Question

which orbital diagram correctly represents fluorine (atomic number 9)?

Explanation:

Step1: Recall electron - filling rules

Electrons fill orbitals following the Aufbau principle (low - energy orbitals first), Pauli exclusion principle (each orbital can hold at most 2 electrons with opposite spins), and Hund's rule (electrons in degenerate orbitals occupy them singly with parallel spins first).

Step2: Determine electron configuration of fluorine

Fluorine has an atomic number of 9, so it has 9 electrons. The electron configuration is $1s^{2}2s^{2}2p^{5}$.

Step3: Analyze orbital diagrams

In the $1s$ orbital, there are 2 electrons with opposite spins (represented by $\uparrow\downarrow$). In the $2s$ orbital, there are 2 electrons with opposite spins. In the $2p$ orbitals (which have 3 degenerate orbitals), 4 electrons pair up in two of the orbitals and 1 electron is unpaired in the third orbital.

Answer:

The second orbital diagram (with $\uparrow\downarrow$ in $1s$, $\uparrow\downarrow$ in $2s$, and $\uparrow\downarrow\uparrow\downarrow\uparrow$ in $2p$) is correct.