* Atomic Number: Sulfur has an atomic number of 16, meaning it has 16 protons in its nucleus. Magnesium has an atomic number of 12, meaning it has 12 protons.

* Electron Configuration: Sulfur has the electron configuration [Ne] 3s² 3p⁴, while magnesium has [Ne] 3s². Both elements have their outermost electrons in the third energy level (n=3).

* Effective Nuclear Charge: Despite being in the same period, sulfur has a higher effective nuclear charge (the net positive charge experienced by an electron) than magnesium. This is because sulfur has more protons in its nucleus attracting its electrons more strongly.

* Shielding Effect: The electrons in the inner shells (n=1 and n=2) shield the outer electrons from the full positive charge of the nucleus. The shielding effect is roughly the same for both elements.

Therefore:

* Higher effective nuclear charge in sulfur pulls its electrons closer to the nucleus, leading to a smaller atomic radius.

* Magnesium's weaker effective nuclear charge allows its electrons to spread out more, resulting in a larger atomic radius.

In general, as you move across a period from left to right, atomic size decreases due to the increasing effective nuclear charge pulling the electrons closer to the nucleus.