Question

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identify the complete electron configuration for an atom of
the element sodium (na):

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$1s^{2}2s^{2}2p^{6}3s^{1}$
$2s^{2}2p^{6}3s^{1}$
$1s^{2}2s^{2}3s^{1}$
$1s^{2}2p^{8}3s^{1}$
$1s^{2}2p^{6}3s^{1}$
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Explanation:

Step1: Find Na's atomic number

Sodium (Na) has an atomic number of 11, so it has 11 electrons.

Step2: Fill orbitals by Aufbau principle

Fill orbitals in order: $1s$ (max 2 e⁻), $2s$ (max 2 e⁻), $2p$ (max 6 e⁻), $3s$ (remaining e⁻).
$1s^2$ uses 2 e⁻, $2s^2$ uses 2 e⁻, $2p^6$ uses 6 e⁻, total $2+2+6=10$ e⁻. 1 remaining e⁻ goes to $3s$: $3s^1$.
Total electrons: $2+2+6+1=11$, matching Na's electron count.

Step3: Eliminate incorrect options

• $2s^2 2p^6 3s^1$: missing $1s^2$ (only 9 e⁻)
• $1s^2 2s^2 3s^1$: missing $2p^6$ (only 5 e⁻)
• $1s^2 2p^8 3s^1$: $2p$ can only hold 6 e⁻
• $1s^2 2p^6 3s^1$: missing $2s^2$ (only 9 e⁻)

Answer:

$1s^2 2s^2 2p^6 3s^1$