Introduction:
The periodic table is divided into several blocks based on the electron configurations of elements. The d-block and p-block elements are two of these blocks. The d-block elements are also known as transition metals and are located in the middle of the periodic table, while the p-block elements are found on the right-hand side.

Differences in Atomic Size:
The size of an atom is determined by its atomic radius, which is the distance between the nucleus and the outermost electron shell. When comparing d-block and p-block elements, we can observe the following differences in atomic size:

1. Shielding Effect:
The d-block elements have a greater number of inner electrons, which provide a shielding effect on the outermost electrons. This shielding effect reduces the attractive force between the nucleus and the outermost electrons, resulting in a larger atomic size for d-block elements.

2. Effective Nuclear Charge:
The effective nuclear charge experienced by the outermost electrons in d-block elements is lower compared to p-block elements. This is because the inner electrons in d-block elements shield the outermost electrons from the full attraction of the nucleus. As a result, the outermost electrons are not pulled in as strongly, leading to a larger atomic size.

3. Electron Configuration:
The electron configuration of d-block elements involves filling electrons in the d-orbitals. These d-orbitals are higher in energy compared to the p-orbitals of p-block elements. The higher energy levels of d-block elements result in larger atomic size.

Exceptions:
It is important to note that there are exceptions to these general trends. For example, the atomic size of zinc (Zn) is smaller than that of gallium (Ga), even though Zn is a d-block element and Ga is a p-block element. This can be attributed to the presence of a completely filled d-orbital in Zn, which leads to greater electron-electron repulsion and a smaller atomic size.

Conclusion:
In conclusion, d-block elements are generally larger than p-block elements due to factors such as shielding effect, effective nuclear charge, and electron configuration. However, it is important to consider exceptions and anomalies in certain elements that may deviate from these trends. The atomic size of an element affects its chemical properties, reactivity, and bonding behavior, making it an important characteristic to consider in understanding the behavior of different elements.