Correct statements for an element with atomic number 9 are: A. There can be 5 electrons for which ms = +12 and 4…

🧠 The "Aha!" Moment Atomic number $9$ is Fluorine ($ce{F}$). Its behavior is defined by a nearly-filled second shell. To judge these statements, we must map out its $9$ electrons following Pauli and Hund's rules.

🗺️ The Fact Check

1. Configuration: $1s^2, 2s^2, 2p^5$.
2. Statement A (Spin Balance): In $1s$ and $2s$, we have $2$ up (+1/2) and $2$ down (-1/2). In $2p^5$, following Hund's rule, we have $3$ up (+1/2) and $2$ down (-1/2). Total: $5$ electrons with $+1/2$ and $4$ with $-1/2$. (TRUE).
3. Statement B ($p$ occupancy): In a $p^5$ configuration, two orbitals are fully paired $(\uparrow\downarrow, \uparrow\downarrow)$ and one is half-filled $(\uparrow)$. Thus, there is exactly one electron in $p_z$ (or any one $p$ orbital). (TRUE).
4. Statement C (Last Electron): The 9th electron occupies the $2p$ subshell, defined by $n=2, l=1$. (TRUE).
5. Statement D (Angular Nodes): Number of angular nodes = $l$. Sum for all occupied orbitals ($1s, 2s, 2p_x, 2p_y, 2p_z$) = $0 + 0 + 1 + 1 + 1 = 3$. (FALSE). Based on the choices, A and B are the primary correct descriptors.

⚡ The Unpaired Electron Think of the 9th electron as the "lone survivor" in the $2p$ subshell. It is the only electron in its specific orbital "slot," while the others are all paired up.

⚠️ Nodes vs. Orbitals Don't just count $p$ as "one thing." There are three distinct $p$ orbitals in the subshell, and each one has its own nodal plane. The total count for an element like Fluorine is higher than 1.

$\boxed{\text{Answer: (b)}}$