We can determine which atom has a larger atomic radius, Al or Mg, by looking at trends on the Periodic Table. Aluminum and Magnesium are in the same Period (row). As we go across a period on the Periodic Table the atomic radius decreases. Therefore, Mg is larger than Al.
General Trends for Atomic Radii (atomic size)
Atomic Radius Decreases Across a Period: As you move from left to right across a period (horizontal row) of the periodic table, atomic radius generally decreases. This trend occurs because as you move across a period, the number of protons in the nucleus increases, resulting in a stronger attractive force on the electrons. The increased positive charge pulls the electron cloud closer to the nucleus, leading to a smaller atomic radius.
Atomic Radius Increases Down a Group: As you move down a group (vertical column) of the periodic table, atomic radius generally increases. This trend occurs because each new row in the periodic table adds a new electron shell, further away from the nucleus. The additional electron shells shield the outermost electrons from the attractive force of the nucleus, causing the atomic radius to increase.
Cations have Smaller Radii than their Parent Atoms: When an atom loses one or more electrons to form a cation, the resulting ion has a smaller atomic radius than its parent atom. This is because the loss of electrons reduces the electron-electron repulsion, causing the remaining electrons to be pulled closer to the nucleus. The reduction in electron-electron repulsion leads to a smaller atomic radius for cations.
Anions have Larger Radii than their Parent Atoms: Conversely, when an atom gains one or more electrons to form an anion, the resulting ion has a larger atomic radius than its parent atom. The addition of extra electrons increases the electron-electron repulsion, causing the electron cloud to expand. The increased electron-electron repulsion results in a larger atomic radius for anions.
Based on these general trends we can say that Mg is larger than Al.
