(i) Due to the lanthanide contraction, thallium (Tl) has a higher first ionization enthalpy compared to gallium (Ga) and aluminum (Al). This is because the contraction results in a higher effective nuclear charge, which increases the ionization energy. (ii) Similarly, the scandide contraction causes gallium to have a higher ionization energy than aluminum due to a similar increase in effective nuclear charge, making electrons harder to remove.
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Approach Solution -2
Step 1: Understanding ionization enthalpy
Ionization enthalpy (or ionization energy) is the amount of energy required to remove the most loosely bound electron from a neutral gaseous atom to form a cation. It generally depends on factors such as atomic size, nuclear charge, shielding effect, and subshell configuration.
Step 2: Elements under consideration
We are comparing elements from Group 13 (the boron family):
- Aluminium (Al, Z = 13)
- Gallium (Ga, Z = 31)
- Thallium (Tl, Z = 81)
All three are in the same group, so their outer electronic configuration is \( ns^2 np^1 \). The variations in their ionization enthalpy arise mainly due to inner electron shielding and relativistic effects.
Step 3: Trends down the group
Generally, ionization enthalpy decreases down a group because atomic size increases and outer electrons are farther from the nucleus. However, in Group 13, this trend is not strictly regular due to additional effects:
- After aluminium, gallium has electrons filling the 3d subshell, which provides poor shielding. The ineffective shielding of 3d electrons increases the effective nuclear charge felt by the outer electrons, thus raising gallium’s ionization enthalpy above the expected value.
- For thallium, the presence of 4f and 5d electrons also causes poor shielding, increasing the effective nuclear charge even more. Moreover, relativistic effects stabilize the outermost electrons, further increasing ionization enthalpy.
Step 4: Explain the order
Taking all effects into account, the effective nuclear charge increases irregularly as follows:
\[
\text{Al} < \text{Ga} < \text{Tl}
\]
Hence, their first ionization enthalpy order becomes:
\[
\text{Tl} > \text{Ga} > \text{Al}
\]
Step 5: Conclusion
The correct order of the first ionization enthalpy is:
Tl > Ga > Al
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