Question

which of the following elements is a noble gas? 1s² 2s² 2p⁶ 3s² 3p⁶, 1s² 2s² 2p⁶ 3s¹, 1s² 2s², 1s² 2s² 2p⁵

Explanation:

Step1: Recall noble - gas electron - configuration

Noble gases have full valence - shell electron configurations. The general rule is that the s and p sub - shells of the outermost energy level are completely filled.

Step2: Analyze each option

• Option 1: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}$ corresponds to argon (Ar), which has a full outer shell (3s²3p⁶), a noble gas.
• Option 2: $1s^{2}2s^{2}2p^{6}3s^{1}$ is sodium (Na), an alkali metal with an incomplete outer shell.
• Option 3: $1s^{2}2s^{2}$ is beryllium (Be), an alkaline - earth metal with an incomplete outer shell.
• Option 4: $1s^{2}2s^{2}2p^{5}$ is fluorine (F), a halogen with an incomplete outer shell.

Answer:

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