Total number of ions from the following with noble gas configuration is _________.
Sr²⁺(z=38), Cs⁺(z=55), La²⁺(z=57), Pb²⁺(z=82), Yb²⁺(z=70) and Fe²⁺(z=26)

Solution

Chemistry ma’am · 3 steps

Subtract the charge from Z and check whether the electron count lands exactly on 2, 10, 18, 36, 54 or 86.

1. Count electrons. For a cation, electrons = Z - charge. An ion has a noble gas configuration only if this count equals a noble gas atomic number: 2, 10, 18, 36, 54 or 86.

   Ion	Electrons	Configuration	Noble gas?
   Sr²⁺	38 - 2 = 36	[Kr]	yes
   Cs⁺	55 - 1 = 54	[Xe]	yes
   La²⁺	57 - 2 = 55	[Xe] 5d¹	no
   Pb²⁺	82 - 2 = 80	[Xe] 4f¹⁴5d¹⁰6s²	no
   Yb²⁺	70 - 2 = 68	[Xe] 4f¹⁴	no
   Fe²⁺	26 - 2 = 24	[Ar] 3d⁶	no

2. Filled is not noble. Yb²⁺ and Pb²⁺ have completely filled subshells, but they carry extra electrons beyond the noble gas core. A stable filled subshell does not make the configuration a noble gas configuration.
3. So. Only Sr²⁺ and Cs⁺ match a noble gas, so the answer is 2.

The common slip: 3. Counting Yb²⁺ as noble gas because its 4f¹⁴ subshell is full gives 3, but 68 electrons is not a noble gas count.

The idea to keep. A noble gas configuration means the electron count equals a noble gas atomic number exactly. Filled d or f subshells on top of the core, as in Pb²⁺ or Yb²⁺, do not count.

Answer from the NTA final answer key. Solution by PrepWiser.