, leading to an increase in atomic size. Electrons in outer energy levels are farther from the nucleus and are shielded by inner electrons, making them easier to remove.

In summary, the trends in ionization enthalpies of d-block elements involve an increase in ionization enthalpy across a period (left to right) due to an increase in effective nuclear charge, decreasing atomic radius, and electron configuration effects. Conversely, ionization enthalpy tends to decrease down a group (top to bottom) due to the addition of energy levels and increased atomic size.