Question

current attempt in progress
why is the 11th electron of the sodium atom located in the third energy level rather than in the second energy level?
○ the 3s orbital is at a lower energy level than the 2d orbital.
○ sodium is unlike every other group 1a element.
○ the 3s orbital must have at least one electron before the 2p can be filled.
○ the first and second energy levels are filled.

Explanation:

The first energy level (\(n = 1\)) can hold a maximum of \(2\) electrons (\(1s^2\)). The second energy level (\(n=2\)) has \(s\) and \(p\) sub - shells. The \(2s\) sub - shell can hold \(2\) electrons (\(2s^2\)) and the \(2p\) sub - shell can hold \(6\) electrons (\(2p^6\)). So, the first two energy levels together can hold \(2 + 2+6=10\) electrons. According to the Aufbau principle (electrons fill orbitals starting from the lowest energy level), once the first two energy levels (\(n = 1\) and \(n = 2\)) are filled (with \(10\) electrons), the \(11^{th}\) electron of sodium must go to the next available energy level (\(n = 3\)).

• The \(2d\) orbital does not exist. The \(d\) orbitals start from \(n = 3\) (\(3d\)).
• Sodium follows the general electron - filling rules for group \(1A\) elements.
• The \(2p\) sub - shell is filled (\(2p^6\)) before electrons start filling the \(3s\) sub - shell.

Answer:

The first and second energy levels are filled.