Answer:-

The atomic radii of d-block elements, also known as transition metals, show some distinct trends as you move across a period (from left to right) and down a group (from top to bottom) in the periodic table. These trends can be explained by various factors:

1. Across a Period (Left to Right):

   • As you move across a period from left to right, the atomic radii of d-block elements generally decrease.
   • This trend can be attributed to an increase in effective nuclear charge (the positive charge felt by the outermost electrons) as you move from left to right. The number of protons in the nucleus increases, but the number of inner-shell electrons remains relatively constant. This increased positive charge pulls the electrons closer to the nucleus, resulting in a smaller atomic radius.

2. Down a Group (Top to Bottom):

   • When you move down a group of d-block elements, the atomic radii generally increase.
   • This increase in atomic size can be explained by the addition of new energy levels or electron shells as you go down the group. Each new energy level is farther from the nucleus, and the inner-shell electrons shield the outer electrons from the increasing positive charge of the nucleus. As a result, the outer electrons are held less tightly and are farther from the nucleus, leading to a larger atomic radius.

3. Transition Metals vs. Non-transition Metals:

   • Transition metals, in general, have smaller atomic radii compared to non-transition metals in the same period. This is because transition metals have a greater effective nuclear charge due to their partially filled d orbitals, which are closer to the nucleus.

4. Variations within Transition Metals:

   • Within the transition metals, there can be variations in atomic radii based on the specific electronic configurations of each element. For example, the atomic radii of some transition metals may be smaller or larger than neighboring elements due to the electron configuration in their outermost energy levels.

5. Exceptions: There can be exceptions to these trends based on the specific electronic configurations and subtle interactions between electrons in the d orbitals. For example, the atomic radii of certain transition metals can be slightly larger or smaller than expected due to factors like electron repulsions and shielding effects.

In summary, the trends in atomic radii of d-block elements involve a decrease in atomic size across a period due to increased effective nuclear charge and an increase in atomic size down a group due to the addition of energy levels and increased electron shielding. These trends help explain the variations in atomic radii observed among the transition metals in the periodic table.