An element has the electron configuration 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6. Which of the following statements about this element is correct?
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Correct answer
B. It is a noble gas in period 6.
Principle or equation
The periodic table is arranged by electron configurations. Elements with completely filled s and p subshells in the outermost shell (ns2 np6) are noble gases. The period number corresponds to the highest principal quantum number (n) of the occupied orbitals.
Why this answer is correct
The configuration ends with 6s2 6p6, meaning the outermost shell (n=6) is completely filled with 8 electrons (2 in 6s and 6 in 6p). This is the characteristic configuration of a noble gas. The element is radon (Rn), which is in period 6 and group 18. It is not a transition metal (those have partially filled d orbitals), not an alkaline earth metal (those end in ns2 only), and not a main-group metal in group 14 (those end in ns2 np2).
Example
For neon (1s2 2s2 2p6), the outermost shell is n=2, completely filled, so it is a noble gas in period 2. Similarly, radon has n=6 completely filled.
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