Which of the following electrons configurations correctly represents an inert element?

Analyze the question: An inert element (noble gas) has a completely filled outermost shell. This is an octet (8 electrons), or a duplet (2) for helium.

Concept: Noble gases (Group 18/0) are unreactive because their outer shells are full, so they neither gain, lose, nor share electrons easily.

Check each option:

• A. 1s²2s²2p⁴: 8 electrons in total, which is oxygen. The outer shell (n = 2) has 2 + 4 = 6 electrons, so it is not full. It needs 2 more electrons. Not inert.
• B. 1s²2s²2p⁶3s²: 12 electrons, which is magnesium. The outer shell (n = 3) has only 2 electrons. It tends to lose them. Not inert.
• C. 1s²2s²2p⁶3s²3p⁴: 16 electrons, which is sulfur. The outer shell has 6 electrons. Not inert.
• D. 1s²2s²2p⁶: 10 electrons, which is neon. The outer shell (n = 2) has 2 + 6 = 8 electrons, a complete octet. Inert.

Answer: Option D, which is the electron configuration of neon.

Exam tip: Look at the outermost shell only. If it ends in s²p⁶ (or is 1s² for helium), the element is a noble gas. You can also count the total electrons: 2, 10, 18, 36 are the atomic numbers of He, Ne, Ar, Kr.