The species that has the same number of electrons as 35 17 Cl is

The species that has the same number of electrons as ³⁵₁₇Cl is

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3216S

3416S+

4018Ar+

3516S2-

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Answer is Wrong!

This question was previously asked in

UPSC NDA-2 – 2017

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The number of electrons in a neutral atom is equal to its atomic number (the subscript). For an ion, the number of electrons is adjusted based on the charge.

The species is ³⁵₁₇Cl. The atomic number is 17, so a neutral Chlorine atom has 17 protons and 17 electrons. We need to find the species with 17 electrons.
A) ³²₁₆S: Atomic number 16. Neutral Sulfur has 16 electrons.
B) ³⁴₁₆S⁺: Atomic number 16. Neutral Sulfur has 16 electrons. S⁺ means it lost 1 electron (16 – 1 = 15 electrons).
C) ⁴⁰₁₈Ar⁺: Atomic number 18. Neutral Argon has 18 electrons. Ar⁺ means it lost 1 electron (18 – 1 = 17 electrons).
D) ³⁵₁₆S^2-: Atomic number 16. Neutral Sulfur has 16 electrons. S²⁻ means it gained 2 electrons (16 + 2 = 18 electrons).
Therefore, ⁴⁰₁₈Ar⁺ has 17 electrons, the same number as neutral ³⁵₁₇Cl. These species are isoelectronic.

The superscript number (mass number) is the sum of protons and neutrons and is irrelevant to the number of electrons in this calculation, unless isotopes are being considered, which is not the case here for electron count. The subscript number is the atomic number, which is crucial as it determines the number of protons and thus the number of electrons in a neutral atom.

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