In an atom of sodium (Na), which has an atomic number of 11, the electron configuration is 1s² 2s² 2p⁶ 3s¹. To determine how many orbitals are completely filled, we can look at the electron configuration:
- 1s² has 2 electrons, filling that orbital.
- 2s² also has 2 electrons, filling that orbital.
- 2p⁶ has 6 electrons, filling all three orbitals in the p subshell.
- 3s¹ has 1 electron, which means this orbital is not completely filled.
From this breakdown, we see that the completely filled orbitals are: 1s, 2s, and 2p. Thus, there are a total of 3 completely filled orbitals in sodium. Therefore, the correct answer is:
3 orbitals.