MCQ
A reduction in atomic size with increase in atomic number is a characteristic of elements of:
- A$D-$block.
- ✓$F-$block.
- CRadioactive series.
- DHigh atomic masses.
✓
Answer
Correct option: B.
$F-$block.
A reduction in atomic size with increase in atomic number is a characteristic of $f-$block elements.
This is due to poor shielding of $f$ electrons. The extent of actinoid contraction is greater than lanthanoid contraction. With increase in atomic number i.e. in moving down a group, the number of the principal shell increases and therefore, the size of the atom increases. But in case of $f$ block elements there is a steady decrease in atomic size with increase in atomic number due to lanthanide contraction. As we move through the lanthanide series$, 4f$ electrons are being added one at each step.
The mutual shielding effect of $f$ electrons is very little. This is due to the shape of the $f$ orbitals. The nuclear charge, however increases by one at each step. Hence, the inward pull experienced by the $4f$ electrons increases. This causes a reduction in the size of the entire 4fn shell.
This is due to poor shielding of $f$ electrons. The extent of actinoid contraction is greater than lanthanoid contraction. With increase in atomic number i.e. in moving down a group, the number of the principal shell increases and therefore, the size of the atom increases. But in case of $f$ block elements there is a steady decrease in atomic size with increase in atomic number due to lanthanide contraction. As we move through the lanthanide series$, 4f$ electrons are being added one at each step.
The mutual shielding effect of $f$ electrons is very little. This is due to the shape of the $f$ orbitals. The nuclear charge, however increases by one at each step. Hence, the inward pull experienced by the $4f$ electrons increases. This causes a reduction in the size of the entire 4fn shell.
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