You're right, sodium (Na) and chlorine (Cl) are both in the same period (Period 3) of the periodic table. However, they are not in the same group. This difference in their placement leads to a significant difference in their atomic sizes. Here's why:

* Atomic Structure: As you move across a period from left to right, the number of protons in the nucleus increases. This increased positive charge attracts the negatively charged electrons more strongly, pulling the electron cloud closer to the nucleus.

* Sodium vs. Chlorine:

* Sodium (Na): Has 11 protons and 11 electrons. Its outermost electron is in the 3s orbital.

* Chlorine (Cl): Has 17 protons and 17 electrons. Its outermost electrons are in the 3s and 3p orbitals.

* Effect on Size: Because chlorine has a greater nuclear charge (more protons), its electrons are drawn in more tightly. This makes chlorine smaller than sodium, even though both elements have their outermost electrons in the same energy level (3rd energy level).

In summary:

* Sodium (Na): Larger atomic size due to weaker attraction between its nucleus and outermost electron.

* Chlorine (Cl): Smaller atomic size due to stronger attraction between its nucleus and outermost electrons.

Let me know if you have any other questions!