As you move down a group (e.g. moving from Al to Ga), the number of electrons/filled orbitals means that the atomic radius of the atom

Understanding Atomic Radius Trends

1. Concept of Atomic Radius: The atomic radius refers to the size of an atom, typically measured as the distance from the nucleus to the outermost electrons.

2. Trend in a Group: As you move down a group in the periodic table, such as from Aluminum (Al) to Gallium (Ga), there is an increase in the atomic radius. This is due to the addition of filled electron shells, which increases the distance between the nucleus and the outermost electrons.

3. Factors Affecting Atomic Radius:

   • Electron Shielding: More inner shells of electrons shield the outer electrons from the positive charge of the nucleus.
   • Principal Quantum Number: Each element in a group has an increasing principal quantum number (n), which contributes to a higher energy level and thus a larger atomic radius.

Summary

As you move down a group in the periodic table from Al to Ga, the atomic radius increases primarily due to the addition of more electron shells, which increases the distance from the nucleus to the outermost electrons, and the effect of electron shielding. This leads to a larger atomic size despite the increase in nuclear charge, as the shielding effect overpowers the attraction between the nucleus and the outer electrons.